CHELONIA
No order of reptiles of the past or present is more sharply and
unequivocally distinguished from all others than the Chelonia or
Testudinata. No order has had a more uniformly continuous and
uneventful history. None now in existence has had a longer known
history, and of none is the origin more obscure. The first known
members of the order, in Triassic times, were turtles in all respects,
as well or nearly as well adapted for their peculiar mode of life as
are those now living, and were they now living they would attract no
especial attention from the ordinary observer and but little from the
naturalist. From time to time some have gone after better things,
and have come to grief, but the main line has remained with fewer
improvements, fewer evolutional changes, than any other group of higher
vertebrates. The turtles seem very early to have adapted themselves
so well to their peculiar mode of life, to have intrenched themselves
so thoroughly in their own province, that no other creatures have
been able to overcome them, or to drive them from it. The remains of
no other air-breathing vertebrates are so omnipresent in the rocks
as those of the turtles; they may be expected wherever fossils of
air-breathing animals are found, though unfortunately often only in
scattered and broken fragments. The loose union of their skeletal bones
and their general habits of life in shallow waters left their bodies as
food for scavengers, or for dismemberment by the tides and currents.
Relationships with other reptiles they really have none. Some have
thought that the plesiosaurs were their first cousins, