The Appendages, Anatomy, and Relationships of Trilobites
by Percy E. Raymond · English · Gutenberg #41695 · 569,380 chars · public domain (US)
The Project Gutenberg eBook of The Appendages, Anatomy, and Relationships of Trilobites
This eBook is for the use of anyone anywhere in the United States and
most other parts of the world at no cost and with almost no restrictions
whatsoever. You may copy it, give it away or re-use it under the terms
of the Project Gutenberg License included with this eBook or online
at www.gutenberg.org. If you are not located in the United States,
you will have to check the laws of the country where you are located
before using this eBook.
Title: The Appendages, Anatomy, and Relationships of Trilobites
Author: Percy E. Raymond
Release date: December 24, 2012 [eBook #41695]
Most recently updated: October 23, 2024
Language: English
Other information and formats: www.gutenberg.org/ebooks/41695
Credits: Produced by Thomas Cosmas. Produced from files made
available on The Internet Archive.
*** START OF THE PROJECT GUTENBERG EBOOK THE APPENDAGES, ANATOMY, AND RELATIONSHIPS OF TRILOBITES ***
Transcriber's Notes
Text emphasis is denoted as _Text_ for italic and =Text= for bold.
Whole and fractional parts are shown as 4-2/3.
OE and oe ligature converted to Oe and oe respectively.
* * * * *
MEMOIRS OF
THE CONNECTICUT ACADEMY
OF ARTS AND SCIENCES
VOLUME VII DECEMBER, 1920
The Appendages, Anatomy, and Relationships of Trilobites
BY
PERCY E. RAYMOND, Ph.D.
ASSOCIATE PROFESSOR OF PALAEONTOLOGY, AND CURATOR OF INVERTEBRATE
PALAEONTOLOGY IN THE MUSEUM OF COMPARATIVE ZOOLOGY,
HARVARD UNIVERSITY
[Illustration: (logo)]
NEW HAVEN, CONNECTICUT
PUBLISHED BY THE
CONNECTICUT ACADEMY OF ARTS AND SCIENCES
AND TO BE OBTAINED ALSO FROM THE
YALE UNIVERSITY PRESS
[Illustration (photo)]
[Illustration (signature)]
MEMOIRS OF
THE CONNECTICUT ACADEMY
OF ARTS AND SCIENCES
VOLUME VII DECEMBER, 1920
The Appendages, Anatomy, and Relationships
of Trilobites
BY
PERCY E. RAYMOND, Ph.D.
ASSOCIATE PROFESSOR OF PALAEONTOLOGY, AND CURATOR OF INVERTEBRATE
PALAEONTOLOGY IN THE MUSEUM OF COMPARATIVE ZOOLOGY,
HARVARD UNIVERSITY
[Illustration: (logo)]
NEW HAVEN, CONNECTICUT
PUBLISHED BY THE
CONNECTICUT ACADEMY OF ARTS AND SCIENCES
AND TO BE OBTAINED ALSO FROM THE
YALE UNIVERSITY PRESS
THE TUTTLE, MOREHOUSE & TAYLOR COMPANY
TO THE MEMORY OF
CHARLES EMERSON BEECHER
SKILLFUL WITH HAND, BRAIN, AND PEN; REVEALER OF THE MYSTERIES
OF TRILOBITES;
THIS MEMOIR IS DEDICATED
FOREWORD.
By CHARLES SCHUCHERT.
Trilobites are among the most interesting of invertebrate fossils and
have long attracted the attention of amateur collectors and men of
science. These "three-lobed minerals" have been mentioned or described
in books at least since 1698 and now several thousand species are
known to palæontologists. To this group of students they are the most
characteristic animals of the seas of Palæozoic time, and even though
they are usually preserved as dismembered parts, thousands upon
thousands of "whole ones" are stored in the museums of the world. By
"whole ones" perfect individuals are not meant, for before they became
fossils the wear and tear of their time and the process of
decomposition had taken away all the softer parts and even most of the
harder exterior covering. What is usually preserved and revealed to us
when the trilobites weather out of the embrace of their entombing
rocks is the test, the hard shell of the upper or dorsal side. From
time to time fragments of the under or limb-bearing side had been
discovered, first by Elkanah Billings, but before 1876 there was no
known place to which one could go to dig out of the ground trilobites
retaining the parts of the ventral side.
Students of trilobites have always wanted specimens to be delivered to
them weathered out of the rock by nature and revealing the ventral
anatomy without further work than the collecting, but the wish has
never been fulfilled. In the Utica black shales, near Rome, New York,
there was finally discovered in 1892 a layer less than ten millimeters
thick, bearing hundreds of _Triarthrus becki_ with most of the ventral
anatomy intact. The collector's first inkling that such were present
in the Utica formation came to him in a chance find in 1884, and for
eight years he sought off and on for the stratum whence this specimen
came. His long search was finally rewarded by the discovery of the
bed, and lo! here were to be had, in golden color, prostrate specimens
with the breathing and crawling legs and the long and beautifully
curved feeling organs all replaced by iron pyrites. Fool's gold in
this case helped to make a palæontologic paradise. The bed contained
not only such specimens of _Triarthrus becki_, but also, though more
rarely, of _Cryptolithus tessellatus_ and exceptionally of _Acidaspis
trentonensis_. This important discovery, which has figured so largely
in unraveling the evolution of the Crustacea and even has a bearing on
that of most of the Arthropoda, was made by Mr. W. S. Valiant, then
curator of the Museum of Rutgers College.
There were, however, great material difficulties to overcome before
the specimens revealed themselves with all of their information
exposed for study. No surgeon was needed, but a worker knowing the
great scientific value of what was hidden, and with endless patience
and marked skill in preparation of fossils. Much could be revealed
with the hammer, because specimens were fairly abundant. A chance
fracture at times showed considerable portions, often both antennæ
entire, and more rarely the limbs protruding beyond the test, but the
entire detail of any one limb or the variation between the limbs of
the head, thorax, and tail was the problem to be solved. No man ever
loved a knotty problem more than Charles E. Beecher. Any new puzzle
tempted him, and this one of _Triarthrus becki_ interested him most of
all and kept him busy for years. From the summer of 1893. when he
quarried out two tons of the pay stratum at Rome, until his death in
1904, his time was devoted in the main to its solution by preparing
these trilobites and learning their anatomical significance.
The specimens of _Triarthrus becki_ from Rome are pseudomorphs
composed of iron pyrites, as has been said, and are buried in a
gray-black carbonaceous shale. A little rubbing of the specimens soon
makes of them bronze images of the former trilobite and while under
preparation they are therefore easily seen. However, as the average
individual is under an inch in length and as all the limbs other than
the antennæ are double or biramous, one lying over the other, and the
outer one fringed with a filamentous beard, the parts to be revealed
by the preparator are so small and delicate that the final touch often
obliterates them. These inherent difficulties in the material were
finally overcome by endless trials on several thousand specimens, each
one of which revealed something of the ventral anatomy. Finally some
500 specimens worthy of detailed preparation were left, and on about
50 of these Beecher's descriptions of _Triarthrus_ and _Cryptolithus_
were based.
The black shale in which the specimens are buried is softer than the
pseudomorphous trilobites, a condition that is of the greatest value
in preparation. With chisel and mallet the trilobites are sought in
the slabs of shale and then with sharp chisels of the dental type they
are revealed in the rough. At first Beecher sought to clean them
further by chemical methods, and together with his friends, the
chemist Horace L. Wells, and the petrologist Louis V. Pirsson, several
solutions were tried, but in all cases the fossils were so much
decomposed as to make them useless in study. Therefore Beecher had to
depend wholly oh abrasives applied to the specimens with pieces of
rubber. Much of this delicate work was done on a dental lathe, but in
the final cleaning most of it was done with patient work by hand.
Rubber has the great advantage of being tough and yet much softer than
either specimen or shale. As the shale is softer than the iron
pyrites, the abrasives (carborundum, emery, or pumice) took away the
matrix more quickly than the trilobite itself. When a part was fully
developed, the rubbers were cut to smaller and smaller dimensions and
the abrading reduced to minute areas. So the work went on and on,
helped along from time to time by the dental chisels. Finally Beecher
became so expert with these fossils that after one side was developed
he would embed the specimen in Canada balsam and fix it on a glass
slide, thus enabling him to cut down from the opposite side. This was
done especially with _Cryptolithus_ because of the great scarcity of
material preserving the limbs, and two of these revealed both sides of
the individuals, though they were then hardly thicker than writing
paper.
Then came illustrations, which at first were camera-lucida drawings in
pencil smoothed out with pen and ink. "In some quarters," however, it
has been said, "his methods unknown, their results were not accepted;
they were regarded as startling, as iconoclastic, and even
unreliable." He therefore decided to rework his material and to
illustrate his publications with enlarged photographs. The specimens
were black, there was little relief between fossil and matrix, and the
ammonium chloride process of coating them white and photographing
under artificial light was unsuitable. Nevertheless, after many
trials, he finally succeeded in making fine enlarged photographs of
the trilobites immersed in liquid Canada balsam, with a contact cover
of glass through which the picture was taken, the camera standing
vertically over the horizontal specimen. Beecher had completed this
work in 1903 and in the winter of 1903-1904 was making the drawings,
nearly all of which are here reproduced. On Sunday morning, February
14, 1904, as he was working at home on a large wash drawing of
_Cryptolithus_, death came to him suddenly, leaving the trilobite
problem but partially solved.
When the writer, in the autumn of 1904, succeeded Professor Beecher in
the chair of Palæontology at Yale, he expected to find considerable
manuscript relating to the ventral anatomy of the trilobites, but
there was only one page. It was Beecher's method first to prepare and
thoroughly study the material in hand, then to make the necessary
illustrations, and between times to read what others had written.
There was no written output until everything had been investigated and
read, certain passages being marked for later reference. Then when all
was assimilated, he would write the headings of topics as they came to
him, later cutting them apart and arranging them in a logical
sequence. When the writer visited him in his home in January 1904, he
was primed for his final trilobite memoir, but the writing of it had
not been begun.
The writer has never made the trilobites his special subjects for
study as he has the brachiopods, and therefore felt that he should not
try to bring to light merely the material things that Beecher had so
well wrought out. It seemed at first an impossible task to find the
specialist and friend to do Beecher justice, but as the years have
passed, one of Beecher's students, always especially interested in
trilobites, has grown into a full appreciation of their structures and
significance, and to him has fallen the continuation of his master's
work. If in the following pages he departs here and there from the
accepted interpretation and the results of others, it is because his
scientific training, in desiring to see with his own eyes the
structures as they are, has led him to accept only those
interpretations that are based on tangible evidence as he understands
such. Furthermore, in seeking the relationship of the trilobites to
the rest of the Arthropoda, his wide study of material and literature,
checked up by the ontogeny of fossil and recent forms, has led him in
places from the beaten path of supposedly ascertained phylogenies. His
results, however, have been won through a detailed study of the
interrelations of the Arthropoda, starting from the fact that the
Trilobita are chronogenetically the oldest and most primitive. The
trilobites are held by him to be the most simple, generalized, ancient
Crustacea known, and the progenitors, directly and indirectly, of all
Arthropoda.
It is now twenty-six years since Professor Beecher began his
publications on the class Trilobita, and in commemoration of him and
his work, Professor Percy E. Raymond of Harvard University presents
this memoir, to bring to fruition the studies and teachings of his
honored guide. It has been with Professor Raymond a labor of love, and
it is for the writer of this foreword a long-desired memorial to the
man to whose position in the Museum and University he had the
privilege of succeeding.
Yale University, New Haven, Connecticut.
PREFACE.
The primary object of this memoir is, as has been stated by Professor
Schuchert, to rescue from oblivion the results of the last few years
of Professor Beecher's investigations on the ventral anatomy of
trilobites. Since he left his data in the form of drawings and
photographs, without even rough notes, it became necessary, in order
to write a text to accompany the plates, to restudy the entire
subject. Under these circumstances, it seemed best to include all that
is known about the appendages of trilobites, thus bringing together a
summary of present information on the subject.
The growth of the memoir to its present size has been a gradual one.
As first completed in 1917, it contained an account of the appendages
only. Thoughts upon the probable use of the appendages led to the
discussion of possible habits, and that in turn to a consideration of
all that is known or could be inferred of the structure and anatomy of
the trilobite. Then followed an inquiry into the relationships to
other Arthropoda, which ultimately upset firmly established
preconceptions of the isolated position of the group, and led to a
modification of Bernard's view of its ancestry.
During the progress of the work, I have had the opportunity of
examining most of the known specimens retaining appendages. From the
Marsh collection in the Yale University Museum were selected the
forty-six specimens showing best the appendages of _Triarthrus_,
_Cryptolithus_, and _Acidaspis_. Dr. Charles D. Walcott very kindly
returned to the Museum of Comparative Zoology the slices of
_Ceraurus_, _Calymene_, and _Isotelus_ which were the basis of his
paper of 1881, and which had been loaned him for further study. He
loaned also eight of the more important specimens of _Neolenus
serratus_, and two of _Triarthrus becki_. At the United States
National Museum I saw the specimens of _Isotelus_ described by
Mickleborough and the isolated limbs of _Calymene_ from near
Cincinnati. The _Isotelus_ at Ottawa I had already studied with some
care while an officer of the Geological Survey of Canada.
This memoir consists, as shown in the table of contents, of four
parts. The appendages of _Neolenus_, _Isotelus_, _Ptychoparia_,
_Kootenia_, _Ceraurus_, _Calymene_, and _Acidaspis_ are discussed, as
fully as circumstances warrant, in the first part, and new
restorations of the ventral surfaces of _Neolenus_, _Isotelus_,
_Triarthrus_, _Ceraurus_ and _Cryptolithus_ are included It is not
supposed that these restorations will be of permanent value in all of
their detail, but they are put forward as the best approximations to
the real structure that the writer is able to present from the
materials so far discovered. I am greatly indebted to Doctor Elvira
Wood for the care and skill with which she has worked up these
restorations from my rather sketchy suggestions. She has put into them
not only a great amount of patient work, but also the results of
considerable study of the specimens.
Part II is a discussion of the internal anatomy of the trilobite and a
brief statement of some of the possible habits and methods of life of
these animals. Part III, which begins with a survey of the
relationships of the trilobites to other Arthropoda, is largely taken
up with an attempt to demonstrate the primitive characteristics of the
former, and their probable ancestral position. The form of the
ancestor of the trilobite is deduced from a study of the morphology,
ontogeny, and phylogeny of the group, and evidence adduced to indicate
that it was a depressed, flattened, free-swimming animal of few
segments.
In Part IV are included somewhat detailed descriptions of a few of the
best specimens of _Triarthrus_ and _Cryptolithus_. Professor Beecher,
while an observer of the minutest details, believed in publishing only
the broader, more general results of his investigations. This method
made his papers brief, readable, and striking, but it also resulted in
leaving in some minds a certain amount of doubt about the correctness
of the observations. In a matter so important as this, it has seemed
that palæontologists are entitled to the fullest possible knowledge of
the specimens on which the conclusions are based. The last part is,
therefore, a record of the data for the restorations of _Triarthrus_
and _Cryptolithus_.
The illustrations in the plates were nearly all made by or under the
supervision of Professor Beecher, as were also text figures 45 and 46.
In conclusion, I wish to express my thanks to Mrs. Charles E. Beecher
for the use of drawings which were the personal property of Professor
Beecher; to Doctor Charles D. Walcott for photographs of the limbs of
_Calymene_, and for his kindness in sending me the slices of
trilobites from Trenton Falls and specimens of _Neolenus_ and
_Triarthrus_; to Doctor R. V. Chamberlin for suggestions and
criticisms in regard to the relationship of trilobites to Insecta,
Arachnida, Chilopoda, and Diplopoda; to Mr. Samuel Henshaw, Director
of the Museum of Comparative Zoology, for permission to use the time
which has been devoted to this work; and to Miss Clara M. Le Vene, for
assistance in the preparation of the manuscript. My greatest debt is
to Professor Charles Schuchert, to whom the work owed its inception,
who has assisted in many ways during its prosecution, and who read the
manuscript, and arranged for its publication. To him I can only
express my warmest thanks for the favors which I have received and for
the efforts which he has put forth to make this a worthy memorial to
our friend and my teacher, Professor Charles Emerson Beecher.
Harvard University, Cambridge, Mass.
November, 1919.
TABLE OF CONTENTS.
Historical review 17
Part I. The appendages of trilobites 20
Terminology 20
The appendages of _Neolenus_ 21
Historical 21
_Neolenus serratus_ (Rominger) 21
Cephalon 21
Thorax 22
Pygidium 23
Epipodites and exites 23
Description of individual specimens 23
Restoration of _Neolenus_ 30
_Nathorstia transitans_ Walcott 31
The appendages of _Isotelus_ 32
Historical 32
_Isotelus latus_ Raymond 34
_Isotelus maximus_ Locke 35
Restoration of _Isotelus_ 37
_Isotelus gigas_ Dekay 37
_Isotelus arenicola_ Raymond 39
The appendages of _Triarthrus_ (see also Part IV) 39
_Triarthrus becki_ Green 39
Historical 40
Restoration of _Triarthrus_ 42
Relation of cephalic appendages to marking on
dorsal surface of glabella 43
Anal plate 44
The appendages of _Ptychoparia_ 45
_Ptychoparia striata_ (Emmrich) 45
_Ptychoparia cordilleræ_ (Rominger) 45
_Ptychoparia permulta_ Walcott 45
The appendages of _Kootenia_ 46
_Kootenia dawsoni_ Walcott 46
The appendages of _Calymene_ and _Ceraurus_ 46
Historical 46
Comparison of the appendages of _Calymene_ and
_Ceraurus_ with those of _Triarthrus_ 47
Spiral branchiæ 48
Ventral membrane 50
Appendifers 51
_Calymene senaria_ Conrad 52
Cephalic appendages 52
Thoracic appendages 53
Pygidial appendages 54
Relation of hypostoma to cephalon in _Calymene_ 55
Restoration of _Calymene_ 56
_Calymene_ sp. ind. 56
_Ceraurus pleurexanthemus_ Green 57
Cephalic appendages 58
Thoracic appendages 59
Pygidial appendages 59
Relation of hypostoma to cephalon 59
Restoration of _Ceraurus pleurexanthemus_ 60
The appendages of _Acidaspis trentonensis_ Walcott 61
The appendages of _Cryptolithus_ (see also Part IV) 61
_Cryptolithus tessellatus_ Green 61
Restoration of _Cryptolithus_ 62
Summary on the ventral anatomy of trilobites 64
Comparison of appendages of different genera 64
Coxopodite 64
Cephalon 64
Thorax 66
Pygidium 67
Caudal rami 68
Homology of cephalic appendages with those of
other Crustacea 69
Functions of the appendages 70
Antennules 70
Exopodites 70
Endopodites 71
Use of the pygidium in swimming 72
Coxopodites 74
Position of the appendages in life 74
Part II. Structure and habits of trilobites 77
Internal organs and muscles 77
Alimentary canal 77
_Ceraurus pleurexanthemus_ 79
_Calymene senaria_ 80
_Cryptolithus goldfussi_ 80
Summary 81
Gastric glands 82
Summary 84
Heart 85
_Illænus_ 85
_Ceraurus_ and _Calymene_ 85
The median "ocellus" or "dorsal organ" 86
Nervous system 89
Various glands 89
Dermal glands 89
Renal excretory organs 90
Reproductive organs 90
Panderian organs 90
Musculature 91
Flexor muscles 92
Extensor muscles 92
Hypostomial muscles 94
Eyes 96
Summary 97
Sex 98
Eggs 98
Methods of life (See also under "Functions of
the Appendages") 98
Habits of locomotion 99
Food and feeding methods 103
Tracks and trails 104
Part III. Relationship of the trilobites to other
Arthropoda 106
Crustacea 106
Branchiopoda 106
_Burgessia bella_ Walcott 108
_Waptia fieldensis_ Walcott 108
_Yohoia tenuis_ Walcott 109
_Opabina regalis_ Walcott 109
Summary 109
Copepoda 110
Archicopepoda 111
Ostracoda 112
Cirripedia 113
Malacostraca 113
Phyllocarida 113
Syncarida 114
Isopoda 114
_Marrella splendens_ Walcott 115
Restoration of _Marrella_ 116
Arachnida 117
Trilobites not Arachnida 117
Merostomata 119
_Sidneyia inexpectans_ Walcott 119
_Emeraldella brocki_ Walcott 119
_Molaria_ and _Habelia_ 120
Araneæ 121
Insecta 122
Chilopoda 123
Diplopoda 124
Primitive characteristics of trilobites 125
Trilobites the most primitive arthropods 125
Limbs of trilobites primitive 125
Summary 128
Number of segments in the trunk 128
Form of the simplest protaspis 132
Origin of the pygidium 134
Width of the axial lobe 137
Presence or absence of a "brim" 137
Segmentation of the glabella 137
Summary 138
The simplest trilobite 138
_Naraoia compacta_ Walcott 139
The ancestor of the trilobites, and the descent
of the Arthropoda 140
Evolution within the Crustacea 142
Summary 144
Evolution of the Merostomata 146
Evolution of the "Tracheata" 147
Summary on lines of descent 147
Final summary 151
Part IV. Description of the appendages of
individual specimens 152
_Triarthrus becki_ Green 152
_Cryptolithus tessellatus_ Green 158
Bibliography 163
LIST OF ILLUSTRATIONS.
1. _Triarthrus becki_ Green. Diagram of limb to show
nomenclature employed 20
2. _Neolenus serratus_ (Rominger). Two thoracic appendages 24
3. The same. An exopodite 26
4. The same. A so-called "epipodite" 26
5. The same. The so-called "exites" 29
6. The same. A cephalic limb 29
7. The same. Restoration of a transverse section 30
8. The same. Restoration of the ventral surface 31
9. _Isotelus_. Restoration of the ventral surface 38
10. _Triarthrus becki_ Green. Restoration of the ventral surface 41
11. The same. Median appendage 44
12. _Ceraurus pleurexanthemus_ Green. Slice showing an exopodite 49
13. _Calymene senaria_ Conrad. Slice showing cephalic coxopodites 53
14. The same. Another similar slice 53
15. The same. Slice showing method of articulation of
the appendages 53
16. The same. Restoration of the ventral surface 55
17. _Ceraurus pleurexanthemus_ Green. Slice showing the method
of articulation of the appendages 58
18. The same. Slice showing an exopodite above an endopodite 58
19. The same. Restoration of a transverse section 60
20. _Cryptolithus tessellatus_ Green. Restoration of the
ventral surface 63
21. _Ceraurus pleurexanthemus_ Green. Slice showing the
abdominal sheath 79
22. The same. Slice showing the large alimentary canal 79
23. _Calymene senaria_ Green. Slice showing the large
alimentary canal 79
24. _Ceraurus pleurexanthemus_ Green. Restoration of a
longitudinal section 81
25. _Cryptolithus tessellatus_ Green. Cheek showing the
genal cæca 84
26. _Illænus._ Volborth's figure of the heart 85
27. Heart of _Apus_ 85
28. _Isotelus gigas_ Dekay. The Panderian organs 91
29. _Ceraurus pleurexanthemus_ Green. Restoration, showing
heart, alimentary canal, and extensor muscles 93
30. The same. Longitudinal section of cephalon 95
31. _Nileus armadillo_ Dalman. Moberg's figure of the
muscle-scars 95
32. _Marrella splendens_ Walcott. Restoration of the
ventral surface 116
33. _Triarthrus becki_ Green. Appendage of the anterior part
of the thorax 126
34. _Apus._ Appendage from the anterior part of the trunk 127
35. _Weymouthia nobilis_ (Ford) 138
36. _Naraoia compacta_ Walcott 145
37. _Pagetia clytia_ Walcott 145
38. _Asaphiscus wheeleri_ Meek 145
39. _Pædeumias robsonensis_ Burling 145
40. _Robergia_ sp. 145
41. Diagram showing possible lines of descent of the Arthropoda 150
42. _Triarthrus becki_ Green. Thoracic appendages 155
43. The same. Pygidial appendages 157
44. The same. Pygidial appendages 158
45. _Cryptolithus tessellatus_ Green. Drawing of the best
single specimen 159
46. The same. Part of the thorax and pygidium, with appendages 162
_Frontispiece._ Charles Emerson Beecher, 1896.
Plates 1-5. Photographs of _Triarthrus becki_, made by C. E. Beecher.
Plate 6. Photographs of _Triarthrus becki_ (figs. 1-3), _Acidaspis
trentonensis_ (fig. 6), and _Cryptolithus tessellatus_ (fig. 7),
made by C. E. Beecher. Photographs of the endopodites of a probable
species of _Calymene_ (figs. 4, 5)
Plates 7-8. Photographs of _Cryptolithus tessellatus_, made by C. E.
Beecher.
Plate 9. Drawings of _Cryptolithus tessellatus_, made by C. E.
Beecher or under his direction.
Plate 10. Photographs of _Isotelus latus_ and _I. maximus_, made by
C. E. Beecher.
Plate 11. Drawing of a restoration of _Ceraurus pleurexanthemus_,
made by Elvira Wood.
HISTORICAL REVIEW.
The beginning of the search for the limbs of trilobites was coeval
with the beginning of scientific study of the group, knowledge of the
appendages being essential to the proper systematic allocation of the
animals.
The early search was so barren of results that negative evidence came
to be accepted as of positive value, and it was for many years
generally believed that such organs as may have been present beneath
the dorsal test were so soft as to be incapable of preservation. This
view is best expressed by Burmeister (1846, p. 43):
There is good proof that the feet of trilobites must have been soft
membranous organs, for the absence of the slightest remains of
these organs in the numerous specimens observed is of itself
evidence of the fact, and it can indeed scarcely be supposed that
hard horny extremities should be affixed to a soft membranous
abdominal surface; since they would not have possessed that firm
basis, which all solid organs of locomotion require, in order that
they may be properly available.
Very well reasoned, and were it not for the discovery of new material
in American localities, Burmeister's views would probably never have
been proved incorrect. One can not escape the suspicion that some of
the accepted hypotheses of today, founded on similar "proof," may
yield in time to the weight of bits of positive evidence.
The history of the study of appendages of trilobites may be divided
into two periods. The first, in which there was a general belief that
the appendages were soft organs, but during which numerous "finds" of
limbs were reported, extended from the time of Linné to the year
(1876) in which Walcott demonstrated the fact that the animals
possessed jointed ambulatory and breathing organs.
The second, much more fruitful period, began with Walcott's
publication of 1881, descriptive of the appendages of _Ceraurus_ and
_Calymene_, and for the purposes of this memoir, closes with his great
contribution on the anatomy of _Neolenus_ (1918). Beecher's brilliant
productions came in the middle of the second period.
In the first period, there were at least two authentic discoveries of
appendages, those of Eichwald (1825) and Billings (1870), but since
neither of these men convinced his confreres of the value of his
finds, the work of neither can be considered as having marked an
especial epoch in the history.
As all the authentic finds will be treated in detail on later pages,
only a brief résumé of the first period will be given here. This has
already been done by Burmeister (1843, 1846) and Barrande (1852,
1872), whose works have been my primary sources of information, but I
have looked up the original papers, copies of nearly all of which are
to be seen in the libraries in Cambridge and Boston. Brig.-Gen. A. W.
Vogdes, U. S. A. (retired), has very kindly placed at my disposal a
number of references and notes.
Linné (1759) was the first to report the discovery of appendages of
trilobites. Törnquist (1896) has pressed for a recognition of the
contribution of the great Swedish naturalist to this problem, but
Beecher (1896 B) doubted the validity of the find. Linné figured a
specimen of _Parabolina spinulosa_ (Wahlenberg), with what he
interpreted as a pair of antennæ attached. He states (translation
quoted from Törnquist): "Most remarkable in this specimen are the
antennæ in the front, which I never saw in any other sample, and which
clearly prove this fossil to belong to the insects." Beecher has shown
as conclusively as can be shown without access to the original
specimen that the supposed antennæ were really only portions of the
thickened anterior border, the appearance being due to imperfect
preservation. Brünnich as early as 1781 called attention to the
imperfection of this specimen, and it is also referred to by
Wahlenberg (1821, p. 39), Brongniart (1822, p. 42), Dalman (1828, p.
73), and Angelin (1854, p. 46).
Audouin (1821) seems to have been the first naturalist with sufficient
knowledge of the Arthropoda to be competent to undertake the study of
the trilobites. He concluded that the absence of ventral appendages
was probably a necessary consequence of the skeletal conformation, and
thought if any were discovered, they would prove to be of a branchial
nature.
Wahlenberg (1821) in the same year expressed his belief that the
trilobites were nearly allied to _Limulus_ and in particular tried to
show that the trilobites could have had masticatory appendages
attached about the mouth as in that modern "insect" (p. 20).
Wahlenberg was also the first to describe an hypostoma of a trilobite
(p. 37, pl. 1, fig. 6), but did not understand the nature of his
specimen, which he described as a distinct species.
Brongniart (1822, p. 40) devoted five pages of his monograph to a
discussion of the affinities of trilobites, concluding that it was
very probable that the animals lacked antennæ and feet, unless it
might be that they had short soft feet which would allow them to creep
about and fix themselves to other bodies.
Schlotheim (1823) thought that the spines on _Agnostus pisiformis_
were segmented and compared them with the antennæ of _Acarus_.
Stokes (1823) was the first who, with understanding, published an
illustration of the ventral side of a trilobite, having figured the
hypostoma of an _Isotelus_. He was followed in the next year (1824) by
Dekay, who also figured the hypostoma of an _Isotelus_, and added some
observations on the structure of trilobites. The researches of
Barrande, Novak, Broegger, Lindstroem, and others have dealt so fully
with the hypostoma that further references to that organ need not be
included here.
Dalman (1826, 1828) reviewed the opinions of his predecessors, and
thought it not impossible that organs of mastication may have been
present under the head shield of the trilobite as in _Limulus_ (1828,
p. 18). In this he of course followed Wahlenberg.
Goldfuss (1828) figured sections of _Dalmanites hausmanni_, _Phacops
macrophthalma_, and _Calymene tristani_, which remind one of some of
Doctor Walcott's translucent slices. So far as one can judge from the
illustrations, it is probable that what he took for limbs were really
fragments of other trilobites. Such is certainly the case in his
figures 9 and 10, where a number of more or less broken thoracic
segments are present. The section of _Encrinurus punctatus_ shown in
figure 7 may possibly exhibit the position and folds of the ventral
membrane beneath the axial lobe, and also, perhaps, the appendages.
His figures 4, 5 and 8 show the hypostoma in section.
Pander (1830) described the hypostoma in greater detail than had been
done by previous authors, but otherwise added nothing to the subject.
Sternberg (1830) thought he had individuals showing appendages, but
judging from his poor figures, he was deceived by fragmentary
specimens.
Green (1839 A, B, C) described specimens of _Phacops_ from Berkeley
Springs, West Virginia, which had the hypostoma in position, and
appear to have had a tubular opening under the axial lobe. While
appendages were not actually present, these specimens suggested fairly
correct ideas about the swimming and breathing organs of trilobites.
They were similar to the ones which Castelnau obtained, and all were
perhaps from the same locality.
It is not worth while to do more than enumerate the other authors of
this period: Hisinger 1837, Emmrich 1839, Milne-Edwards 1841, for they
all shared the same views, and added nothing to what was already
known.
Castelnau (1843) described and figured a _Phacops_ said to come from
Cacapon Springs, West Virginia, which he thought possessed remains of
appendages. There is nothing in the description or figures to indicate
exactly what was present, but it is very unlikely that any limbs were
preserved. The broad thin "appendage" figured may have been a fragment
of a thoracic segment. This specimen was evidently described by
Castelnau before 1843, as is inferred from a reference in the Neues
Jahrbuch, 1843, P. 504, but I have not seen the earlier publication.
Burmeister (1843-1846), in his "Organization of the Trilobites,"
reviewed in _extenso_ the history of the search for appendages, and
concluded that they must have been so soft as to preclude the
possibility of their being preserved as fossils. "Their very absence
in fossils most distinctly proves their former real structure" (p.
10). In figures 7 and 8 on plate 6 he gave a restoration of the
ventral surface of an _Asaphus_, the first restoration of the ventral
anatomy to be attempted. Since he chose modern branchiopods as his
model, he did not go so far wrong as he might have done. Still, there
is little in the figure that would now be accepted as correct. The
following quotation will serve to give the opinion of this zoologist,
who from his knowledge of the Crustacea, was the most competent of the
men of his time to undertake a restoration of the appendages of the
trilobites:
... in giving a certain form to the feet in the restored figure, I
have done so rather intending to indicate what they might have
resembled, than with any idea of assuming their actual form. I
merely assert that these organs were soft, membranous, and fringed,
adapted for locomotion in water, placed on the abdominal portion of
the body, and extending sidewise beneath the lateral lobes of the
rings, as shown in the ideal transverse section. These feet were
also indented, and thus divided into several lobes at the open
lower side, and each separate lobe was furnished at the margin with
small bristles serving as fins. The last and external lobe was
probably longer, smaller, and more movable, and reached to the
termination of the projecting shell lobe, bearing a bladder-shaped
gill on the inner side (1846, p. 45).
McCoy (1846) observed in several trilobites a pair of pores situated
in the dorsal furrows near the anterior end of the glabella. He showed
that the pits occupy precisely the position of the antennæ of insects
and suggested that they indicated the former presence of antennæ in
these trilobites (chiefly _Anipyx_ and "_Trinucleus_"). The evidence
from _Cryptolithus_, set forth on a later page, indicates the
correctness of McCoy's view.
Richter (1848, p. 20, pl. 2, fig. 32) described and figured what he
took to be a phyllopod-like appendage found in a section through a
_Phacops_. Without the specimen it is impossible to say just what the
structure really was. The outline figure is so obviously modeled on an
appendage of _Apus_ that one is inclined to think it somewhat
diagrammatic. In calling attention to this neglected "find," Clarke
(1888, p. 254, fig.) interprets the appendage as similar to the spiral
branchiæ of _Calymene senaria_, and adds that he himself has seen
evidence of spiral branchiæ in the American Phacops rana.
Beyrich (1846) described a cast of the intestine of "_Trinucleus_,"
and Barrande (1852) further elaborated on this discovery.
Corda (1847) made a number of claims for appendages, but all were
shown by Barrande (1852) to be erroneous.
Barrande (1852, 1872) gave a somewhat incomplete summary of the
various attempts to describe the appendages of trilobites, concluding
that none showed any evidence of other than soft appendages, until
Billings' discovery of 1870.
Volborth (1863) described a long chambered tubular organ in _Illænus_
which he believed to represent a cast of the heart of a trilobite, but
which has since been likened by writers to the intestinal tract in
"_Trinucleus_."
PART I.
THE APPENDAGES OF TRILOBITES.
Terminology.
The terminology employed in the succeeding pages is essentially the
same as that used by Beecher, with two new terms added. Beecher
assigned to the various segments of the limbs the names suggested by
Huxley, but sometimes used the name protopodite instead of coxopodite
for the proximal one. It is obvious that he did not use protopodite in
the correct sense, as indicating a segment formed by the fusion of the
coxopodite and basipodite. The usage employed here is shown in figure
1.
[Illustration: Fig. 1.--_Triarthrus becki_ Green. Diagram of one of
the limbs of the thorax, viewed from above, with the endopodite in
advance of the exopodite. 1, coxopodite, the inner extension being
the endobase (gnathobase on cephalon); 2, basipodite, springing from
the coxopodite, and supporting the exopodite, which also rests upon
the coxopodite; 3, ischiopodite; 4, meropodite; 5, carpopodite; 6,
propodite; 7, dactylopodite, with terminal spines.]
The investigation of _Ceraurus_ showed that the appendages were
supported by processes extending downward from the dorsal test,
and on comparison with other trilobites it appeared that the same was
true in _Calymene_, _Cryptolithus_, _Neolenus_, and other genera. Thin
sections showed that these processes were formed by invagination of
the test beneath the dorsal and glabellar furrows. While these
processes are entirely homologous with the entopophyses of _Limulus_,
I have chosen to apply the name _appendifer_ to them in the
trilobites.
The only other new term employed is the substitution of _endobase_ for
gnathobase in speaking of the inner prolongation of a coxopodite of
the trunk region. The term gnathobase implies a function which can not
in all cases be proved.
The individual portions of which the limbs are made up are called
_segments_, and the articulations between them, _joints_. Such a
procedure is unusual, but promotes clearness.
The Appendages of Neolenus.
HISTORICAL.
The first mention of _Neolenus_ with appendages preserved was in
Doctor Walcott's paper of 1911, in which two figures were given to
show the form of the exopodites in comparison with the branchiæ of the
eurypterid-like _Sidneyia_. In 1912, two more figures were presented,
showing the antennules, exopodites, and cerci. The specimens were
found in the Burgess shale (Middle Cambrian) near Field, in British
Columbia. This shale is exceedingly fine-grained, and has yielded a
very large fauna of beautifully preserved fossils, either unknown or
extraordinarily rare elsewhere. It was stated in this paper (1912 A)
that trilobites, with the exception of _Agnostus_ and _Microdiscus_,
were not abundant in the shale.
In discussing the origin of the tracks known as _Protichnites_,
Walcott presented four figures of _Neolenus_ with appendages, and
described the three claw-like spines at the tip of each endopodite.
Three new figures of the appendages were also contributed to the
second edition of the Eastman-Zittel "Text-book of Paleontology"
(1913, p. 701). Later (1916, pl. 9) there was published a photograph
of a wonderful slab, bearing on its surface numerous Middle Cambrian
Crustacea. Several of the specimens of _Neolenus_ showed appendages.
Finally, in 1918, appeared the "Appendages of Trilobites," in which
the limbs of _Neolenus_ were fully described and figured (p. 126),
and a restoration presented. Organs previously unknown in trilobites,
epipodites and exites, attached to the coxopodites, were found.
=Neolenus serratus= (Rominger).
(Text fig. 2-8.)
Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1911, p. 20,
pl. 6, figs. 1, 2 (exopodites of thorax and cephalon);--Ibid., vol.
57, 1912, p. 191, pl. 24, figs. 1, la (antennules, caudal rami, and
endopodites of thorax);--Ibid., vol. 57, 1912, p. 277, pl. 45,
figs. 1-4 (antennules, endopodites of cephalon and thorax, caudal
rami);--Text-book of Paleontology, edited by C. R. Eastman, 2d ed.,
vol. 1. 1913, p. 701, fig. 1343 (exopodites), p. 716, fig. 1376
(abdominal appendages), fig. 1377 (appendages of thorax and
pygidium);--Ann. Rept. Smithson. Inst. for 1915, 1916, pl.
9;--Smithson. Misc. Coll., vol. 67, 1918, pp. 126-131 et al., pl.
14, fig. 1; pls. 15-20; pl. 21, fig. 6; pls. 22, 23; pl. 31
(restoration); pl. 34, fig. 3 (restored section); pl. 35, fig. 4;
pl. 36, fig. 3 (hypostoma).
The following description of the appendages of _Neolenus_ is
summarized from Walcott's paper of 1918, and from a study of the eight
specimens mentioned below.
_Cephalon._
The antennules are long, slender, and flexible, and lack the formal
double curvature so characteristic of those of _Triarthrus_. There are
short fine spines on the distal rims of the segments of the proximal
half of each, thus giving great sensitiveness to these organs. In the
proximal portion of each, the individual segments are short and wider
than long, and in the distal region they are narrow and longer than
wide.
There are four pairs of biramous cephalic appendages, which differ
only very slightly from the appendages of the thorax. All are of
course excessively flattened, and they are here described as they
appear.
The coxopodites, shown for the first time in Walcott's paper of 1918,
are broad, longer than wide, and truncated on the inner ends, where
they bear short, stout, unequal spines similar to those along the
anterior margin. The gnathobases are but slightly modified to serve as
mouth parts, much less so than in _Triarthrus_, but the coxopodites
of the cephalon are shorter and wider than those of the thorax.
At the distal end of the coxopodite arise the endopodite and
exopodite. The endopodite consists of six segments, the distal ones,
propodite and dactylopodite, more slender than the others, the last
bearing three terminal spines. The first endopodite is shorter than
the others and slightly more slender (pl. 16, fig. 1)[1] and the
anterior appendages turn forward more or less parallel to the sides of
the hypostoma (pl. 22). The basipodite, ischiopodite, meropodite, and
carpopodite are, in their flattened condition, roughly rectangular,
only a little longer than wide, taper gradually distally, each bears
small spines on the outer rim, and some of the proximal ones usually
have a row along the margin.
[Footnote 1: _Nota bene!_ All references in this section are to the
plates of Doctor Walcott's paper in 1918.]
The exopodites of the cephalon, as of the body of Neolenus, are very
different from those of any other trilobite whose appendages were
previously known. As shown in the photographs (pl. 20, fig. 2; pl.
22), each exopodite consists of a single long, broad, leaf-like blade,
not with many segments as in _Triarthrus_, but consisting of a large
basal and small terminal lobe. It bears on its outer margin numerous
relatively short, slender, flat setæ. The long axes of the exopodites
point forward, and the setæ are directed forward and outward. They
stand more nearly at right angles to the shaft on the cephalic
exopodites than on those of the thorax. This same type of broad-bladed
exopodite is also found on the thorax and pygidium.
The number of functional gnathobases on the cephalon is unknown. That
four endopodites were present on one side is shown pretty clearly
by specimen 58591 (pl. 16, fig. 3) and while no more than two well
preserved exopodites have been seen on a side, there probably were
four. Specimen 65513 (pl. 16, fig. 1) shows gnathobases on the second
and third appendages of that individual as preserved, but there is
no positive evidence that these are really the second and third
appendages, for they are obviously displaced. The hypostoma of
Neolenus is narrow but long, several specimens showing that it
extended back to the horizon of the outer ends of the last pair of
glabellar furrows. It is not as wide as the axial lobe, so that, while
gnathobases attached beneath the first pair of furrows would probably
not reach back to the posterior end of the hypostoma, they might lie
parallel to it and not extend beneath. It seems possible, then, that
there were four pairs of endobases but that the second rather than the
first pair served as mandibles, as seems to be the case in Ceraurus.
_Thorax._
The thorax of _Neolenus_ consists of seven segments, and the
appendages are well shown (pl. 17, fig. 1; pl. 18, figs. 1, 2; pl. 20,
fig. 1.), The endopodites of successive segments vary but little,
all are slender but compact, and consist of a long coxopodite with
six short, rather broad segments beyond it. In the figures, the
endopodites extend some distance in a horizontal direction beyond the
edges of the dorsal test, as many as four segments being in some cases
visible, but measurements show that the appendages tended to fall
outward on decay of the animal. The dactylopodites are provided
with terminal spines as in _Triarthrus_. The coxopodites are long,
straight, and slender. They are well shown on only one specimen (pl.
18), where they are seen to be as wide as the basipodite, and the
endobases are set with spines on the posterior and inner margins. They
are so long that those on opposite sides must have almost met on the
median line. The segments of the endopodites are mostly but little,
if any, longer than broad, and at the distal end each shows two or
more spines. The propodite and dactylopodite are notably more slender
than the others. The exopodites of the thorax are broad and flat, and
each shaft has two distinct parts with different kinds of setæ. The
posterior edge of the proximal lobe is fringed with a slender, flat,
overlapping hairs which are a little longer than the width of the
lobe, and stand at an angle of about 60 degrees with the direction of
the axis of the appendage. The outer lobe is at an angle with the main
one, and has short, very fine setæ oh the margin. One or two specimens
show some evidence of a joint between the inner and outer lobes,
but in the great majority of cases they seem to be continuous; if
originally in two segments, they have become firmly united. The
exopodites of the thorax, like those of the cephalon, are directed
diagonally forward and outward. (pl. 21, fig. 6; pl. 22.)
_Pygidium._
The pygidium of _Neolenus serratus_ is large, and usually shows five
rings on the axial lobe and four pairs of ribs on the sides. There are
five pairs of biramous appendages belonging to this shield, and behind
these a pair of jointed cerci. That the number of abdominal appendages
should correspond to the number of divisions of the axial lobe rather
than to the number of ribs on the pleural lobes is of interest, and in
accord with other trilobites, as first shown by Beecher.
The endopodites of the pygidium have the same form as those of the
thorax, are long, and very much less modified than those of any other
trilobite whose appendages are known. On some specimens, they extend
out far beyond the dorsal test, so that nearly all the segments are
visible (pl. 17, fig. 3; pl. 18; pl. 19; pl. 20, fig. 1), but in these
cases are probably displaced. The segments are short and wide, the
whole endopodite tapering gradually outward. The dactylopodite bears
terminal spines, and the individual segments also have outward-directed
spines.
The cerci appear to have been long, slender, very spinose organs much
like the antennules, but stiff rather than flexible. They are a little
longer than the pygidium (pl. 17, figs. 1, 2), and seem to be attached
to a plate on the under surface of the posterior end and in front of
the very narrow doublure. The precise form of this attachment can not
be determined from the published figures. They bear numerous fine
spines (pl. 17, fig. 3).
_Epipodites and Exiles._
Doctor Walcott has found on several specimens of _Neolenus_ remains of
organs which he interprets as epipodites and exites attached to the
coxopodites. A study of the specimens has, however, convinced me that
both the large and small epipodites are really exopodites, and that
the exites are badly preserved and displaced coxopodites. Detailed
explanation of this interpretation is given below in the description
of the several specimens involved.
_Description of Individual Specimens._
Doctor Walcott was kind enough to send me eight of the more important
specimens of _Neolenus_ figured by him, and since my interpretation
of them does not agree in all respects with his, I have thought it
fairer to the reader to present here rather full notes explaining
the position I have taken. I understand that since I communicated my
interpretation of the epipodites and exites to him, Doctor Walcott has
submitted the specimens to several palæontologists, who consider that
epipodites are really present. Since I am not able to convince myself
that their conclusion is based upon sound evidence, I give here my own
interpretation. There is of course, no a priori reason why trilobites
should not have had epipodites.
Specimen No. 58589.
Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, pl. 45,
fig. 2;--Zittel-Eastman Text-book of Paleontology, vol. 1, 1913,
fig. 1377;--Smithson. Misc. Coll., vol. 67, 1918, pl. 18, fig. 1;
pl. 20, fig. 1.
This is one of the most important of the specimens, as it shows the
coxopodites of three thoracic limbs and the well preserved endopodites
of six thoracic and five pairs of pygidial appendages.
The appendages are all shifted to the left till the articular socket
of the coxopodite is about 8 mm. outside of its proper position. The
endopodites extend a corresponding amount beyond the edge of the
dorsal test and are there so flattened that they are revealed as a
mere impression. The coxopodites, which are beneath the test, seem to
have been somewhat protected by it, and while hopelessly crushed, are
not flattened, but rather conformed to the ridges and grooves of the
thorax.
[Illustration: Fig. 2. _Neolenus serratus_ (Rominger). A sketch of the
coxopodites and endopodites of two thoracic segments. Note notch for
the reception of the lower end of the appendifer. × 3.]
The coxopodite of the appendage of the last thoracic segment is best
preserved. It is rectangular, about one third as wide as long, with a
slight notch in the posterior margin near the outer end. The inner end
is obliquely truncated and shows about ten sharp spines which do not
appear to be articulated to the segment, but rather to be direct
outgrowths from it. There are similar spines along the posterior
margin, but only two or three of what was probably once a continuous
series are now preserved. On the opposite margin of the coxopodite
from the slight depression mentioned above, there is a slight
convexity in the outline, which is better shown and explained by the
coxopodite just in front of this. That basal segment has the same form
as the one just described, but as its posterior margin is for the
greater part of its length pushed under the one behind it, the spines
are not shown. On the posterior margin, two-thirds of the length from
the proximal end, there is a shallow notch, and corresponding to it, a
bulge on the anterior side. From analogy with Ceraurus and _Calymene_
it becomes plain that the notch and bulge represent the position of
the socket where the coxopodite articulated with the appendifer. Since
these structures have not been shown in previous illustrations, a
drawing giving my interpretation of them is here inserted (fig. 2).
It is evident from the position of the notch that the row of spines
was on the dorsal (inner) side of the coxopodite and that the
truncation was obliquely downward and outward.
The endopodite of the last thoracic appendage is well preserved and
may be described as typical of such a leg in this part. The basipodite
is as wide as the coxopodite, and it and the three succeeding
segments, ischiopodite, meropodite, and carpopodite, are all
parallel-sided, not expanded at the joints, and decrease regularly in
width. The propodite and dactylopodite are also parallel-sided, but
more slender than the inner segments, and on the end of the
dactylopodite there are four little spines, three of them--one large
and two small--articulated at the distal end, and the fourth
projecting from the posterior outer angle. Each segment has one or
more spines on the outer articular end, and the ischiopodite has
several directed obliquely outward on the posterior margin. All of the
four proximal segments show a low ridge parallel to and near the
anterior margin, and several endopodites of the pygidium have a
similar ridge and a row of spines along the posterior margin of some
of the segments. These features indicate that the segments in question
were not cylindrical in life, but compressed. From the almost
universal location of the spines on the posterior side of the limbs as
preserved, it seems probable that in the natural position the segments
were held in a plane at a high angle with the horizontal, the ridge
was dorsal and anterior and the row of spines ventral and posterior.
Because the spines on the endobases are dorsal it does not follow that
those on the endopodites were, for the position of the coxopodite in a
crushed specimen does not indicate the position of the endopodite of
even the same appendage.
The endopodites of the pygidium are similar to the one just described,
except that some of them have spines on the posterior margin of the
segments, and a few on the right side have extremely fine, faintly
visible spines on the anterior side. The specimen shows fragments
of a few exopodites, but nothing worth describing. In the middle
of the right pleural lobe there is a small organ which Walcott has
interpreted as a small epipodite. It is oval in form, broken at the
end toward the axial lobe, and has exceedingly minute short setæ on
the posterior margin. From analogy with other specimens, it appears
to me to be the outer end of an exopodite.
_Measurements:_ The entire specimen is about 64 mm. long and
52 mm. wide at the genal angles. The thorax is about 41 mm. wide
(disregarding the spines) at the seventh segment, and the axial lobe
about 13 mm. wide at the same horizon. The measurements of the
individual segments of the seventh left thoracic limb are:
Coxopodite, 9 mm. long, 3 mm. wide, the middle of the notch 8 mm.
from the inner end, measured along the bottom, and 6 mm. measured
along the top.
Basipodite, 5 mm. long, 3 mm. wide
Ischiopodite, 4 " " 3 " "
Meropodite, 3.5 " " 2.5 " "
Carpopodite, 3.5 " " 2 " "
Propodite, 3 " " 1.25 " "
Dactylopodite, 2 " " 1.25 " "
The five distal segments of the last pygidial endopodite are together
10.5 mm. long. The whole six segments of the endopodite of the third
thoracic segments are together 21 mm. long. The distance from the
appendifer of the third segment to the outer end of the spine is 17
mm. From the center of the notch in the coxopodite to the outer end
is 1.5 mm., which, added to the length of the endopodite, 21 mm.,
makes a distance of 22.5 mm. from the appendifer to the tip of
the dactylopodite, showing that if projected straight outward, the
endopodites of the thorax would project 5.5 mm. beyond the test,
including spines.
The distance across the axial lobe from appendifer to appendifer on
the seventh thoracic segment is 12.5 mm. Measured along the top of
the coxopodite, it is 6 mm. from the middle of the notch to the inner
end, and measured along the bottom it is 8 mm. From the truncated form
of the ends it is evident that the coxopodites extended inward and
downward from the appendifers, and with the dimensions given above,
the inner toothed ends would practically meet on the median line.
Measurements on the appendages of the pygidia show that on this
specimen they extend back about twice as far beyond the edge of the
pygidium as they should, all being displaced.
Specimen No. 65514.
Illustrated: Walcott, Smithson. Misc. Coll., vol. 67, 1918,
pl. 19, figs. 1-3.
This specimen is so twisted apart that it is not possible to determine
to what segments the appendages belong, but it exhibits the best
preserved exopodites I have seen. The best one is just in front of the
pygidium on the matrix, and shows a form more easily seen than
described (our fig. 3). There is a broad, flat, leaf-like shaft, the
anterior side of which follows a smooth curve, while in the curve on
the posterior side, which is convex backward, there is a re-entrant,
setting off a small outer lobe whose length is about one third the
length of the whole. This lobe seems to be a continuation of the
shaft, and the test of the whole is wrinkled and evidently very thin.
The main and distal lobes of the shaft both bear numerous delicate
setæ, but those of the outer lobe are much shorter and finer than
those on the main portion. The latter are flattened and blade-like.
[Illustration: Fig. 3. Exopodite of _Neolenus serratus_ (Rominger), to
show form of the lobes of the shaft, and the setæ. × 4.]
[Illustration: Fig. 4. _Neolenus serratus_ (Rominger). One of the
so-called epipodites of specimen 65515, showing that it has the same
outline as an exopodite (compare figure 3) and fragments of setæ on
the margin. × 3.]
The anterior edge of the shaft shows a narrow stiffening ridge and the
setæ are but little longer than its greatest width. The second segment
of the pygidium has another exopodite like this one, but shows faintly
the line between the two lobes, as though there were two segments.
This specimen also shows some very well preserved endopodites, but
they differ in no way from those described from specimen No. 58589.
Walcott mentions two large epipodites projecting from beneath the
exopodites. I judge that he has reference to the distal lobes of the
exopodites, but as these are continuous with the main shaft, there can
be no other interpretation of them than that which I have given above.
_Measurements:_ The pygidium is 19 mm. long (without the spines) and
about 34 mm. wide at the front. The exopodites show faintly beneath
the pygidial shield, but their proximal ends are too indistinct to
allow accurate measurement. Apparently they were just about long
enough to reach to the margin of the shield. The best preserved one,
that of the second segment in the pygidium, is about 11 mm. long, 2.5
mm. wide at the widest; the distal lobe is 2.5 mm. long, and the
longest setæ of the main lobe 3.5 mm. long. The pleural lobe of the
pygidium is just 11 mm. wide at this point.
The endopodites project from 8 to 12 mm. beyond the pygidium, showing
about four segments.
The thoracic exopodite described above is 11 mm. long and 2.75 mm.
wide at the widest part. The distal lobe is 3.5 mm. long and 2.25 mm.
wide, and the longest setæ on the main lobe 3 mm. long.
Specimen No. 65519.
Illustrated: Walcott, Zittel-Eastman Text-book of Paleontology,
vol. 1, 1913, fig. 1343;--Smithson. Misc. Coll., vol. 67, 1918,
pl. 21, fig. 6.
This specimen is somewhat difficult to study but is very valuable as
showing the natural position of the exopodites of the anterior part of
the thorax. Walcott's figures are excellent and show the broad
leaf-like shafts, the distal lobes with the re-entrant angles in the
posterior margin, and the long fine setæ of the main lobes. None of
the distal lobes retains its setæ. All extend back to the dorsal
furrows, but the proximal ends are not actually shown.
The specimen is especially important because it shows the same distal
lobes as specimen No. 65514, and demonstrates that they are a part of
the exopodite and not of any other structure.
_Measurements:_ The exopodite belonging to the fourth thoracic segment
is 23 mm. long and 4 mm. wide at the widest part. The longest setæ are
7 mm. in length.
Specimen No. 65520.
Illustrated: Walcott, Smithson. Misc. Coll., vol. 67, 1918, pl. 20,
fig. 2; pl. 22, fig. 1.
This is a practically entire specimen, on two blocks, one showing the
interior of the shell, and the other the one figured by Walcott, a
cast of the interior. The first shows the low rounded appendifers at
the anterior angle of each axial tergite. They are almost entirely
beneath the dorsal furrows and do not project so far into the axial
lobe as those of Ceraurus and _Calymene_. In fact, only those at the
anterior end of the thorax project inward at all. As expected, there
are five pairs on the pygidium. The cephalon is unfortunately so
exfoliated that the appendifers there are not preserved. The doublure
of the pygidium is extremely narrow.
The cast of the interior shows, rather faintly, the exopodites of the
right side of the thorax and of the left side of the cephalon, and,
still more faintly, the caudal rami and a few pygidial endopodites.
The exopodites on the right side are in what seems to be the customary
position, directed obliquely forward and outward, and the tips of
their distal lobes project slightly beyond the edge of the test. These
lobes were interpreted by Walcott as epipodites, but after comparing
them with the terminal lobes of the exopodites of specimens No. 65519
and 65514 I think there can be no doubt that they represent the same
structure. The pleura of the individual thoracic segments on this side
of the specimen have an unusual appearance, for they are bluntly
rounded or obtusely pointed, instead of being spinose.
The interpretation of the appendages of the cephalon is somewhat
difficult. At the left of the glabella there are two large exopodites,
the anterior of which lies over and partially conceals the other.
These show by their position that they belong to the fourth and fifth
cephalic appendages. In front of these lie two appendages which may be
either endopodites or exopodites, but which I am inclined to refer to
the latter. Both are narrow and shaped like endopodites, but bear on
their outer edges close-set fine setæ. They also show what might be
considered as faint traces of segmentation. If the first of these ran
under the end of the exopodite behind it, as shown in Walcott's figure
(pl. 22), then it would be necessary to interpret it as an endopodite,
but it really continues down between the exopodite and the glabella,
and seems to be attached opposite the middle of the eye. The specimen
does not indicate clearly whether this appendage is above or below
the exopodite behind it, but one's impression is that it is above, in
which case it also must be an exopodite. The appendage in front, being
similar, is similarly interpreted. If this be correct, then the
exopodites of the second and third cephalic appendages are much
shorter and narrower than those of the fourth and fifth. All of these
appendages are obviously out of position, for the cheek has been
pushed forward away from the thorax, though still pivoting on its
inner angle at the neck-ring, till the eye has been brought up to the
dorsal furrow. In this way the anterior exopodites have been thrust
under the glabella and all the appendages have been moved to the right
of their original position. The anterior exopodite is very poorly
shown, but seems to be articulated in front of the eye. The posterior
exopodites are very similar to those on the thorax. The distal lobe is
shown only by the second from the last. It has the same form as the
distal lobes on the thoracic exopodites, and like them has much finer
setæ than the main lobe, but it does not stand at so great an angle
with the axis of the main lobe, nor yet is it so straight as shown in
Walcott's figure.
_Measurements:_ The specimen is about 72 mm. long and 54 mm. wide at
the genal angles. The pygidium is 22 mm. long and 37 mm. wide. The
doublure is 1.5 mm. wide. The exopodite of the third thoracic segment
is 19.5 mm. long. The pleural lobe at this point is 13 mm. wide
without the spines and 18.5 mm. wide with them. The third exopodite of
the cephalon was apparently about 15 mm. long when complete.
Specimen No. 65515.
Illustrated: Walcott, Smithson. Misc. Coll., vol. 67, 1918, pl. 20,
figs. 3, 4.
This is a small piece of the axial portion of a badly crushed
Neolenus, showing appendages on the left side as viewed from above. On
the posterior half there are three large appendages which have the
exact form of the exopodites of other specimens. There is a broad,
oval, proximal lobe and a distal one at an angle with it. The proximal
part of the shaft has fine setæ or the bases of them, and the distal
lobe faint traces of much finer ones. The form, and the setæ so far as
they are preserved, are exactly like those of the exopodites on the
specimens previously described. (See fig. 4, page 26.) Beneath them
there are slender, poorly preserved endopodites.
In front of the exopodites and endopodites lie a series of structures
which Walcott has called exites, but for which I can see another
explanation. Walcott has shown them as four broad rounded lobes, but
his figure must be looked upon as a drawing and not as a photograph,
for it has been very much retouched.
For convenience of discussion, these lobes may be called Nos. 1, 2, 3,
and 4, the last being the posterior one (fig. 5). This lobe is best
shown on the matrix, where the anterior end is seen to be margined by
stout spines, while the posterior end lies over the endopodite and
under the exopodite behind it. No. 3 is sunk below the level of the
others, and only a part of it has been uncovered. Its margin bears
strong spines of different sizes. Its full shape can not be made out,
but it has neither the shape nor the form of spines shown in figure 3,
plate 20 (1918). Lobes 2 and 1 and another lobe in front of 1 seem to
form a continuous series and to be part of a single appendage. They
are all in one plane, arc so continuous that the joints between them
can be made out with difficulty and if they do belong together, can
easily be explained.
[Illustration: Fig. 5.--A sketch of the so-called exites of _Neolenus
serratus_ (Rominger), to show the form and the character of the
spines. × 2.]
[Illustration: Fig. 6.--Endopodite of a cephalic appendage of
_Neolenus serratus_ (Rominger), showing the very broad coxopodite.
× 2.]
Before calling these structures new organs not previously seen on
trilobites, it is of course necessary to inquire if they can be
interpreted as representing any known structures. That they can not be
exopodites is obvious, since they are bordered by short stout spines
instead of setæ. The same stout spines that negate the above possible
explanation at once suggest that they are coxopodites (compare fig 6).
At first sight, the so-called exites seem too wide and too rounded to
be so interpreted, but if reference be had to the specimens rather
than the figures, it will be noted that the only well preserved
structure (No. 2) is longer than wide, has spines only on one side and
one end, and does not differ greatly from the coxopodite of specimen
No. 58589 (pl. 18, 1918). If structures 2, 1, and the segment ahead of
1 are really parts of one appendage, it can only be an endopodite, of
which No. 2 is the coxopodite, No. 1 the basipodite, and the next
segment the ischiopodite. If one looks carefully, there are no traces
of spines on either end of No. 1, but only on the margin. The extreme
width of No. 2 is against this interpretation as a coxopodite (see,
however, fig. 6), but it may be rolled out very flat, as this is an
unusually crushed specimen. No. 2 is 10 mm. long and 6 mm. wide at
the widest point. No. 1 is 5 mm. long and 3.5 mm. wide.
The crucial point in this determination is whether 2 and 1 are parts
of the same appendage. I believe they are, but others may differ.
Specimen No. 65513.
Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, pl. 45,
fig. 3;--Ibid., vol. 67, 1918, pl. 16, figs. 1, 2.
This is nearly all of the right half of an entire specimen, but the
only appendages of any interest are those of the cephalon. Five
endopodites emerge from beneath that shield, but as all are displaced
it is not possible to say how many belong to the head. When held at
the proper angle to the light, the second and third from the front
show faintly the partial outlines of the coxopodites. The anterior
side and end of the best preserved one shows irregular stout spines of
unequal sizes, and the inner end is truncated obliquely (fig. 6).
These coxopodites are like those on the thorax of specimen No. 58589,
but shorter and wider. This of course suggests that the "exite" No. 2
of specimen No. 65515 may be a cephalic coxopodite. The endopodite of
this appendage, like the others on this cephalon, is shorter and
stouter than the thoracic or pygidial endopodites of the others
described.
[Illustration: Fig. 7.--A restored section across the thorax of
_Neolenus serratus_, showing the probable form of attachment of the
appendages, their relation to the ventral membrane, and the jaw-like
endobases of the coxopodites.]
_Measurements:_ The cephalon is 24 mm. long and about 60 mm. wide. The
coxopodite of the third appendage is about 10 mm. long and 5.5 mm.
wide at the widest point. The corresponding endopodite is 19 mm. long
and projects 11 mm. beyond the margin, which is about 5 mm. further
than it would project were the appendage restored to its proper
position.
RESTORATION OF NEOLENUS.
(Text figs. 7, 8.)
This restoration is based upon the information obtained from the
studies which have been detailed in the preceding pages, and differs
materially from that presented by Doctor Walcott. The appendages are
not shown in their natural positions, but as if flattened nearly into
a horizontal plane. The metastoma is added without any evidence for
its former presence.
The striking features of the appendages are the broad unsegmented
exopodites which point forward all along the body, and the strong
endopodites, which show practically no regional modification. Although
the exopodites have a form which is especially adapted for use in
swimming, their position is such as to indicate that they were not so
used. The stout endopodites, on the other hand, probably performed the
double function of natatory and ambulatory legs.
[Illustration: Fig. 8.--_Neolenus serratus_ (Rominger). A restoration
of the ventral surface, with the endopodites omitted from one side, to
permit a better exposition of the exopodites. The position and number
of the appendages about the mouth are in considerable doubt. Restored
by Doctor Elvira Wood under the supervision of the writer. About
one-half larger than the average specimen.]
=Nathorstia transitans= Walcott.
Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, pl. 28,
fig. 2.
The badly preserved specimen on which this genus and species was
based is undoubtedly a trilobite, but for some reason it does not
find a place in Walcott's recent article on "Appendages" (1918). The
preservation is different from that of the associated trilobites,
being merely a shadowy impression, indicating a very soft test. The
general outline of the body, the position of the eye, and even a
trace of spines about the pygidium (in the figure) are similar
to those of _Neolenus_, and I would venture the suggestion that
_Nathorstia transitans_ is a recently moulted _Neolenus serratus_,
still in the "soft-shelled" condition. Even if not a Neolenus, it is
probable, from the state of preservation, that it is an animal which
had recently cast its shell.
Walcott describes such fragments of appendages as remain, as follows:
Head. A portion of what may be an antenna projects from beneath the
right anterior margin; from near the left posterolateral angle a
large four-jointed appendage extends backward. I assume that this
may be the outer portion of the large posterior appendage (maxilla)
of the head.
Thorax. Traces of several slender-jointed thoracic legs project
from beneath the anterior segments and back of these on the right
side more or less of six legs have been pushed out from beneath the
dorsal shield; these are composed of three or four long slender
joints; fragments of the three proximal joints indicate that they
are shorter and larger and that they have a fringe of fine setæ.
Indications of a branchial lobe (gill) are seen in two specimens
where the legs are not preserved. This is often the case both among
the Merostomata (pl. 29, fig. 3, _Molaria_) and Trilobita (pl. 24,
fig. 2, _Ptychoparia_).
Two caudal rami project a little distance beneath the posterior
margin of the dorsal shield.
This latter feature of course suggests _Neolenus_. The other
appendages are too poorly preserved to allow comparison without seeing
the specimen.
The specific name was given "on account of its suggesting a transition
between a Merostome-like form, such as _Molaria spinifera_, and the
trilobites." In what respect it is transitional does not appear.
Formation and locality: Same as that of _Neolenus serratus_. One
nearly complete specimen and a few fragments were found.
The Appendages of Isotelus.
HISTORICAL.
The first specimen of _Isotelus_ with appendages was described orally
by Billings before the Natural History Society of Montreal in 1864,
and in print six years later (1870, p. 479, pls. 31, 32). The specimen
is described in detail on a later page. Billings recognized the
remains of eight pairs of legs on the thorax, a pair for each segment,
and he inferred from the fact that the appendages projected forward
that they were ambulatory rather than natatory organs. He was unable
to make out the exact number of the segments in the appendages, but
thought each showed at least four or five.
Having examined the individual sent to London by Billings, Woodward
(1870, p. 486, fig, 1) reviewed the collection from the American
Trenton in the British Museum and found a specimen in the "Black
Trenton limestone," from Ottawa, Ontario, in which, alongside the
hypostoma, was a jointed appendage, which he described as the "jointed
palpus of one of the maxillæ." This has always been considered an
authentic "find," but I am informed by Doctor Bather that the specimen
does not show any real appendage. For further discussion, see under
_Isotelus gigas_.
In 1871, Billings' specimen was examined by Professors James D. Dana
(1871, p. 320), A. E. Verrill, and Sydney I. Smith, who agreed
that the structures identified by Billings as legs were merely
semicalcified arches of the membrane of the ventral surface, which
opinion seems to have been adopted by zoologists generally in spite of
the fact that the most elementary consideration of the structure of
the thorax of a trilobite should have shown its falsity. While the
curvature of the thoracic segments was convex forward, that of the
supposed ventral arches was convex backward, and the supposed arches
extended across so many segments as to have absolutely prevented any
great amount of motion of the segments of the thorax on each other.
Enrollment, a common occurrence in _Isotelus_, would have been
absolutely impossible had any such calcified arches been present.
Walcott, in his study of trilobites in thin section (1881, pp. 192,
206, pl. 2, fig. 9), obtained eleven slices of _Isotelus gigas_ which
showed remains of appendages. He figured one of the sections, stating
that it "shows the basal joint of a leg and another specimen not
illustrated gives evidence that the legs extended out beneath the
pygidium, as indicated by their basal joints."
The second important specimen of an _Isotelus_ with appendages was
found by Mr. James Pugh in strata of Richmond age 2 miles north of
Oxford, Ohio, and is now in the U. S. National Museum. It was first
described by Mickleborough (1883, p. 200, fig. 1-3). In two successive
finds, a year apart, the specimen itself and its impression were
recovered. Since I am redescribing the specimen in this memoir (see
p. 35), it only remains to state here that Mickleborough interpreted
the structures essentially correctly, though not using the same
terminology as that at present adopted. His view that the anterior
appendages were chelate can not, however, be supported, nor can his
idea that the sole appendages of the pygidium were foliaceous
branchial organs.
Walcott (1884, p. 279, fig. 1) studied the original specimens and
presented a figure which is much more detailed and clear than those of
Mickleborough. By further cleaning the specimen he made out altogether
twenty-six pairs of appendages. He stated that one of these belonged
to the cephalon, nine to the thorax,[1] and the remaining sixteen to
the pygidium. He showed that the endopodites of the pygidium were of
practically the same form as those on the thorax, and stated that the
"leg beneath the thorax of the Ohio trilobite shows seven joints in
two instances; the character of the terminal joint is unknown." His
figure shows, and he mentions, markings which are interpreted as
traces of the fringes of the exopodites.
[Footnote 1: The posterior one of these he believed to have been
crowded forward from beneath the pygidium.]
In the same year Woodward (1884, p. 162, fig. 1-3) reproduced all of
Mickleborough's figures, and suggested that the last seven pairs of
appendages on the pygidium of _Calymene_ and _Isotelus_ were probably
"lamelliform branchiferous appendages, as in _Limulus_ and in living
Isopoda."
Professor Beecher published, in 1902, an outline taken from
Mickleborough's figure of this specimen, to call attention to certain
discontinuous ridges along the axial cavity of the anterior part of
the pygidium and posterior end of the thorax. These ridges are well
shown in Mickleborough's figure, though not in that of Walcott, and
their presence on the specimen was confirmed by a study by Schuchert,
who contributed a diagrammatic cross-section to Beecher's paper (1902,
p. 169, pl. 5, figs. 5, 6). Beecher summarized in a paragraph his
interpretation of this specimen:
The club-shaped bodies lying within the axis are the gnathobases
attached at the sides of the axis; the curved members extending
outward from the gnathobases are the endopodites; the longitudinal
ridges in the ventral membrane between the inner ends of the
gnathobases are the buttresses and apodemes of the mesosternites;
the slender oblique rod-like bodies shown in the right pleural
region in Walcott's figure are portions of the fringes of the
exopodites.
In 1910, Mr. W. C. King of Ottawa, Ontario, found at Britannia, a few
miles west of Ottawa, the impression in sandstone of the under surface
of a large specimen of _Isotelus arenicola_, described on a later page
(p. 39).
Finally (1918, p. 133, pl. 24, figs. 3, 3a; pl. 25), Walcott has
redescribed the specimen from Ohio, presenting a new and partially
restored figure. He refers also to the specimen from Ottawa under the
name _Isotelus covingtonensis?_ Foerste (not Ulrich). He advances the
view, which I am unable to share, that the cylindrical appearance of
the segments of the appendages of _Isotelus_ is due to post-mortem
changes.
=Isotelus latus= Raymond.
(pl. 10, fig. 1.)
Illustrated: _Asaphus platycephalus_ Billings, Quart. Jour. Geol.
Soc., London, vol. 26, 1870, pl. 31, figs. 1-3; pl. 32, figs. 1,
2.--Woodward, Geol. Mag., vol. 8, 1871, pl. 8, figs. 1,
1a.--Gerstäcker, in Bronn's "Klassen u. Ordnungen d. Thier-Reichs,"
1879, pl. 49, fig. 1.--von Koenen, N. Jahrb. f. Min., etc., vol. 1,
1880, pl. 8, fig. 8.--Milne-Edwards, Ann. Sci. Nat., Zoologie, ser.
6, vol. 12, 1881, pl. 12, fig. 45.
_Isotelus latus_ Raymond, Bull. Victoria Mem. Mus., Geol. Survey
Canada, No. 1, 1913, p. 45 (species named).
_Isotelus covingtonensis?_ Walcott (not Foerste), Smithson. Misc.
Coll., vol. 67, 1918, p. 134.
Knowledge of the appendages of this species is derived from the
specimen which Billings described in 1870. It was found in the
Trenton, probably the Middle Trenton, near Ottawa, Ontario, and is
preserved in the Victoria Memorial Museum at Ottawa.
Viewed from the upper surface, it shows a large part of the test,
but is broken along the sides, so that parts of the free cheeks,
considerable of the pleural lobes of the thorax, and one side of the
pygidium are missing. Viewed from the lower surface, the appendages
are practically confined to the cephalon and thorax.
A short time before his death, Professor Beecher had this specimen and
succeeded in cleaning away a part of the matrix so that the appendages
show somewhat more clearly than in Billings' time, but they are not so
well preserved as on the Mickleborough specimen, found in Ohio
somewhat later.
The hypostoma is in place and well preserved; the posterior points are
but 3 mm. in advance of the posterior margin of the cephalon. Behind
the hypostoma there are only two pairs of cephalic appendages, the
first of which is represented by the coxopodite and a trace of the
endopodite. The outer end of the coxopodite is close to the outer
margin of one of the prongs of the hypostoma and about 3 mm. in front
of its posterior end. The gnathobase curves backward and inward, and
appears to pass under the tip of the hypostoma. There were probably
two appendages in front of this, whose gnathobases projected under the
hypostoma, but the specimen shows nothing of them unless it be that
one small fragment about 2 mm. back of the center is really a part of
a gnathobase.
The specimen retains only the coxopodite and basipodite of the
posterior cephalic appendage on the left side. The coxopodite is
long and apparently cylindrical, the cross-section being of uniform
diameter throughout the length. The inner portion is nearly straight,
while the outer part is curved gently forward.
It is possible to make out remains of eight pairs of appendages on the
thorax, some of them represented by coxopodites only, but most with
more or less poorly preserved endopodites as well. No exopodites are
visible. The coxopodites of the thorax seem to be of the same form
as the last one on the cephalon, but slightly less curved. All are
long and heavy, and there seems to be no decrease in size toward the
pygidium. The endopodites are very imperfectly shown. They seem to be
longer than those of _Isotelus maximus_, and the segments, while of
less diameter than the coxopodites, do not show so great a contrast to
them as do those of that species. The direction of the endopodites is
diagonally forward, and the outer portions do not appear to be curved
backward as in _Isotelus maximus_. It would appear also that the
endopodites were nearly or quite long enough to reach the outer margin
of the dorsal test. On no endopodite can more than three segments be
definitely distinguished, but the longest ones are the most obscurely
segmented.
No appendages are preserved on the pygidium, but at one side of the
median groove there are two projections which may be processes to
which the appendages were attached.
_Measurements:_ Total length of specimen, 109 mm. Probable length when
complete, 116 mm. Length of cephalon, 40 mm.; width at genal angles,
restored, about 62 mm. (Billings' restoration). Width of doublure of
front of cephalon on median line, 17 mm.; length of hypostoma, 20 mm.
Length of coxopodite of last appendage on left side of cephalon,
10.5 mm.; length of basipodite of the same appendage, 5 mm. Diameter
of coxopodite, 2 mm.; diameter of basipodite, 1.5 mm. Length of
coxopodite on left side of the second segment of the thorax, 11 mm.;
diameter, about 2.5 mm. Length of basipodite of the same, 5 mm.;
diameter, about 1.5 mm. Length of ischiopodite, 3.5 mm.; diameter,
about 1.5 mm. Length of meropodite, 2.5 mm. (this may be less than
the total length as the segment is not completely exposed.) Distance
between proximal ends of gnathobases of the fifth thoracic segment,
about 7 mm. Distance between outer ends of the coxopodites of the
first thoracic segment (estimated from measurements on the left side),
27 mm Distance apart of the dorsal furrows at the first thoracic
segment, 27 mm. Length of the longest exopodite which can be traced,
about 20 mm.
=Isotelus maximus= Locke.
(pl. 10, fig. 2.)
Illustrated: Mickleborough, Jour. Cincinnati Soc. Nat. Hist., vol.
6, 1883, p. 200, figs. 1-3 (endopodites and coxopodites). Walcott,
Science, vol. 3, 1884, p. 279, fig. 1 (endopodites, coxopodites,
and traces of exopodites). Woodward, Geol. Mag., dec. 3, vol. 1,
1884, p. 162, figs. 1-3 (copies of Mickleborough's figures).
Bernard, The Apodidæ, 1892, text fig. 49. Beecher, Amer. Jour.
Sci., vol. 13, 1902, p. 169, pl. 5. figs. 5, 6 (outline from one of
Mickleborough's figures and an original figure). Walcott, Smithson.
Misc. Coll., vol. 67, 1918, p. 133, pl. 24, figs. 3, 3a; pl. 25,
fig. 1.
This specimen, which conies from the Richmond strata 2 miles north of
Oxford, Ohio, is the best preserved of the specimens of _Isotelus_
with appendages which has so far been found. The individual consists
of two parts, the actual specimen, and the impression of the ventral
side.
To describe it I am using very skillfully made plaster reproductions
of both parts, presented to the Museum of Comparative Zoology by
Doctor Charles D. Walcott, and presumably made after he cleaned the
specimen as described in Science (1884). I have also an enlarged
photograph (pl. 10, fig. 2) which seems to have been made after some
later period of cleaning, probably by Professor Beecher, and I have
examined the original specimens in Washington.
Viewed from the dorsal side, it is seen that the individual is very
imperfect, the greater part of the cephalon being removed by a
diagonal break which cuts off the anterior third of the left eye and
extends to the front of the second thoracic segment on the right side.
The ends of the pleura of both sides of the thorax are broken away, as
are also the greater parts of the pleural lobes and the posterior end
of the pygidium. On the ventral side, merely the posterior tips of the
hypostoma remain, but the distal ends of the appendages were so far
within the outer margin that the appendagiferous area is quite fully
retained.
The most conspicuous feature of this specimen is the presence of nine
pairs of large coxopodites behind the hypostoma, and of the remains of
ten pairs of endopodites, making in all ten pairs of appendages which
are easily seen. The apportionment of these segments to cephalon,
thorax, and pygidium is not agreed upon by the people who have
examined the specimens, but if one remembers that it is the outer
and not the inner end of the coxopodite which articulates with the
appendifer, it at once becomes evident that the first two pairs of
appendages on the specimen are the last two pairs belonging to the
cephalon, and that the next eight pairs are those of the thorax.
The impressions of fourteen pairs of coxopodites are readily counted
on the pygidium, and as Doctor Walcott noted sixteen pairs on the
actual specimens, his number was probably correct.
_Cephalon._
Projecting the line of the back of the cephalon through from the
dorsal side, it is found that the posterior tips of the hypostoma are
7 mm. in front of the posterior margin of the cephalon, and that the
points of attachment of the posterior pair of cephalic appendages
(the second pair shown on the specimen) are just within the posterior
margin. The gnathobases of this pair of appendages extend back some
distance beneath the thorax, and so give the impression that they
belong to that part of the body. So far as can be determined, the
cephalic appendages do not differ in any way from those of the thorax.
On the mould of the ventral surface, just outside of the lateral edge
of the right lobe of the hypostoma, is the somewhat imperfectly shown
impression of the endopodite of the third cephalic appendage. The
point of junction of the endopodite and coxopodite is about 2 mm. in
front of the tip of the adjacent branch of the hypostoma, and the
gnathobase is curved around just behind it. This accounts for three of
the pairs of cephalic appendages. The second cephalic appendages must
have thrust their gnathobases under the prongs of the hypostoma, and
the endopodites were probably close to its edge. No trace of this pair
appears on the specimen.
_Thorax._
The thoracic appendages are the best preserved of any, and show the
large coxopodites and the more slender endopodites which do not extend
to the outer margin of the test. The latter extend forward and outward
for about one half their length, then turn backward in a graceful
curve.
Walcott's figure in Science shows hair-like markings on the under
side of the right half of the thorax. These were interpreted by both
Walcott and Beecher as fringes of the exopodites, but since the
setæ of those organs on all other trilobites are always above the
endopodites, while these are represented as below them, it would seem
doubtful if this interpretation can be sustained. Furthermore, I find
no trace of them on either cast or mould, and the actual specimen does
not now show them.
_Pygidium._
The coxopodites and endopodites of the pygidium seem to be similar
to those on the thorax, but both are shorter and more slender, and
the former decrease in length rapidly toward the posterior end. As
mentioned above, it is not perfectly plain how many appendages are
present, but I have accepted Doctor Walcott's count of sixteen pairs.
Of the endopodites only the barest traces are seen, and of exopodites
nothing.
One point of considerable interest in this specimen is the thickness,
as it probably gives some measure of the space occupied by the animal.
In _Triarthrus_ and other trilobites from Rome, New York, the
appendages are pressed directly against the dorsal test, but in this
specimen a considerable space intervenes between the plane of the
appendages and the shell. Between the central furrow and the inner
surface of the dorsal test at the anterior end of the thorax is a
distance of 13 mm. and under the dorsal furrows the thickness is about
7 or 8 mm., no accurate measurement being possible in the present
state of the specimen.
_Measurements:_ Length of specimen on median line, 121 mm.; probable
original length, about 195 mm. (Walcott's restoration). Length of
thorax, 58 mm.[1] Width of axial lobe at the first thoracic segment,
45 mm.; total width as preserved, 92 mm.; width as estimated from the
mould of the ventral surface, no mm.; Walcott's restoration, 105 mm.
[Footnote 1: If this specimen had the same proportions as specimens of
_Isotelus maximus_ from Toronto, the total length would be only 174
mm. The cephalon would be about 52 mm. long, the thorax 58 mm., and
the pygidium about 64 mm. long.]
Length of coxopodite of fourth left cephalic appendage, about 18 mm.;
diameter, about 2.5 mm. Length of coxopodite of last left cephalic
appendage, about 18.5 mm. Distance apart of inner ends of gnathobases
of fourth cephalic appendages, about 4 mm. Distance apart of inner
ends of endobases of first thoracic segment, about 6 mm. Distance
apart of outer ends of coxopodites of first thoracic segment, about 43
mm.
Length of coxopodite of seventh left thoracic appendage 16 mm.,
diameter about 3.5 mm.; length of basipodite of the endopodite of the
same appendage 6 mm.; diameter about 2 mm.; length of ischiopodite 5
mm.; length of meropodite 4.5 mm.; length of carpopodite 4.5 mm.;
length of propodite 3 mm.; length of dactylopodite 2.75 mm.; total
length of endopodite 25.75 mm.
Length of coxopodite of fourth left thoracic appendage 20 mm.,
diameter 4 mm.; length of five proximal joints of the endopodite 25
mm.; diameter of basipodite about 2 mm.
RESTORATION OF ISOTELUS.
(Text fig. 9.)
The exopodites have been omitted from this restoration since nothing
is known of their actual form. The chief reason for the figure is to
contrast the greatly developed coxopodites of the posterior part of
the cephalon and thorax with those of other trilobites. The antennules
and first two pairs of biramous appendages of the cephalon are more or
less hypothetical, and less is known of the appendages of the pygidium
than is shown here. The restoration is based somewhat upon Walcott's
figure in Science. The outline is that of a specimen of _Isotelus
maximus_ from Toronto, Ontario.
=Isotelus gigas= Dekay.
Illustrated: Woodward, Quart. Jour. Geol. Soc., London, vol. 26,
1870, text fig. 1; Geol. Mag., dec. 3, vol. 1. 1884, p. 78, text
fig. Milne-Edwards, Ann. Sci. Nat, Zoologie, ser. 6, vol. 12, 1881,
pl. 12, fig. 46. Walcott, Bull. Mus. Comp. Zool., Harvard Coll.,
vol. 8, 1881, pl. 2, fig. 9; Geol. Mag., dec. 4, vol. 1, 1894, pl.
8, fig. 9; Proc. Biol. Soc. Washington, vol. 9, 1894, pl. 1, fig.
9.
The specimen in the British Museum which Woodward called _Asaphus
platycephalus_, is, in all probability, an _Isotelus gigas_. Woodward
says of it:
I was at once attracted by a specimen of _Asaphus_, from the Black
Trenton Limestone (Lower Silurian), which has been much eroded on
its upper surface, leaving the hypostoma and what appear to be
the appendages belonging to the first, second, and third somites,
exposed to view, united along the median line by a longitudinal
ridge. The pseudo-appendages, however, have no evidence of any
articulations. But what appears to me to be of the highest
importance, as a piece of additional information afforded by
the Museum specimen, is the discovery of what I believe to be
the _jointed palpus_ of one of the maxillæ, which has left its
impression upon the side of the hypostoma--just, in fact, in that
position which it must have occupied in life, judging by other
Crustaceans which are furnished with an hypostoma, as _Apus_,
_Serolis_, etc.
The palpus is 9 lines in length, the basal joint measures 3 lines,
and is 2 lines broad, and somewhat triangular in form.
There appear to be about 7 articulations in the palpus itself,
above the basal joint, marked by swellings upon its tubular stem,
which is 1 line in diameter.
[Illustration: Fig. 9.--A restored composite of _Isotelus maximus_ and
_I. latus_. The exopodites are left out because entirely unknown.
Drawn by Doctor Elvira Wood. Natural size.]
Desiring to know more of this individual, I wrote to Doctor Bather
and was surprised to learn that the specimen which was the basis of
Woodward's observations is so badly preserved as to be of no real
value. With his permission, I append a note made by Doctor Bather
some years ago when selecting fossils to be placed on exhibition:
_Asaphus gigas_ Dekay. Ordovician, Trenton Limestone. N. America,
Canada. Descr. H. Woodward, 1870, Q. J. G. S., XXVI, pp. 486-488,
text fig. 1, as _Asaphus platycephalus_. Coll. and presd. J. J.
Bigsby, 1851. Regd. I 14431.
This specimen is in the Brit. Mus. Geol. Dept. I 14431. The
supposed hypostome is exceedingly doubtful; it lies dorsad of the
crushed glabellar skeleton. The "appendage" is merely the edge of
a part in the head-shield; the maxilla is some calcite filling,
between two such laminæ.
13 Sept. 1911. (Signed) F. A. BATHER.
Walcott figured a slice of _Isotelus gigas_ from Trenton Falls, New
York, which shows a few fragments of appendages, but is of particular
importance because it shows the presence of well developed appendifers
beneath the axial lobe.
=Isotelus arenicola= Raymond.
Illustrated: Ottawa Nat, vol. 24, 1910, p. 129, pl. 2, fig. 5.
The following quotations from my paper are inserted here to complete
the record of appendage-bearing specimens:
A rather remarkable specimen of this species was found by W. C.
King, Esq., on the shore of Lake Deschenes at Britannia [near
Ottawa, Ontario]. This specimen is an impression of the lower
surface of the trilobite, and shows a longitudinal ridge
corresponding to the central furrow along the axis of the ventral
side of the animal, ten pairs of transverse furrows, and the
impression of the hypostoma. The doublure of the pygidium has
also left a wide smooth impression, but in the cephalic region
the hypostoma is the only portion of which there are any traces
remaining. The specimen was found on a waterworn surface of the
beach, partially covered by shingle....
The transverse furrows are the impressions left by the gnathobases
of the basal joints of the legs. They were evidently long and very
heavy, but the specimen has been so abraded that all details are
obscured. The first six pairs of impressions are longer and deeper
than the four behind. The first eight pairs seem to pertain to the
thoracic appendages, while the last two belong to the pygidium.
From the posterior tips of the hypostoma to the first gnathobases
of which traces are present there is a distance of about 22 mm.
without impressions. In _Isotelus gigas_ the hypostoma normally
extends back to the posterior margin of the cephalon, so that it
seems that in this specimen the impressions of the first two pairs
of gnathobases under the thorax may not have been preserved. In
that case, the six pairs of strong impressions may represent the
last six pairs of thoracic segments, and the pygidium might begin
with the first of the fainter ones.
_Horizon and locality:_ From the sandstone near the base of the Aylmer
(Upper Chazy) formation at Britannia, west of Ottawa, Ontario.
Specimen in the Victoria Memorial Museum, Geological Survey of Canada,
Ottawa.
The Appendages of Triarthrus.
=Triarthrus becki= Green.
(Pls. 1-5; pl. 6, figs. 1-3; text figs. 1, 10, 11, 33, 42.)
(Also see Part IV.)
Illustrated: Matthew, Amer. Jour. Sci., vol. 46, 1893, pl. 1, figs.
1-7;--Trans. N. Y. Acad. Sci., vol. 12, pl. 8, figs. 1-7.--Beecher,
Amer. Jour. Sci., vol. 46, 1893, text figs. 1-3;--Amer. Geol., vol.
13, 1894, pl. 3;--Amer. Jour. Sci., vol. 47, pl. 7, text fig.
1;--Amer. Geol., vol. 15, 1895, pls. 4, 5;--Ibid., vol. 16, 1895,
pl. 8, figs. 12-14; pl. 10. fig. 1;--Amer. Jour. Sci.,
vol. 1, 1896, pl. 8; Geol. Mag., dec. 4, vol. 3, 1896, pl.
9;--Eastman-Zittel Text-book of Paleontology, vol. 1, 1900, text
figs. 1267-1269;--2d ed., 1913, fig. 1375; Studies in Evolution,
1901, reprint of all previous figs.;--Amer. Jour. Sci., vol. 13,
1902, pl. 2, figs. 1-5; pl. 3, fig. 1; pl. 4, fig. 1; pl. 5, figs.
2-4;--Geol. Mag., dec. 10, vol. 9, 1902, pls. 9-11, text figs.
1-3.--Walcott, Proc. Biol. Soc. Washington, vol. 9, 1894, pl. 1
figs. 1-6;--Geol. Mag., dec. 4, vol. 1, 1894, pl. 8;--Smithson.
Misc. Coll., vol. 67, 1918, pl. 29, figs. 1-11; pl. 30, figs.
17-20; pl. 32; pl. 34, figs. 4-7; pl. 35, fig. 5.--Bernard, Quart.
Jour. Geol. Soc., London, vol. 50, 1894, text figs. 11,
12.--Oehlert, Bull. Soc. Géol. France, ser. 3, vol. 24, 1896,
text figs. 1-17, 34.--Jaekel, Zeits. d. d. geol. Gesell., vol. 53,
1901, text fig. 24. Moberg, Geol. Fören. Förhandl., vol. 29, pl. 5,
1907, pl. 4, fig. 2; pl. 5, fig. 1.--Handlirsch, Foss. Insekten,
1908, text fig. 6.--Tothill, Amer. Jour. Sci., vol. 42, 1916, p.
380, text fig. 5.--Crampton, Jour. N. Y. Entomol. Soc., vol. 24,
1917, pl. 2, fig. 20.
Historical.
Specimens of _Triarthrus_ retaining appendages were first obtained by
Mr. W. S. Valiant from the dark carbonaceous Utica shale near Rome,
New York, in 1884, but no considerable amount of material was found
until 1892. The first specimens were sent to Columbia University, and
were described by Doctor W. D. Matthew (1893). This article was
accompanied by a plate of sketches, showing for the first time the
presence of antennules in trilobites and indicating something of the
endopodites and exopodites of the appendages of the cephalon, thorax,
and pygidium. Specimens had not yet been cleaned from the lower side,
so that no great amount could then be learned of the detailed
structure. Matthew concluded that "The homology with _Limulus_ seems
not to be as close in _Triarthrus_ as in the forms studied by Mr.
Walcott; but the characters seem to be of a more comprehensive type,
approaching the general structure of the other Crustacea rather than
any special form."
Professor Beecher's first paper, dated October 9, 1893, merely
mentioned the fact that the Yale University Museum had obtained
material from Valiant's locality, but was quickly followed by a paper
read before the National Academy of Sciences on November 8, and
published in December, 1893. This paper described particularly the
thoracic appendages.
This was followed in January (1894 A) by an article in which some
information about the mode of occurrence of the specimens was added,
and in April (1894 B), the limbs of the pygidium were described and
figured. The determination of the structure of the appendages of the
head evidently presented some difficulty, for the article describing
this portion of the animal did not appear until the next February
(1895 A). This cleared up the ventral anatomy of _Triarthrus_, and was
followed by a short article (1896 A) accompanied by a restoration of
the trilobite showing all the appendages.
This ended Professor Beecher's publications on _Triarthrus_ until his
final paper in 1902, although he contributed some of his results and
figures to his chapter on the trilobites in the Eastman-Zittel
Text-book of Paleontology in 1900.
The discovery of these excellent specimens had of course excited very
great interest. Doctor Walcott also studied a number of specimens from
Valiant's locality, and published in 1894, with some original figures,
the results of his comparison of the appendages of _Triarthrus_ with
those of _Calymene_ and _Ceraurus_.
In his article on the "Systematic Position of the Trilobites," Bernard
(1894) used the results of Professor Beecher's studies of 1893, and
also quoted the papers by Matthew (1893) and Walcott (1894), though
the article by the latter appeared too late to be used except for a
note added while Bernard's paper was in press. A final footnote quoted
from Professor Beecher's paper of April, 1894 (1894 B).
Oehlert (1896) gave an excellent summary in French of the work of
Beecher and Walcott on _Triarthrus_, with reproductions of many of
their figures.
Valiant (1901) in a non-technical article described his long search
for trilobites with antennas. The discovery of the wonderful pyritized
trilobites at Cleveland's Glen near Rome was not the result of a lucky
accident, but the culmination of eight years of labor in a locality
especially selected on account of the fineness of grain of the shale.
[Illustration: Fig. 10.--_Triarthrus becki_ Green. A new restoration,
modified from Professor Beecher's, to incorporate the results of his
later work. The inner ends of the endobases are probably too far
apart, as it was not discovered until after the drawing had been made
that the appendifers projected within the dorsal furrows. Drawn by
Doctor Elvira Wood. × about 3.8.]
After 1896, Professor Beecher turned his attention largely to the
problem of the classification of trilobites, and while he continued
the arduous task of cleaning the matrix from specimens of
_Triarthrus_ and _Cryptolithus_ he did not again publish upon the
subject of appendages until forced to do so by the doubts cast by
Jaekel (1901) upon the validity of his earlier conclusions. Because of
certain structures which he thought he had interpreted correctly from
a poorly preserved specimen of _Ptychoparia_, Jaekel came to the
conclusion that Beecher's material was not well preserved. Professor
Beecher would have taken much more kindly to aspersions upon his
opinions than to any slight upon his beloved trilobites, and his
article on the "Ventral Integument of Trilobites" of 1902 was designed
not only as an answer to Jaekel, but also to show by means of
photographs the unusually perfect state of preservation of the
specimens of _Triarthrus_. This article, like so many describing the
appendages of trilobites, beginning with Matthew's, was published in
two places (Beecher 1902).
Most of Beecher's papers, except the last one, were reprinted in
the volume entitled "Studies in Evolution," published by Charles
Scribner's Sons at the time of the Yale Bicentennial in 1901. The
part pertaining particularly to _Triarthrus_ is on pages 197 to 219.
Moberg (1907), in connection with a specimen of _Eurycare angustatum_
which he thought preserved some appendages, described and illustrated
some of the appendages of _Triarthrus_.
The most recent discussion of _Triarthrus_, with some new figures,
is by Walcott (1918, p. 135, pls. 29, 30). He gives a summary of
Beecher's work with numerous quotations. The principal original
contribution is a discussion of the form and shape of the appendages
before they were flattened out in the shale. He found also what
he thought might possibly be the remains of epipodites on three
specimens, one of which he illustrated with a photograph. I have seen
nothing which could be interpreted as such an organ in the many
specimens I have studied.
A point in which Walcott differs from Beecher in the interpretation of
specimens is in regard to the development of the endopodites of small
pygidia. Beecher (1894 B, pl. 7, fig. 3) illustrated a series of
endopodites which he likened to the endites of a thoracic limb of
_Apus_. Doctor Walcott finds that specimens in the United States
National Museum show slender endopodites all the way to the back of
the pygidium, and thinks that Beecher mistook a mass of terminal
segments of exopodites for a series of endopodites. On careful
examination, however, the specimen shows, as Beecher indicated, a
series of endopodites in undisturbed condition (No. 222, our pl. 4,
fig. 5).
_Restoration of Triarthrus._
One of the more important points noted in the later studies of
_Triarthrus_ is that the gnathites of the cephalic appendages are much
less like the endobases under the thorax than Beecher earlier thought,
and showed in his restored figures and in his model. The four
gnathites of each side are curved, flattened, not club-shaped, and
so wide and so close together that they overlap one another. The
metastoma is somewhat larger and more nearly circular than Beecher's
earlier preparations led him to suppose.
The restoration here presented is modified only slightly from the
one designed by Professor Beecher, and the modifications are taken
principally from figures published by him. The gnathites are drawn in
form more like that shown by the specimens and his figures in the
American Geologist (1895 A), and the metastoma is taken from one of
the specimens. On the thorax the chief modification is in the addition
of a considerable number of spines to the endopodites. In spite of the
trivial character of most of these changes, they emphasize one of the
important characteristics of _Triarthrus_ the regional differentiation
of the appendages.
It should be pointed out that although _Triarthrus_ is usually
considered to be a very primitive trilobite, its appendages are more
specialized than those of any of the others known. This is shown in
their great length, the double curvature of the antennules, the
differentiation of four pairs of endobases on the cephalon as
gnathites, and the flattening of the segments of the posterior
endopodites. These departures from the uniformity existing among the
appendages of the other genera lead one to question whether the genus
is really so primitive as has been supposed.
_Relation of the Cephalic Appendages to the Markings on the Dorsal
Surface of the Glabella._
_Triarthrus becki_ is usually represented as having four pairs of
glabellar furrows, but the two pairs at the front are exceedingly
faint and the first of them is hardly ever visible, though that it
does exist is proved by a number of authentic specimens. The neck
furrow is narrow and sharply impressed, continuing across the glabella
with a slightly backward curvature. In front of it are two pairs of
linear, deeply impressed furrows which in their inward and backward
sweep are bowed slightly forward, the ends of the corresponding
furrows on opposite sides nearly meeting along the crest of the
glabella. In front of these, near the median line, is a pair of slight
indentations, having the appearance and position of the inner ends of
a pair of furrows similar to those situated just behind them.
In front of and just outside this pair are the exceedingly faint
impressions of the anterior pair of furrows, these, as said above,
being but seldom seen. They are short, slightly indented linear
furrows which have their axes perpendicular to the axis of the
cephalon, and do not connect with each other or with the dorsal
furrows. The latter are narrow, sharply impressed, and merge into a
circumglabellar furrow at the front. In front of the circumglabellar
furrow is a very narrow rounded ridge, but the anterior end of the
glabella is very close to the margin of the cephalon.
Specimen No. 214, which was cleaned from the dorsal side, shows the
posterior tip of the hypostoma, apparently in its natural position,
3.5 mm. back from the anterior margin. The entire length of the
cephalon is 6 mm., so that the hypostoma reaches back slightly over
one half the length (0.583). The greater part of it has been cleaned
off, and one sees the proximal portions of the antennules, which are
apparently attached just at the sides of the hypostoma, 2.5 mm. apart
and 2.25 mm. back from the anterior edge of the cephalon. This
position is distinctly within the outline of the glabella and
corresponds approximately to the location of the second pair of
glabellar furrows. Specimens 214, 215, 216, 217, and 219 all seem to
show the same location for the bases of the antennules. Specimen 220
is the one in which the basal shafts are best preserved and the points
of attachment seem to be further apart in it than in any of the
others. This specimen is 38 mm. long, and the bases of the antennules
are 5.5 mm. apart and 4 mm. behind the anterior margin. As the
specimen is cleaned from the ventral side, the dorsal furrows do not
show distinctly, but another specimen of about the same size (No. 228,
38.5 mm. long) has the dorsal furrows 8 mm. apart 4 mm. back of the
anterior margin.
On the same slab with specimens 209 and 210 there is an individual
which, although retaining the test, has had the proximal ends of the
antennules so pressed against it that the course of the one on the
left side is readily visible. It originates in a small oval mound
whose posterior margin impinges upon the third glabellar furrow near
the middle of its course, and just outside the outer end of the second
glabellar furrow. The cephalon of this specimen is 5 mm. long, and the
point of origin of the left antennule is 2.75 mm. in front of the
posterior margin and 0.75 mm. from the dorsal furrow.
It is therefore evident that the antennules in this species are not
attached beneath the dorsal furrows, but within them and opposite the
second pair of glabellar furrows.
All cephalic appendages behind the antennules are attached somewhat
within the dorsal furrows, the first pair as far forward as the
antennules and the last pair apparently under the anterior edge of
the neck ring. They do not appear to correspond in position to the
posterior glabellar furrows and neck ring, being more crowded. The
last pair is attached to appendifers beneath the nuchal segment, and
the first pair beneath the third glabellar furrows. There are no
depressions on the dorsal surface corresponding to the points of
attachment of the mandibles.
Anal Plate.
Professor Beecher, during his first studies of _Triarthrus_, found no
appendages pertaining to the anal segment, but later evidently came
upon a spinose anal plate which he caused to be figured. The specimen
(No. 201) on which this appendage is preserved is cleaned from the
dorsal side, and the anal plate is a small, bilaterally symmetrical,
nearly semicircular structure margined with small spines. Specimen 202
also shows the same plate (pl. 5, fig. 6), but it is imperfectly
preserved. It has a large perforation in the anterior half. Both of
these specimens are in the Yale University Museum.
[Illustration: Fig. 11.--_Triarthrus becki_ Green. Anal plate of
specimen 65525 in the U. S. National Museum. Drawn by Doctor Wood. ×
20.]
The anal plate is especially well shown by specimen 65525 in the
United States National Museum (fig. 11). This specimen is from Rome,
New York, and two photographs of it have been published by Walcott
(1918, pl. 29, fig. 6; pl. 30, fig. 19). It is developed from the
dorsal side, and the anal plate is displaced, so that it projects
behind the end of the pygidium. It is semicircular in shape, with a
hemispheric mound at the middle of the anterior half. Two furrows
starting from the anterior edge on either side of the mound border its
sides, and, uniting back of it, continue as an axial furrow to the
posterior margin. The mound is perforated for the opening of the
posterior end of the alimentary canal. The lateral borders of the
plate bear five pairs of short, symmetrically placed spines. The plate
is 1 mm. wide and 0.5 mm. long, and the entire trilobite is 11.5 mm.
long.
THE APPENDAGES OF PTYCHOPARIA.
=Ptychoparia striata= (Emmrich).
Illustrated: Jaekel, Zeits. d. d. geol. Gesell., 1901, vol. 53,
part 1, pls. 4, 5.
Jaekel has described a specimen of this species obtained from the
Middle Cambrian near Tejrovic, Bohemia, which on development showed
beneath the test of the axial lobe, certain structures which he
believed represented the casts of proximal segments of appendages.
On the basis of this specimen he produced a new restoration of the
ventral surface of the trilobite, in which he showed three short wide
segments in the place occupied by the coxopodite of an appendage of
_Triarthrus_. He also made the mouth parts considerably different from
those of the latter genus. Beecher (1902) showed that the structures
which Jaekel took for segments of appendages were really the fillings
between stiffening plates of chitin on the ventral membrane, and
demonstrated the fact that similar structures existed in _Triarthrus_.
It cannot be said, therefore, that any appendages are really known in
_Ptychoparia striata_, but some knowledge of the internal anatomy of
the species is supplied by the specimen.
=Ptychoparia cordilleræ= (Rominger).
Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, p. 192,
pl. 24, fig. 2;--Ibid., vol. 67, 1918, pl. 21, figs. 3-5 (corrected
figure).
Walcott has figured a single individual of this species showing
appendages, the accompanying description being as follows (1918, p.
144):
Ventral appendages. Only one specimen has been found showing the
thoracic limbs. This indicates very clearly the general character
of the exopodite and that it is situated above the endopodite,
although there are only imperfect traces of the latter....
The exopodites are unlike those of any trilobite now known. They
are long, rather broad lobes extending from the line of the union
of the mesosternites and the pleurosternites. At the proximal end
they appear to be as wide as the axial lobe of each segment, and to
increase in width and slightly overlap each other nearly out to the
distal extremity.... They are finely crenulated along both the
anterior and dorsal margins, which indicates the presence of fine
setæ.
The specimen is quite imperfectly preserved, but seems to indicate
that the exopodite of Ptychoparia had a long, rather narrow
unsegmented shaft.
_Measurements_ (from Walcott's figure): The specimen is a small one,
about 9.5 mm. long, an individual exopodite is about 2 mm. long and
the shaft 0.33 mm. wide.
_Horizon and locality:_ Middle Cambrian, Burgess shale, between Mount
Field and Wapta Peak, above Field, British Columbia.
=Ptychoparia permulta= Walcott.
Illustrated: Walcott, Smithson. Misc. Coll., vol. 67, 1918, p. 145,
pl. 21, figs. 1, 2.
Walcott figured one individual of this species showing long slender
antennules projecting in front of the cephalon. It is of especial
interest because one of the antennules shows almost exactly the same
sigmoid curvature which is so characteristic of the related
_Triarthrus_. The individual segments are not visible.
_Measurements:_ The specimen is 23 mm. long and the direct distance
from the front of the head to the anterior end of the more perfect
antennule is 9.5 mm. Measured along the curvature, the same antennule
is about 11 mm. long.
_Horizon and locality:_ Same as the preceding.
The Appendages of Kootenia.
=Kootenia dawsoni= Walcott.
Illustrated: Walcott, Smithson. Misc. Coll., vol. 67, 1918, pl. 14,
figs. 2, 3.
One specimen figured by Doctor Walcott shows the distal ends of some
of the exopodites and endopodites of the right side. He compares the
exopodites with those of Neolenus, stating that the shaft consists
of two segments, the proximal section being long and flat, fringed
with long setæ, while the distal segment has short fine setæ. The
endopodite best shown is very slender, and the segments are of uniform
width and only slightly longer than wide.
Measurements (from Walcott's figures): Length of specimen, about 41
mm. Length of five distal segments of an endopodite, 7.5 mm. Since
the pleural lobe is only 7 mm. wide, the endopodites, and probably
the exopodites also, must have projected a few millimeters beyond the
dorsal test when extended straight out laterally.
Formation and locality: Burgess shale, Middle Cambrian, on the west
slope of the ridge between Mount Field and Wapta Peak, above Field,
British Columbia.
The Appendages of Calymene and Ceraurus.
HISTORICAL.
All of the work on these species has been done by Doctor Walcott, who
summarized his results in 1881.
In the first of his papers (1875, p. 159), Walcott did not describe
any appendages but paved the way for further work by a detailed and
accurate description of the ventral surface of the dorsal shell of
Ceraurus. He demonstrated the presence in this species of strongly
buttressed processes which extend directly downward from the test just
within the line of the dorsal furrows. One pair of these is seen
beneath each pair of the glabellar furrows, each segment of the thorax
has a pair, and there are four pairs on the pygidium. He pointed out
also that these projections were but poorly developed on that part of
the glabella which is covered by the hypostoma. He called them axial
processes, the only name which appears to have been suggested thus
far.
The first announcement of the discovery of actual appendages in
_Ceraurus_ and _Calymene_ was made by the same investigator in a
pamphlet published in 1876 in advance of the 28th Report of the New
York State Museum of Natural History, the publication of the whole
report being delayed till 1879. The results were obtained by the
process of cutting translucent slices of enrolled trilobites derived
from the Trenton limestone at Trenton Falls, New York. Since he
summarized all the results of this study in one paper at a later
date, it is not necessary to follow the stages of the work.
A second preliminary paper was published in pamphlet form in
September, 1877, and in final form in 1879, when the first figures
were presented.
In his important paper of 1881, Walcott reviewed all that was known of
the appendages of trilobites to that time, and gave the results of
seven years of study of sections of enrolled specimens. Slices had
been made of 2,200 individuals from Trenton Falls, which resulted in
obtaining 270 which were worthy of study. Of these, 205 were from
_Ceraurus pleurexanthemus_, 49 from _Calymene senaria_, 11 from
_Isotelus gigas_, and 5 from _Acidaspis trentonensis_.
Walcott's views on certain portions of the anatomy can best be set
forth in the form of a few extracts (1881, pp. 199-208):
_The Ventral Membrane._--In those longitudinal sections in which the
ventral membrane is most perfectly preserved, it is shown to have been
a thin, delicate pellicle or membrane, strengthened in each segment by
a transverse arch, to which the appendages were attached. These arches
appear as flat bands separated by a thin connecting membrane, somewhat
as the arches in the ventral surface of some of the Macrouran
Decapods....
In by far the greater number of sections, both transverse and
longitudinal, the evidence of the former presence of an exterior
membrane, protecting the contents of the visceral cavity, rests on the
fact that the sections show a definite boundary line between the white
calcspar, filling the space formerly occupied by the viscera, and the
dark limestone matrix. Even the thickened arches are rarely seen.
The mode of attachment of the leg to the ventral surface is shown [in
transverse and longitudinal sections of _Ceraurus_ and _Calymene_].
These illustrations are considered as showing that the point of
articulation was a small, round process projecting from the posterior
surface of the large basal joint, and articulating in the ventral arch
somewhat as the legs of some of the Isopods articulate with the arches
in the ventral membrane. The arches of the ventral membrane in the
trilobite ... afford a correspondingly firm basis for the attachment
of the legs.
Branchial appendages.--The branchiæ have required more time and labor
to determine their true structure than any of the appendages yet
discovered. They were first regarded as small tubes arranged side by
side, like the teeth in a rake; then as setiferous appendages, and
finally as elongate ribbon-like spirals and bands attached to the side
of the thoracic cavity, the epipodite being a so-called branchial arm.
All of these parts are now known to belong to the respiratory system,
but from their somewhat complex structure, and the various curious
forms assumed by the parts when broken up and distorted, it was a long
time before their relations were determined.
The respiratory system is formed of two series of appendages, as found
beneath the thorax. The first is a series of branchiæ attached to the
basal joints of the legs, and the second, the branchial arms, or
epipodites.
The branchiæ, as found in _Calymene_, _Ceraurus_, and _Acidaspis_,
have three forms. In the first they bifurcate a short distance from
the attachment to the basal joint of the leg, and extend outward and
downward as two simple, slender tubes, or ribbon-like filaments.
In the second form they bifurcate in the same mariner, but the two
branches are spirals. These two forms occur in the same individual
but, as a rule, the more simple ribbon-like branchia is found in the
smaller or younger specimens, and the spiral form in the adult.... The
spiral branchiæ of Ceraurus are usually larger and coarser than those
of _Calymene_.
The third type of the branchiæ [consists of rather long straight
ribbons arranged in a digitate manner on a broad basal joint]. As far
as yet known, this is confined to the anterior segments of the thorax.
The epipodite or branchial arm was attached to the basal joints of the
thoracic legs and formed of two or more joints. This has been called a
branchial arm, not that it carried a branchia, but on account of its
relation to the respiratory system. It is regarded as an arm or
paddle, that, kept in constant motion, produced a current of water
circulating among the branchiæ gathered close beneath the dorsal
shell. . . .
Of the modification the respiratory apparatus underwent beneath the
pygidium, we have no evidence.
In his latest publication (1918, pp. 147-153, pls. 26-28, 33), Walcott
has reviewed his earlier work on _Calymene_ and _Ceraurus_, and
presented a new restoration of the former. The coxopodites are now
interpreted as being similar to those of _Triarthrus_ and Neolenus,
but the exopodites are still held to be spiral and the setiferous
organs labelled as epipodites rather than exopodites.
Comparison of the Appendages of Calymene and Ceraurus with those of
Triarthrus.
As one may see by reading the above quotations from Doctor Walcott's
descriptions, he found certain branchial organs in _Ceraurus_ and
_Calymene_ which have not been found in other trilobites but otherwise
the essential features of the appendages of all are in agreement.
Spiral Branchiæ.
It is now necessary to inquire if the thin sections can not be
interpreted on the basis of trilobites with the same organs as
_Triarthrus_. The interpretation of the structures seen in these
translucent slices is exceedingly difficult, and Doctor Walcott
deserves the utmost praise for the acumen with which he drew his
deductions. Even with the present knowledge of _Triarthrus_,
_Isotelus_, and _Neolenus_ as a guide, I do not think it is safe to
speak dogmatically about what one sees in them.
Walcott has summarized his results in his restoration of the
appendages of _Calymene_ (1918, pl. 33). The coxopodite supports a
slender six-jointed endopodite as in _Triarthrus_, dorsal to which is
a short setiferous epipodite which differs from the exopodite of
_Triarthrus_, in being less long, unsegmented, and in having shorter
setæ. Arising from the same part of the coxopodite with this epipodite
is the bifurcate spiral branchia which has not been seen in this form
in other trilobites. The evidence on which the existence of this organ
is postulated consists of a series of sections across the thorax, the
best of them figured by Walcott in his plates 2 and 3 (1881) and plate
27 (1918).
The specimens sliced were all partially or quite enrolled, and in that
position one would expect to find the appendages so displaced that it
would be only rarely that a section would be cut, either by chance or
design, in such a direction as to show any considerable part of any
one appendage. This expectation has proved true in regard to the
endopodites, the sections rarely showing more than two or three
consecutive segments. Sections like those shown in figures 1 and 2
in plate 2 (1881) seem to be unique. On the other hand, there are
numerous slices showing the so-called spiral branchiæ. They show for
the most part as a succession of rectangular to kidney-shaped spots
of clear calcite.[1] Usually these clear spots are isolated, not
confluent, but in a small number of specimens, perhaps three or four,
the spots are connected in such a way as to show a zig-zag band which
suggests a spiral. Such an explanation is of course entirely
reasonable, but it would be surprising if so slender a spiral should
be cut in such a way as to exhibit the large series of successive
turns shown in many of these thin sections. Continuous sections of
such organs should be no more common than continuous sections of
endopodites.
[Footnote 1: In looking at Walcott's figures of 1881, it should be
remembered that the dark portions of the figures are clear calcite in
the specimens, while the light part is the more or less opaque
matrix.]
One of the arguments against the interpretation of these series of
spots as sections across spiral arms is that of probabilities. It
is known from flattened specimens that _Neolenus_, _Kootenia_,
_Ptychoparia_, _Triarthrus_, and _Cryptolithus_ all have a single type
of exopodite, consisting of a simple setiferous shaft. All these
genera have been examined in a way that permits no doubt about the
structure, and no trace of spiral arms has been detected. On the other
hand, Walcott found spiral arms in three unrelated genera, _Calymene_,
_Ceraurus_, and _Acidaspis_, all of the trilobites in which he found
exopodites by the method of sectioning. What are the probabilities
that genera of three different families, studied by means of sections,
should agree in having a type of exopodite different from that of the
five genera about whose interpretation there can be no doubt?
Another argument against the interpretation of the sections as spirals
is that in any one line the individual spots are of roughly uniform
size. This means of course that the spiral has been cut by a plane
parallel to the tangent plane. This might happen once, just as once
Doctor Walcott cut all six segments of a single endopodite, but that
it should happen repeatedly is highly improbable. Moreover, there is
a limit to the diameter of the section which may be made from these
slender spirals. Most of the spots have one diameter about one half
greater than the other, but others are from three to six times as long
as wide. These last could obviously be cut only from a very large
spiral, and they are therefore interpreted by Walcott as setæ of
epipodites. Yet all gradations are found among the sections, from the
long setæ to the short dots. (See pl. 27, 1918.) In referring to one
slice, Walcott says (1918, p. 152):
In the latter figure and in figure 13, plate 27, the setæ of several
epipodites appear to have been cut across so as to give the effect
of long rows of setæ. The same condition occurs in specimens of
_Marrella_ when the setæ of several exopodites are matted against each
other.
[Illustration: Fig. 12.--A slice of _Ceraurus pleurexanthemus_ in
which the exopodite happened to be cut in such a way as to show a part
of the shaft and some of the setæ in longitudinal section. Specimen
80. × 4.]
This is certainly an apt comparison, and equally true if _Neolenus_,
_Triarthrus_, or _Cryptolithus_ were substituted for _Marrella_.
Now consider the "epipodites." They are well shown in _Calymene_ in
the specimens illustrated on plate 27, figure 11 (1918), and plate 3,
figure 3 (1881), and less clearly in one or two others. Slices 22 (pl.
27, fig. 12, 1918) and 80 (our fig. 12) show what is called the same
organ in Ceraurus. It will be noted that all of these slices are cut
in the same way, that is, more or less parallel to the under surface
of the head, or, at any rate, on a plane parallel to a plane which
would be tangent to the axial portion of the coiled shell. The
sections which show the spirals best are those which are cut by a
plane perpendicular to the long axis of the body. If one were to
attempt to cut an enrolled _Triarthrus_ in such a way as to get a
section showing the length of the setæ, one would not cut a section
perpendicular to the axis of the animal, nor, in fact, would he cut
one parallel to the ventral plane, but it is obvious that in this
latter type of section he would stand a better chance of finding a
part of the plane of the exopodite coincident with the plane of his
section than in the former. And that seems to be what has happened in
these sections of _Calymene_ and _Ceraurus_. If the exopodites were
preserved, transverse sections were bound to cut across many sets of
fringes, and the resultant slice would show transverse sections of the
setæ as a series of overlapping spots. A few fortunately located
sections in a more nearly horizontal plane might cut the setæ and
occasionally the shaft of one or more exopodites in the longitudinal
plane, and the resulting effect would produce the so-called
"epipodites." A careful study has shown that no one of these
epipodites is complete, and they do not have the palmate form shown in
Walcott's figures.
And the last and most important argument against the spiral appendages
is that certain slices, of both _Calymene_ and _Ceraurus_, show
definitely exopodites of exactly the type found in other trilobites.
These are discussed later in the detailed description of the various
slices.
If these series of spots are interpreted on the basis of the known
structure of _Triarthrus_, they are of course a series of sections
through the setæ of the exopodites. It will be shown in Part IV
that these setæ are not circular in section, but flattened, in
_Cryptolithus_ even blade-like, and that they overlap one another. A
section across them would give the same general appearance as, for
instance, that shown in figures 4, 6, 9, and 10 of Walcott's plate 3
(1881).
When both endopodites and the "spiral branchiæ" are present in the
same section (pl. 1, fig. 4; pl. 2, figs. 1, 2), the "spiral branchiæ"
are dorsal to the endopodites, as the setæ of the exopodites would be
expected to be. The specimens which show the clear spots connected,
and which suggest a spiral (pl. 3, fig. 5), may seem at first sight to
bear evidence against this interpretation, but one has only to think
of the effect of cutting a section along the edge where the setæ are
attached to the shaft of the exopodite of _Triarthrus_ to see that
such a zig-zag effect is entirely possible. One would expect to cut
just this position only rarely, and, in fact, the zig-zags are seen in
only three or four sections. The bifurcation of the basal segment of
the "spiral branchiæ" (pl. 3, fig. 10, 1881) is probably more apparent
than real, if indeed these basal segments have anything to do with the
succeeding one.
A second peculiarity of _Calymene_, shown in Walcott's restoration, is
the great enlargement of the coxopodites and of the distal segments of
the endopodites of the fifth pair of appendages of the cephalon. This
is based on the sections of plate 3, figures 6, 7, 8, 9, 10 (1881).
After a study of the specimens I regret to find myself still
unconvinced that the posterior cephalic appendages were any larger
than those in front.
Ventral Membrane.
The most striking value of the thin sections of _Ceraurus_ and
_Calymene_, and therein they have a great superiority over all the
other forms so far investigated, is that they show the extent of the
body cavity and the position, though not the substance, of the ventral
membrane. Transverse sections through _Ceraurus_ (Walcott's pl. 1.
figs. 1-5; pl. 2, figs. 1, 3, 1881) and _Calymene_ (pl. 3, figs. 9,
10, 1881) show that the body cavity was almost entirely confined to
the axial lobe. The longitudinal sections of _Ceraurus_ (pl. 2, figs.
6, 8; pl. 4, fig. 8) and of _Calymene_ (pl. 2, figs. 5, 7; pl. 5,
figs. 1-4) show that the ventral membrane was exceedingly thin and was
wrinkled transversely when the shell was enrolled.
The specimens of figures 1-3, plate 5 (1881) show the form of the
ventral membrane more distinctly than any of the others. The section
of figure 1 was cut just inside the dorsal furrow on the right side,
and figure 2, which is on the opposite side of the same slice, is
almost exactly on the median line. Figure 3 shows a section just
inside the left dorsal furrow. Section 2 did not cut any of the
appendages, and the ventral membrane is shown as a thickened,
probably chitinous sheet thrown into low sharply crested folds equal
in number to, and pointing in a direction just the reverse of, the
crests of the segments of the thorax. Under the pygidium, where there
would of course be less wrinkling, the folds are hardly noticeable. In
the actual specimens one sees more plainly than in the figures the
line of separation between the ventral membrane and the appendages,
but the state of preservation of everything beneath the dorsal shell
is so indefinite that one does not feel sure just what the connection
between the appendages and the membrane was. In the original of figure
5, plate 2, which seems to have been cut so as to cross the appendages
at their line of junction with the ventral membrane, there appear to
be narrow chitinous (?) plates extending from the ventral membrane to
the dorsal test.
Appendifers.
In Ceraurus there are regular calcareous processes which extend down
from the dorsal test just inside the line of the dorsal furrow, and
which undoubtedly serve as points of attachment of the appendages.
These processes, which for convenience I have designated as
"appendifers," are broken off in most specimens showing the lower
surface of _Ceraurus pleurexanthemus_, but on certain ones cleaned
with potash they are well preserved. Doctor Walcott showed them well
in his figures of the lower surface of this species (1875, pl. 11;
1881, pl. 4, fig. 5), while the attempt of Raymond and Barton (1913,
pl. 2, fig. 7) to show them by photography was not so successful.
There is one pair of appendifers on each of the thoracic segments and
four pairs on the pygidium. On the cephalon there is one pair under
the neck furrow, and a pair under the posterior glabellar furrows.
These are not concealed by the hypostoma. Further forward, and
completely covered by the hypostoma, are two much less strongly
developed but similar ones, so that there are in all four pairs of
appendifers on the cephalon, though it is extremely doubtful if the
appendages were articulated directly to all of them. On a specimen of
_Ceraurus pleurexanthemus_ 30 mm. long on the median line, the dorsal
furrows are 7.5 mm. apart at the anterior end of the thorax, and the
tips of the appendifers of this segment are only 4 mm. apart. Each
consists of a straight slender rod with a knoblike end projecting
directly downward from the dorsal test, and supported by a thin
calcareous plate which runs diagonally forward to the anterior edge of
the segment directly under the dorsal furrow. On the pygidium three
pairs of the appendifers have this form, while the fourth pair consist
of low rounded tubercles which are concealed by the doublure. These
appendifers are probably cut in many of Walcott's sections of
Ceraurus, but owing to the state of preservation it is not always
possible to determine what part is appendage, what part is body
cavity, and what part is appendifer.
Nearly forty years ago Von Koenen (1880, p. 431, pl. 8, figs. 9, 10)
described and figured the appendifers of Phacops latifrons. He found
them to be calcareous projections on the hinder margin of each
segment, converging inward, and about 1.5 mm. long. He correctly
considered them as supports (Stützpunkte) for the feet.
Appendifers are well developed also in Pliomerops, and in well
preserved specimens of _Calymene senaria_ from Trenton Falls they are
present, but instead of being rod-like processes, they are rather
thick, prominent folds of the shell. They are also well shown in some
of the thin sections. A specimen of _Triarthrus_ (No. 229, our pl. 5,
fig. 2) has broad processes extending downward from the lower side of
the test below the dorsal furrows, much as in _Calymene_, and the
individual of _Cryptolithus_ shown in plate 8, figure 1, possesses
slender appendifers. Two other specimens (Nos. 237 and 242) show them
quite well. They were probably present in all trilobites, but seldom
preserved. The appendifers have the same origin as the entopophyses of
_Limulus_, and like them, may have relatively little effect on the
dorsal surface.
_Calymene senaria_ Conrad.
(Text figs. 13-16, 23.)
Illustrated: Walcott, Bull. Mus. Comp. Zool., Harvard Coll., vol.
8, 1881, pl. 1, figs. 6-10; pl. 2, figs. 5-7, 10; pl. 3, figs. 1,
3, 8-10; pl. 4, figs. 3, 7; pl. 5, figs. 1-6; pl. 6, figs. 1
(restoration), 2;--Proc. Biol. Soc. Washington, vol. 9, 1894, pl.
1. fig. 7 (restoration);--Geol. Mag., dec. 4, vol. 1. 1894, pl. 8,
figs. 7, 8;--Smithson. Misc. Coll., vol. 67, 1918, pl. 26, figs.
1-7, 9-13; pl. 27, figs. 4, 5 (not 5a), 11 (not 12, _Ceraurus_),
13, 14, 15 (not _Ceraurus_); pl. 28, figs. 7, 8; pl. 33, fig. 1
(restoration); pl. 34, fig. 2; pl. 35, fig. 6.--Dames, N. Jahrb. f.
Min., etc., vol. 1, 1880, pl. 8, figs. 1-5.--Milne-Edwards, Ann.
Sci. Nat., Zoologie, ser. 6, vol. 12, 1881, pl. 11, figs. 19-32;
pl. 12, figs. 33-41.--Packard, Amer. Nat., vol. 16, 1882, p. 796,
fig. 12.--Bernard, The Apodidæ, 1892, text figs. 50, 52,
54;--Quart. Jour. Geol. Soc., London, vol. 50, 1894, text figs. 13,
15, 17.--Oehlert, Bull. Soc. Géol. France, ser. 3, vol. 24, 1896,
fig. 12.--Beecher, Amer. Jour. Sci., vol. 13, 1902, pl. 5, fig. 7.
In both of Walcott's accounts (1881, 1918) of the appendages of
_Calymene_ and _Ceraurus_, he has described them together, so that
those who have not taken time to study the illustrations and
disentangle the descriptions are very apt to have a confused notion in
regard to them. I have therefore selected from the original specimens
those slices of _Calymene_ which are most instructive, and bearing in
mind the probable appearance of the appendages of an enrolled
_Triarthrus_, have tried to interpret them. In such a method of study,
I have of course started with a pre-formed theory of what to expect,
but have tried to look for differences as well as likenesses.
_Cephalic Appendages._
_Antennules._--The evidence of antennules rests on a single slice (No.
78). The appendage in question is exceedingly slender and arises at
the side of the hypostoma near its posterior end. It shows fine,
slender segments, and curves first outward and then forward. If it is
in its natural position, it is not an antennule, but the endopodite of
the second or third pair of cephalic appendages. It is short, only
about one-third the length of the hypostoma, but is doubtless
incomplete. The two distal segments show a darker filling, indicating
that they were hollow. Judging from analogy with other trilobites, the
appendage is probably an endopodite and not an antennule. There can be
no reasonable doubt, however, that _Calymene_ possessed antennules.
Some idea of the form of the coxopodites of the cephalic appendages
may be obtained from sections which cut in approximately the plane of
the hypostoma. Such sections are shown in Walcott's photographs (pl.
26, figs. 4, 6, 11, 1918). Specimens 50 (fig. 4, our fig. 13), 51
(fig. 6), 6 (fig. 11), and 40 (our fig. 14) agree in showing two
pairs of slender coxopodites which are attached at the sides of the
hypostoma and run backward parallel and close to it, and two pairs of
larger coxopodites which are behind the hypostoma, although the point
of attachment of the third pair is in front of its tip. The anterior
pair are apparently under-developed and no longer function as mouth
parts, while the posterior two pairs are large and armed on their
inner ends with spines. Specimen 78, which has already been mentioned
in connection with the antennules, shows a second very slender
appendage back of the so-called antennule, which is equally slender,
but is directed outward instead of forward. It seems not improbable,
from their position and similarity, that these two are the endopodites
of the first two appendages on one side of the hypostoma. Specimen 6
shows rather inadequately the endopodites of the second and third
cephalic appendages. I have not found other slices showing endopodites
of the cephalon. Walcott, in both his restorations, has shown
enlarged, paddle-shaped dactylopodites on the distal ends of the
fourth cephalic endopodites. The evidence for this rests principally
on three slices, No. 38 (pl. 26, figs. 9, 10), 53 (pl. 26, fig. 12),
and 43 (pl. 26, fig. 13). Of these, No. 43 may be dismissed at once as
too poorly preserved to be interpreted. No. 53 does show a section of
an appendage which seems to have an unusually wide dactylopodite, but
this slice presents no evidence at all as to the appendage to which
the dactylopodite appertains, nor can one even be sure that there has
not been a secondary enlargement. Specimen 43 shows this feature
much less definitely than is indicated by the published photograph
and drawing. The segment in question is strongly curved, with a
constriction possibly dividing it into two. If it is in its natural
position in this section, it obviously belongs to one of the thoracic
segments and not to the cephalon. With evidence of difference so
unsatisfactory, I prefer to reconstruct the posterior cephalic
endopodites on the same plan as those of the thorax.
[Illustration: Fig. 13.--Slice through _Calymene senaria_ in the plane
of the hypostoma, showing the very slender coxopodites beside that
organ, the spines on the inner end of one of the maxillulæ, and the
anterior position of the attachment of all these appendages. From a
photographic enlargement. Specimen 50. × 4.]
[Illustration: Fig. 14.--Slice through the hypostoma and thorax of
_Calymene senaria_ Conrad, showing the small size of the coxopodites
nearest the hypostoma. Shell in black, appendages and filling of
abdominal cavity dotted. From a photographic enlargement. Specimen 40.
× 3.8.]
[Illustration: Fig. 15.--Transverse section of _Calymene_, showing
method of articulation with the appendifer. The shell is in solid
black, the filling of the appendage and appendifer stippled. Traced
from a photographic enlargement of the slice. Specimen 63. × 7.]
_Exopodites._--Walcott admits that there is no direct evidence of spiral
exopodites in the cephalon of _Calymene_. No one of the sections
cutting through the plane of the hypostoma shows any trace of
appendages which could be interpreted as exopodites.
_Thoracic Appendages._
The large coxopodites of the anterior thoracic appendages are well
shown in many specimens cut longitudinally, of which Nos. 23, 50, and
55 may be mentioned, since photographs of them have been published by
Walcott (pl. 26, figs. 1-4, 1918). The endobases of all taper toward
the proximal ends. Transverse slices show sections of the coxopodites
which are no wider than those in longitudinal sections, indicating
that they were not compressed but probably cylindrical. This is borne
out by an individual (pl. 28, fig. 7, 1918) which is not a slice but
an actual specimen, the body cavity of which was hollow, and, opened
from above, shows the impressions of the last two coxopodites of the
cephalon, and the first four of the thorax.
One transverse section (No. 63, see our fig. 15) is especially
valuable, as it shows the method of articulation of the coxopodites
with the dorsal skeleton. Another specimen (No. 73) shows that
appendifers are present in _Calymene_, and while the appendifer does
not retain its original form in slice No. 63, the section does show
clearly that there was a notch in the inner (upper) side of the
coxopodite into which the lower end of the appendifer fitted, thus
giving a firm, articulated support for the appendage. This notch
appears to be slightly nearer the outer than the inner end of the
coxopodite, and since it must have made a kind of ball-and-socket
joint, considerable freedom of movement was allowed. The appendage
must have been held in place by muscles within the coxopodite and
attached to the appendifer.
No slice which I have seen shows a continuous section through all the
segments of an endopodite, but many, both longitudinal and transverse,
show one, two, or as many as three segments.
Such sections as No. 120 show that the endopodites of the thorax
were slender and composed of segments of rather uniform diameter.
Other sections, notably No. 83, 154, and in, show that they tapered
distally, and bore small spines at the outer end of each segment.
The exopodites of course furnish the chief difficulty in
interpretation. Doctor Walcott finds two sets of structures attached
to the coxopodite, a long, slender, spiral exopodite, and a short,
broad epipodite with a fringe of long setæ. Since he has given the
same interpretation for _Calymene_, _Ceraurus_, and _Acidaspis_, I
have considered the question of all three together on a preceding page
(p. 48), and given my reasons for regarding both structures as due to
sections in different directions across setiferous exopodites.
Sections like those shown in figures 11, 13, and 14 of plate 27 (1918)
happen to be cut in or near the plane of the setæ of an exopodite, and
so show hairs of considerable length. Such sections are, as would be
expected, very few in number, while sections like those shown on
figures 4, 5, 7, and 9 of plate 27, which cut the setæ more nearly at
right angles, are very common. Slices which give any definite idea of
the form of the shaft of the exopodite are exceedingly rare. Perhaps
the most satisfactory one is No. 23 (pl. 3, fig. 3, 1881), which shows
the proximal part of a long, slender, unsegmented shaft, with the
bases of a number of slender setæ. The organ is not complete, as would
be inferred from the published figure, but the section cuts diagonally
across it, and the total length is unknown. It is directed forward,
like the exopodites of Neolenus, but whether or not this is a natural
position is yet to be learned.
The proximal, non-setiferous portion of the exopodite is evidently
at an angle with the setiferous part. Another similar exopodite is
apparently shown by specimen 29 (pl. 3, fig. 9, 1881), which has a
similar angulated shaft and just a trace of the bases of the setæ.
_Pygidial Appendages._
That appendages were present under the pygidium is shown by
longitudinal sections, but nothing is known of the detail of
structure.
[Illustration: Fig. 16. Restoration of _Calymene senaria_ Conrad,
based upon data obtained from the study of the translucent sections
made by Doctor Walcott. Prepared by Doctor Elvira Wood, under the
supervision of the author. About twice natural size.]
_Relation of Hypostoma to Cephalon in Calymene._
In _Calymene_ the shape of the hypostoma bears little relation to the
shape of the glabella, and it is relatively smaller, both shorter and
narrower, than in Ceraurus. In shape, neglecting the side lappets at
the front, it is somewhat rectangular, but rounded at the back, where
it is bifurcated by a shallow notch. The anterior edge has a narrow
flange all across, which is turned at almost right angles to the plane
of the appendage, and which fits against the doublure of the free
cheeks at the sides and against the epistoma in the middle. The side
lappets show on their inner (upper) surface shallow pits, one on each
lappet, which fit over projections that on the dorsal surface show as
deep pits in the bottom of the dorsal furrows in front of the anterior
glabellar furrows. The appendifers on the head in _Calymene_ take the
form of curving projections of shell underneath the glabellar and neck
furrows, and owing to the narrowness of the hypostoma, all these are
visible from the ventral side, even with it in position. This shield
extends back about 0.6 of the length of the cephalon, and to a point
a little behind the second glabellar furrow from the back of the head.
In Doctor Walcott's restoration of _Calymene_ he has represented
all four pairs of biramous appendages as articulating back of the
posterior end of the hypostoma. I think his sections indicate that
the gnathobases of two pairs of these appendages rested alongside or
beneath it, and in particular, the longitudinal sections (1881, pl. 5)
would appear to show that the mouth was some distance in advance of
its posterior end.
_Restoration of Calymene._
(Text fig. 16.)
From what has been said above, it is evident that for a restoration of
the appendages of _Calymene_ considerable dependence must be placed
upon analogy with other trilobites. Nothing is positively known of the
antennules, the exopodites of the cephalon, or any appendages, other
than coxopodites, of the pygidium, but all were probably present. It
is inferred from the slices that the first two pairs of cephalic
appendages were poorly developed, the endopodites short and very
slender, the coxopodites lying parallel to the sides of the hypostoma
and nearly or quite functionless. The gnathites of the last two pairs
of cephalic appendages are large, closely approximated at their inner
ends, and bear small tooth-like spines. The endopodites are probably
somewhat better developed than the anterior ones and more like those
on the thorax.
The coxopodites of the thorax appear to have had nearly cylindrical
endobases which tapered inward. The endopodites were slender, tapering
gradually outward, and probably did not extend beyond the dorsal test.
Small spines were present on the distal end of each segment. Each
exopodite had a long, slender, unsegmented shaft, to which were
attached numerous long, overlapping, flattened setæ. The shaft may
have been angulated near the proximal end, and may have been directed
somewhat forward and outward as in Neolenus, but the evidence on this
point is unsatisfactory. The number of pairs of appendages is that
determined by Walcott from longitudinal sections, namely, four pairs
on the cephalon beside the antennules, thirteen pairs in the thorax,
and nine pairs on the pygidium.
=Calymene= sp. ind.
(pl. 6, figs. 4, 5.)
Illustrated: Walcott, Bull. Mus. Comp. Zool., Harvard Coll., vol.
8, 1881, pl. 6, figs. 5a, b;--Proc. Biol. Soc. Washington, vol. 9,
1894, pl. 1, fig. 10;--Geol. Mag., dec. 4, vol. 1, 1894, pl. 8,
fig. 10;--Smithson. Misc. Coll., vol. 67, 1918, pl. 36, figs. 1, 2,
2a-d.--Milne-Edwards, Ann. Sci. Nat., Zoologie, ser. 6, vol. 12,
1881; pl. 12, figs. 44a, b.
In the United States National Museum there is a thin piece of
limestone, about 3 inches square, which has on its surface eight
jointed objects that have been called legs of trilobites. Two of these
were figured by Walcott (1881, pl. 6, fig. 5). The slab contains
specimens of _Dalmanella_ and _Cryptolithus_, in addition to the
appendages of trilobites, and is said by Doctor Ulrich to have come
from the tipper part of the Point Pleasant formation (Trenton) on the
bank of the Ohio River below Covington, Kentucky.
The specimens are all endopodites of long slender form, similar to
those of _Triarthrus_, but since that genus does not occur in the
Point Pleasant, it is necessary to look upon some other trilobite as
the former possessor of these organs. Both _Isotelus_ and _Calymene_
occur at this horizon, and as the specimens obviously do not belong
to _Isotelus_ or _Cryptolithus_, it is probable that they were
formerly part of a _Calymene_.
All the endopodites are of chitinous material, and the various
specimens show, according to the perfection of their preservation,
from four to six segments. The endopodite as a whole tapers but
slightly outward, and the individual segments are of nearly equal
length. They appear to be but little crushed, and are oval in section,
with a crimped anterior and posterior margin. One or two show a median
longitudinal ridge, such as is seen in some appendages of
_Triarthrus_. Each segment is parallel-sided, with a slight expansion
at the distal end, where the next segment fits into it.
Under the heading "Ordovician Crustacean Leg," Walcott (1918, p. 154,
pl. 36, figs. 1,2) has recently redescribed these specimens, and
thinks that they do not belong to _Calymene_, nor, indeed, to any
trilobite. He concludes that they were more like what one would expect
in an isopod. Passing over the fact that the oldest isopod now known
is Devonian, the fossils in question seem to me quite trilobite-like.
Walcott says:
The legs are associated with fragments of _Calymene meeki_ but it
is not probable that they belong to that species; if they did, they
are unlike any trilobite leg known to me. The very short coxopodite
and basopodite are unknown in the trilobites of which we have the
legs, as they are fused into one joint forming the long protopodite
in the trilobite. The distal joint is also unlike that of the
trilobite legs known to us.
A great deal of Doctor Walcott's difficulty probably arises from his
homology of the coxopodite of the trilobite with the protopodite of
the higher Crustacea. The coxopodite of the trilobite is not fused
with the basipodite, this latter segment always remaining free.
Indeed, Walcott himself says of _Neolenus_ (1918, p. 128):
Each thoracic leg (endopodite) is formed of a large elongate
proximal joint (protopodite), four strong joints each about 1.5
times as long as wide (basopodite, ischiopodite, meropodite and
carpopodite); two slender elongate joints (propodite and
dactylopodite) and a claw-like, more or less tripartite
termination.
Walcott's drawing (pl. 36, fig. 1) is a composite one, and while it
shows eight segments, I was not able to count more than seven on any
of the specimens themselves. In regard to the terminal segment,
the dactylopodite of the limb shown in his plate 36, figure 2, is
unusually long, and a comparison with other photographs published on
the same plate shows that such long segments are unusual.
Proof that these are appendages of a _Calymene_ is of course wanting,
but there is no particular reason so far to say that they are not.
_Measurements:_ Two of the more complete specimens, each showing six
segments, are each 8 mm. long.
Somewhat similar to the specimens from Covington are the ones
described by Eichwald (1825, p. 39, 1860, pl. 21), the specimens being
from the Silurian of Gotland. The figure copied by Walcott (1881, pl.
6, fig. 4) has never been looked upon as entirely satisfactory
evidence of the nature of the specimen, and so far as I know, the
fossil has not been seen by any modern investigator.
=Ceraurus pleurexanthemus= Green.
(pl. 11; text figs. 12, 17-19, 21, 22, 24, 29, 30.)
Illustrated: Walcott, Ann. Lye. Nat. Hist. New York, vol. II, 1875,
pl. 11;--31st Ann. Rept. New York State Mus. Nat. Hist, 1879, pl.
1, fig. 3;--Bull. Mus. Comp. Zool., Harvard Coll., vol. 8, 1881,
pl. 1, figs. 1-5; pl. 2, figs. 1-4, 6-8; pl. 3, figs. 2, 4-7; pl.
4, figs. 1, 2, 4-6, 8; pl. 6, fig. 3; Smithson. Misc. Coll., vol.
67, 1918, pl. 26, figs. 8, 14, 15; pl. 27, figs. 1-3, 5a, 6-9, 12
(not _Calymene_), (not 15, _Calymene_); pl. 28, figs. 1-5; pl. 34,
fig. 1; pl. 35, fig. 7.--Milne-Edwards, Ann. Sci. Nat., Zoologie,
ser. 6, vol. 12, 1881, pl. 10, figs. 1-18.--Bernard, The Apodidæ,
1892, text figs. 46, 51.
_Cephalic Appendages._
No trace of antennules has yet been found.
I find only three sections cut through the plane of the hypostoma of
Ceraurus which show anything of the cephalic appendages, and no one of
them is very satisfactory. The best is No. 22, the one figured by
Walcott (pl. 3, fig. 2, 1881; pl. 27, fig. 12, 1918), but one should
remember that this section is not actually cut in the plane of the
hypostoma but is a slice diagonally through the head, cutting through
one eye and the posterior end of the hypostoma. It shows what seem to
be the coxopodites of the second, third, and fourth pairs of cephalic
appendages, the exopodites of the third and fourth pairs, and the
metastoma. If this interpretation is correct, the first pair of
gnathites lay alongside the hypostoma or under its edge, and were
feebly developed, the second pair were attached in front of the tip of
the hypostoma, curved back close to it, and their inner ends reached
the sides of the metastoma. The third and fourth pairs were back of
the metastoma, the third pair was stronger than the second, and the
fourth probably like the third.
[Illustration: Fig. 17. Transverse section of _Ceraurus
pleurexanthemus_, showing the relation of the coxopodite to the
appendifer. Traced from a photographic enlargement of the slice.
Specimen 128. × 4/5.]
[Illustration: Fig. 18. Slice of _Ceraurus pleurexanthemus_, showing a
nearly continuous section of an endopodite and an exopodite above it.
The latter is so cut as to show only the edge of the shaft and the
bases of a few setæ. Traced from a photographic enlargement. Specimen
in. × 4.]
Specimen 92 shows traces of the slender endopodites belonging to the
cephalon, but no details. Specimen 22 shows on one side exopodites
(epipodites of Walcott) belonging to the third and fourth cephalic
appendages. That belonging to the third shows some long setæ and a
trace of the shaft, while the one on the fourth appendage (third
coxopodite) has a portion of a broad shaft and a number of long setæ.
It should again be remembered that the slice does not cut through the
plane of the exopodite, but across it at a low angle, so that a part
but not all of the shaft is shown. On the other side of this slice
there is a fairly good section of one of the thoracic exopodites. It
is, however, turned around in the opposite direction from the others,
as would be expected in an enrolled specimen.
Specimens 4 and 5 (pl. 1, figs. 4, 5, 1881) are slices cut diagonally
through the head of Ceraurus, in front of the posterior tip of the
hypostoma. They show fragments of endopodites and exopodites which may
be interpreted as practically identical in form with those of the
thorax. Due to the diagonal plane in which the section is cut, slice 5
shows the coxopodites of two pairs of appendages, one lying nearer
the median cavity than the other. It is extremely difficult to
visualize the interpretation of such sections.
_Thoracic Appendages._
A transverse section through a thoracic segment (No. 128, our fig. 17)
shows the relation of coxopodite to appendifer to be the same as in
_Calymene_, the upper side of the coxopodite having a notch a little
outward from the middle. After seeing that specimen, it is possible to
understand slice No. 168, which shows longitudinal sections through a
number of coxopodites of the thorax, with fragments of both exopodites
and endopodites articulated at the distal ends. These and longitudinal
vertical sections like No. 18 (pl. 2, fig. 8, 1881) show that the
endobases taper inward, and the general uniformity in width in
sections taken at various angles indicates that the coxopodites were
not greatly flattened.
A unique slice (No. 111, pl. 2, fig. 2, 1881; pl. 27, fig. 1, 1918;
our fig. 18) shows a nearly complete thoracic endopodite, and above it
a part of the proximal end of the exopodite of the same segment. When
one considers that out of over two thousand sections only this one
shows the six successive segments of an endopodite, one realizes how
futile it is to expect that dozens of the equally slender "spirals"
should be cut so as to show practically all their turns.
This endopodite is slender, all the segments have nearly the same
length and diameter, though there is a slight taper outward, each
segment is expanded distally for the articulation of the next, and
there are small spines on the distal ends of some of them. There is
probably a terminal spine present, though it is neither so long nor so
plainly visible as in Walcott's photograph.
The exopodite on this same specimen was evidently cut diagonally
across near the setiferous edge, showing a section through the shaft
and the bases of seven setæ (fig. 18). This section is so exactly what
would be obtained by cutting similarly an exopodite of either Neolenus
or _Triarthrus_ that it should in itself dispose of the
"spiral-exopodite" theory.
Several sections have already been illustrated showing sections across
the setæ of the exopodites (pl. 3, figs. 4-6, 1881; pl. 27, figs. 3,
4, 9, 1918), and similar sections are not uncommon. Only a very few,
however, show sections in the plane of the exopodite. If only No. 111,
described above, were known, it would be inferred that the exopodite
had a slender shaft as in _Calymene_, but another good slice, No. 80
(fig. 12, ante) shows that the blade was rather broad, though not so
broad as in Neolenus. The other specimen is No. 22, which has already
been discussed. The thoracic exopodite of this specimen has been very
incorrectly figured by Walcott, as it shows no such palmate shaft as
he has indicated, but a long blade-like one is outlined, though its
entire width is not actually shown.
_Pygidial Appendages._
Sections 14 and 18 (pl. 2, figs. 4, 8, 1881) prove the presence under
the pygidium of three pairs of appendages, the coxopodites and
fragments of endopodites of which are shown. Nothing is known of the
exopodites.
_Relation of Hypostoma to Cephalon._
In Ceraurus the body portion and posterior end of the hypostoma are
roughly oval, about as wide as the glabella at its broadest part, and
the posterior edge extends back to within 0.5 to 1 mm. of the neck
furrow. The posterior pair of appendifers are behind the hypostoma,
while the second pair are in front of its posterior end but escape
being covered by it on account of its oval shape. At the anterior end
the hypostoma is widened by the presence of two side lappets which
extend beyond the boundaries of the glabella. In both Ceraurus and
Cheirurus the anterior edge of the hypostoma fits against the doublure
at the anterior margin of the head and the epistoma is either entirely
absent or is so narrow as not to be seen in specimens in the ordinary
state of preservation. A section across the cephalon of _Ceraurus
pleurexanthemus_ at the horizon of the eyes shows the sides of the
hypostoma fitting closely against the sides of the glabella (Walcott's
pl. 1, fig. 1). Further back on the head it is not in contact with the
dorsal test, and the gnathobases extend beneath it.
Restoration of _Ceraurus pleurexanthemus_. (pl. 11; text fig. 19.)
The restoration of the appendages of _Ceraurus pleurexanthemus_ is a
tentative one, based upon a careful study of the translucent sections
prepared by Doctor Walcott. In no case among these sections is the
actual test of any appendage preserved, and the real form of each part
is generally obscured by the crystallization of the calcite which
fills the spaces formerly occupied by animal matter.
[Illustration: Fig. 19. Restoration of a transverse section of the
thorax of _Ceraurus pleurexanthemus_ Green, showing the relation of
the appendages to the appendifers and the ventral membrane. The
probable positions of the heart and alimentary canal are indicated.]
No section shows anything which can be identified as any part of the
antennules, so that these organs have been supplied from analogy with
_Triarthrus_.
There are undoubtedly four pairs of biramous Cephalic appendages, but
their points of attachment are not so obvious. There are two pairs of
conspicuous appendifers on the posterior part of the cephalon and
another pair almost concealed by the hypostoma. It is probable that
the appendages of the cephalon were not attached directly beneath
them, as the four pairs have to be placed within the space occupied by
the three pairs of appendifers. As the mouth is in front of the
posterior end of the hypostoma, the gnathites of the first pair of
biramous appendages may have extended beneath that organ, or they may
have lain beside it, and only become functional when the hypostoma was
dropped down in the feeding position. The second pair of gnathites
reached just to the tip of the hypostoma, and the other two pairs
seemingly curved backward behind it.
The points of attachment on the thorax, as shown clearly in sections,
were directly beneath the lower ends of the appendifers. The
endopodites were long enough to reach to or a little beyond the outer
extremities of the pleural spines, while the exopodites were
apparently somewhat shorter. Each endopodite consisted of six short,
fairly stout segments, each with at least two spines on the somewhat
expanded distal ends. The exact form of the exopodites could not be
made out. The shaft was apparently rather short, unsegmented, and
fairly broad. The setæ appear from the sections to have been more or
less blade-shaped and to have overlapped, as do those of the
exopodites of _Cryptolithus_. Judging from their position in the
sections, the setæ not only bordered the posterior side of the shaft,
but radiated out from the end as well.
The pygidium shows three pairs of functional appendifers, hence three
pairs of appendages have been supplied. There is a fourth pair of
rudimentary appendifers, but as they are beneath the doublure they
could not have borne ambulatory appendages.
The Appendages of Acidaspis trentonensis Walcott.
(pl. 6, fig. 6.)
A single individual of _Acidaspis trentonensis_, obtained from the
same locality and horizon as the specimens of _Triarthrus_ and
_Cryptolithus_, when cleaned from the ventral side shows a number
of poorly preserved endopodites which seem very similar in shape and
position to those of _Triarthrus_. One endopodite on the right side
of the head and the first five on the right side of the thorax are the
best shown. All are slender, are directed first forward at an angle of
about 45 with the axis, then, except in the case of the cephalic
appendage, turn backward on a gentle curve and extend a little
distance beyond the margin of the test, but not as far as the tips of
the lateral spines of the thoracic segments.
The individual segments of the endopodites can not be seen clearly
enough to make any measurements. On the fourth and fifth endopodites
of the thorax, some of the segments seem to be broad and triangular as
in _Triarthrus_. All that can be seen indicates that _Acidaspis_ had
appendages entirely similar to those of _Triarthrus_, but perhaps not
quite so long, as they seem not to have projected beyond the limits of
the lateral spines. There are no traces of antennules nor,
unfortunately, of exopodites.
_Measurements:_ Length 8 mm.
Walcott (1881, p. 206) stated that his sections had shown the presence
in this species of legs "both cephalic and thoracic" and also the
"spiral branchiæ." His specimens were from the Trenton at Trenton
Falls, New York.
The Appendages of Cryptolithus.
=Cryptolithus tessellatus= Green.
(pl. 6, fig. 7; pls. 7-9; text figs. 20, 25, 45, 46.)
(See also Part IV.)
Illustrated: Beecher, Amer. Jour. Sci., vol. 49, 1895, pl. 3.
When Professor Beecher wrote his short article on the "Structure
and Appendages of _Trinucleus_" (1895), he had only three specimens
showing appendages. In his later work he cleaned several more, so that
there are now thirteen specimens of _Trinucleus_ = _Cryptolithus_
available for study, though some of these do not show much detail. In
his last and unpublished study, Beecher devoted the major part of his
attention to this genus, and summarized his findings in the drawings
which he himself made of the best individuals (text figs. 45, 46).
Valiant (1901) stated that he had found a _Trinucleus_ with antennæ in
the Frankfort shale south of Rome, New York. The specimen has not been
figured.
None of the specimens shows much more of the appendages of the
cephalon than, the hypostoma and the antennules, so that we are still
in ignorance about the mouth parts.
The most striking characteristics of the appendages are as follows:
the antennules are long, and turn backward instead of forward; none
of the limbs projects beyond the margin of the dorsal test; the
exopodites extend beyond the endopodites, reaching very nearly to the
margin of the test; the endopodites are not stretched out at right
angles to the axis, but the first three segments have a forward and
outward direction as in _Triarthrus_, while the last four turn back
abruptly so that they are parallel to the axis; the limbs at the
anterior end of the thorax are much more powerful than the others; the
dactylopodites of the endopodites show a fringe of setæ instead of
three spines as in _Triarthrus_ and _Neolenus_. All these would, as
Beecher has already suggested, seem to be adaptations to a burrowing
habit of life, the antennules being turned backward and the other
appendages kept within the shelter of the dorsal test in order to
protect them, and the anterior endopodites enlarged and equipped with
extra spines to make them more efficient digging and pushing organs.
_Restoration of Cryptolithus._
(Text fig. 20.)
It should be definitely understood that the present figure is a
restoration and not a drawing of a specimen, and that there are many
points in the morphology of _Cryptolithus_ about which no information
is available, especially about the appendages under the central
portion of the cephalon. The information afforded by all the figures
published in this memoir is combined here. As gnathites are preserved
on none of the specimens, those represented in the figure are purely
conventional.
A person who is acquainted only with _Cryptolithus_ preserved in
shale, or with figures, usually has a very erroneous idea of the
fringe It is not a flat border spread out around the front of the
head, but stands at an angle about 45 in uncrushed specimens of most
species. When viewed from the lower side, there is a single outer,
concentric row of the cup-shaped depressions, bounded within by a
prominent girder. This row is in an approximately horizontal plane,
while the remainder of the doublure of the fringe rises steeply into
the hollow of the cephalon. Since the front of the hypostoma is
attached to this doublure, it stands high up within the vault and
under the glabella. Two specimens, Nos. 231 and 233, show something of
the hypostoma, and they are the only ones known of any American
trinucleid. That of specimen 233, the better preserved, is very small,
straight across the front, and oval behind. It seems that it is
abnormally small in this specimen and I should not be surprised if in
other specimens it should be found to be larger.
In the Bohemian _Trinucleoides reussi_ (Barrande), the oldest of the
trinucleids, the hypostoma is very commonly present, and is of the
proper size to just cover the cavity of the glabella, seen from the
lower side, and has, toward the anterior end, side flaps which reach
out under the prominent glabellar lobes. This large size of the
hypostoma would cause the antennules to be attached outside the dorsal
furrows, and the position in which they are attached in the American
species of _Cryptolithus_ may be explained as an inherited one, since
with the small hypostoma they might have been within the glabella, as
in _Triarthrus_.
The antennules are seen in three specimens, and in all cases are
directed backward. The particular course in which they are drawn in
the restoration is purely arbitrary. The second pair of cephalic
appendages are represented as directed downward and forward, since in
one or two specimens fragments of forward-pointing endopodites were
seen near the front of the cephalon, and because in other trilobites
the second pair of appendages is always directed forward. The
remaining three pairs have a more solid basis in observed fact, for
the two or three specimens retaining fragmentary remains of them
indicate that they turn backward like those on the thorax, and that
the individual segments are longer and more nearly parallel-sided than
those of the more posterior appendages. The gnathites of all the
cephalic appendages are admittedly purely hypothetical. None of the
specimens shows them. As drawn, they are singularly inefficient as
jaws, but if, as is suggested by the casts of the intestines of
trinucleids found in Bohemia, these trilobites were mud-feeders,
inefficient mouth-parts would be quite in order.
[Illustration: Fig. 20. _Cryptolithus tessellatus_ Green. A
restoration of the appendages drawn by Doctor Elvira Wood from the
original specimens and from the photographs made by Professor Beecher.
× 9.]
The appendages of the thorax and pygidium can fortunately be taken
quite directly from the photographs of the dorsal and ventral sides of
well preserved specimens. There is of course a question as to the
number and the exact form of those on the pygidium, but I think the
present restoration is fairly well justified by the specimens. As
would be expected from the narrow axial lobe, the gnathobases of the
coxopodites are short and small.
Summary on the Ventral Anatomy of Trilobites.
COMPARISON OF APPENDAGES OF DIFFERENT GENERA.
Since the appendages of _Triarthrus_, _Cryptolithus_, _Neolenus_,
_Calymene_, and _Ceraurus_ are now known with some degree of
completeness, those of _Isotelus_ somewhat less fully, and something
at least of those of _Ptychoparia_, _Kootenia_, and _Acidaspis_, these
forms being representatives of all three orders and of seven different
families of trilobites, it is of some interest to compare the
homologous organs of each.
All in which the various appendages are preserved prove to have a pair
of antennules, four pairs of biramous limbs on the cephalon, as many
pairs of biramous limbs as there are segments in the thorax, and
a variable number of pairs on the pygidium, with, in the case of
_Neolenus_ alone, a pair of tactile organs at the posterior end. Each
limb, whether of cephalon, thorax, or pygidium, consists of a
coxopodite, which is attached on its dorsal side to the ventral
integument and supported by an appendifer, an exopodite, and an
endopodite. The exopodite is setiferous, and the shaft is of variable
form, consisting of one, two, or numerous segments. The endopodite
always has six segments, the distal one armed with short movable
spines.
_Coxopodite._
The coxopodite does not correspond to the protopodite of higher
Crustacea, the basipodite remaining as a separate entity. The inner
end of the coxopodite is prolonged into a flattened or cylindrical
process, which on the cephalon is more or less modified to assist in
feeding, and so becomes a gnathobase or gnathite. The inner ends of
the coxopodites of the thorax and pygidium are also prolonged in a
similar fashion, but are generally somewhat less modified. These
organs also undoubtedly assisted in carrying food forward to the
mouth, but since they probably had other functions as well, I prefer
to give them the more non-committal name of endobases.
In _Triarthrus_ and _Neolenus_ the endobases are flattened and taper
somewhat toward the inward end. In _Isotelus_, _Calymene_ and
_Ceraurus_, they appear to have been cylindrical. In other genera they
are not yet well known. In all cases, particularly about the mouth,
they appear to have been directed somewhat backward from the point of
attachment. As it is supposed that these organs moved freely forward
and backward, the position in which they occur in the best preserved
fossils should indicate something of their natural position when
muscles were relaxed.
_Cephalon._
_Antennules._--Antennules are known in _Triarthrus_, _Cryptolithus_,
_Neolenus_, and _Ptychoparia_. In all they are long, slender, and
composed of numerous segments, which are spiniferous in _Neolenus_,
and very probably so in the other genera.
In _Triarthrus_, _Neolenus_, and _Ptychoparia_ they project ahead of
the cephalon, emerging quite close together under the front of the
glabella, one on either side of the median line. In _Cryptolithus_
they turn backward beneath the body, but since only three or four
specimens are known which retain them, it is possible that other
specimens would show that these organs were capable of being turned
forward as well as backward. The proximal ends of the antennules being
ball-like, it is probable, as Doctor Faxon has suggested to me, that
these "feelers" had considerable freedom of motion. The antennules of
_Triarthrus_ are apparently somewhat less flexible than those of the
other genera, and have a double curvature that is seen among the
others only in Ptychoparia. The proximal end of an antennule in
_Triarthrus_ is a short cylindrical shaft, apparently articulating in
a sort of ball-and-socket joint. The proximal end in the other genera
is still unknown. The points of attachment in _Triarthrus_ seem to be
under the inner part of the second pair of glabellar furrows. In
_Cryptolithus_ they appear to be beside the anterior lobe of the
glabella under what have long been known as the antennal pits. In the
other genera the location is not definitely known, but in _Neolenus_
it seems to be under the dorsal furrows near the anterior end of the
glabella. Viewed from the under side, the point of attachment is
probably always beside the middle or anterior part of the hypostoma,
just behind the side wings.
_Paired biramous appendages._--Behind the antennules all the appendages
except those on the anal segment are biramous, consisting of a
coxopodite with an inward-directed endobase and an outward-directed
pair of branches, the exopodite above, and the six-jointed endopodite
beneath. The basipodite really bears the exopodite, but the latter
also touches the coxopodite. This structure has been seen in
_Triarthrus_, _Cryptolithus_, _Neolenus_, _Kootenia_, _Calymene_,
_Ceraurus_, and _Ptychoparia_. In _Triarthrus_, _Neolenus_,
_Acidaspis_, _Ptyclioparia_, and Kootenia, the appendages extend
beyond the margins of the dorsal test. In _Cryptolithus_ and
_Isotelus_ none (other than antennules) does so. In _Isotelus_ and
_Acidaspis_ only the endopodites have been seen. In _Triarthrus_,
_Calymene_, _Ceraurus_, and _Neolenus_ there are four pairs of
appendages behind the antennules. The other genera probably had the
same number, but the full structure of the under part of their cephala
is not known. In _Triarthrus_ the endopodites of the cephalon are
slender, the individual segments parallel-sided, the inner ones
flattened, the outer ones cylindrical in section. They project
slightly beyond the edge of the cephalon when fully extended, and each
terminates in three small spines. In _Cryptolithus_ the endopodites of
the cephalon are longer than those of the thorax, but with the
possible exception of the first pair, are bent backward at the
carpopodite, and do not ordinarily project beyond the brim of the
test. In _Neolenus_ the endopodites of the cephalon are rather thick
and wide, but are long, project forward, and extend beyond the brim.
The individual segments are flattened, probably compressed oval in
section. The terminal segment of each is furnished with three strong
spines at its distal end. In _Calymene_ and _Ceraurus_ the endopodites
appear to consist of slender segments which are oval or circular in
section. In _Calymene_ Walcott believed the three distal segments of
the last endopodites of the head to be greatly enlarged, giving these
appendages a paddle-like form similar to some of the appendages of
eurypterids. The evidence for this does not seem to me to be good. The
cephalic endopodites of _Isotelus_ are entirely similar to those of
the thorax, and are rather short, consisting of a series of short
cylindrical segments which do not taper greatly toward the distal end.
The endopodites of the cephalon of _Acidaspis_, _Kootenia_, and
_Ptychoparia_ are still unknown.
The exopodites of the cephalon seem in all known cases (_Triarthrus_,
_Cryptolithus_, _Neolenus_, and Ceraurus) to be like those of the
thorax. They point more directly forward in most cases, project beyond
the margin of the head normally only in Triarthrus, and usually occupy
the region under the cheeks (fixed and free).
The endobases of the coxopodites of the appendages of the cephalon
probably in all cases function as mouth-parts (gnathites), and are
especially modified for this purpose in Triarthrus, being flattened,
shoe-shaped in outline, and so arranged that they work over one
another in a shearing fashion. While the more anterior of the
coxopodites are attached in front of the posterior tip of the
hypostoma, the gnathites of Triarthrus bend backward so that all are
behind the hypostoma. In _Calymene_ and _Ceraurus_, two or three pairs
of the gnathites are back of the hypostoma, and one or more pairs may
be beside or under the hypostoma. In these genera the mouth is
probably in front of the tip of the upper lip. In _Isotelus_, the
mouth seems to have been situated in the notch between the two
branches of the hypostoma, and the gnathites of two or three pairs of
the appendages probably worked under its forks. Since the length of
the hypostoma differs in the various species of _Isotelus_, there
would be a variable number of gnathites projecting under its forks,
according to the species. In this genus the gnathites are of the same
long form, cylindrical in cross-section, as the endobases of the
thoracic segments, but each is bowed back considerably from the point
of attachment.
The gnathites of _Neolenus_ are like the endobases of the thorax, but
broader. The great length of the hypostoma makes it probable that the
mouth was far back and that some of the gnathites were in front of it.
The gnathites of _Cryptolithus_ are unknown. Professor Beecher in his
drawing shows some fragments with toothed ends near the hypostoma, and
it may be that they are inner ends of gnathites, but I see nothing
to substantiate such an interpretation. If, as some suppose,
_Cryptolithus_ was a mud feeder, the gnathites were probably poorly
developed. Of the gnathites of _Kootenia_, _Ptychoparia_, and
_Acidaspis_ also nothing is known.
_Thorax._
In each genus there is a pair of appendages for each segment of
the thorax. When the axial lobe is narrow, the endobases of the
coxopodites are small and short (_Cryptolithus_, _Ceraurus_,
_Calymene_). When the axial lobe is wide, the endobases are long and
stout (_Isotelus_, _Triarthrus_). The exopodites always lie above
and in front of the corresponding endopodites. In Triarthrus the two
branches are of practically equal length. In _Cryptolithus_ the
exopodites are much the longer. In _Neolenus_, _Calymene_, _Ceraurus_,
_Kootenia_, and _Ptychoparia_, the exopodites are shorter than the
endopodites.
The exopodites in Triarthrus consist of a proximal shaft, succeeded by
numerous short segments, and ending distally in a long, grooved,
somewhat spatula-shaped segment. Along the anterior margin of the
shaft there are many small spines. Along the posterior margin there
are numerous flattened setæ, which all lie in one plane and which seem
to be more or less united to one another like the barbs of a feather.
The setæ are short, not much longer than the width of one of the
thoracic segments, and point backward and outward. In _Cryptolithus_
the shaft does not seem to be made up of small segments, and is
narrow, with a decided backward curve. The setæ are considerably
longer and much more flattened than in Triarthrus. In _Calymene_ the
state of preservation does not allow a very full knowledge of the
exopodites, but they appear to have a slender, unjointed shaft and
short and delicate setæ. The coiled branches of the exopodites as
described by Walcott seem to me to be only ordinary Triarthrus-like
organs, and this, as I understand from Schuchert, was also the view of
Beecher. In _Ceraurus_ the exopodite seems to have been somewhat
paddle-shaped, expanded at the distal end, and to have had rather
thick, blade-like setæ.
The exopodite of _Neolenus_ is decidedly leaf-like, and reminds one
somewhat of the exites of some of the phyllopods. The shaft is a
broad unsegmented blade. The setæ are slender, delicate, flattened,
and a little longer than the width of the shaft. The exopodites
of this genus point forward all along the body. In _Kootenia_ the
exopodites are like those of _Neolenus_, but with a narrower shaft.
The exopodites of _Ptychoparia_ appear to be very much like those of
Triarthrus, but the shaft is probably not segmented.
The endopodites of the thorax of _Triarthrus_, _Cryptolithus_, and
_Acidaspis_ show progressive modification from front to back in the
broadening of the individual segments and the assumption by them of
a triangular form. Not only do the individual segments become more
triangular from front to back, but more of the segments of each
endopodite become triangular. This modification has so far been seen
in these three genera only. The individual segments, except the distal
ones, seem to be flattened in all these genera. The distal end of the
terminal segment of each endopodite of _Triarthrus_ bears three small
movable spines, and each of the segments usually bears three or
more spines, located in sockets along the dorsal surface and at
the anterior distal angle of each segment. The endopodite of
_Cryptolithus_ is bent backward at the carpopodite and this segment
is always thickened. At the distal end of the dactylopodite there
is a tuft of spines, the triangular segments have tufts of spines on
their posterior corners, and there are groups of spines also in the
neighborhood of the articulations.
The endopodites of _Ceraurus_, _Calymene_, and _Isotelus_ are all
relatively slender, the segments are parallel-sided, and there seems
to be no particular modification from front to back of the thorax. The
endopodites of _Isotelus_ are short, the entire six segments of one
being but little longer than the coxopodite of the same appendage. The
segments of the endopodites of _Neolenus_ are mostly short and wide,
and at the distal end of the terminal segment there are three stout
spines. In _Kootenia_ the endopodites are long and very slender. The
endopodites of Ptychoparia are too poorly preserved to show details,
and those of the thorax of _Acidaspis_ likewise reveal little
structure, but they seem to have the triangular modification, and to
turn back somewhat sharply at about the position of the carpopodite.
_Pygidium._
Beecher showed that in _Triarthrus_ there was a pair of appendages on
the pygidium for every segment of which it is composed except the last
or anal segment (protopygidium). Walcott has since shown that in
_Neolenus_ this segment bears a pair of cerci, and Beecher's drawings
show that in his later studies he recognized a spinous plate, the
possible bearer of cerci, on the anal segment of _Triarthrus_. The
appendages of the anal segment have not yet been seen on other species
of trilobites.
The appendages of the pygidium do not show any special modifications,
but seem in all cases to be similar to those of the posterior part of
the thorax. In _Cryptolithus_ all the pygidial appendages are short
and remain beneath the cover of the dorsal test, while in _Triarthrus_
and _Neolenus_ they extend behind it.
In the latter genus the endopodites of the pygidial appendages appear
to be practically identical in form with those of the thorax, the
individual segments being perhaps a little more nearly square in
outline. Like those of the thorax, the segments of the pygidial
endopodites bear numerous short spines. The caudal cerci are richly
segmented, slightly flexible, spinous tactile organs. They are
symmetrically placed, nearly straight when in their natural position,
and make an angle of about 75 with one another. They appear to be
attached to a narrow rim-like plate which seems to fit in just ahead
of the doublure of the pygidium, or perhaps over it.
In _Ceraurus_, _Calymene_, and _Isotelus_, the endopodites of the
pygidium are similar to those of the thorax, but seemingly more
slender, with less well developed coxopodites, and with, in the
last-named genus, slender cylindrical segments. Exopodites are not
known on the pygidia of any of these genera, but since they are
present and like those of the thorax in _Triarthrus_, _Cryptolithus_,
_Neolenus_, and _Ptychoparia_, there is little reason to think that
they were absent in _Ceraurus_ or _Calymene_, though there is some
question about _Isotelus_.
The limbs are largest and longest on the anterior part of the thorax
of a trilobite, and diminish regularly in length and strength to the
posterior end of the pygidium. This regular gradation shows, as
Beecher was the first to point out, that
Showing first 200,000 characters. Full text: gutenberg.org/ebooks/41695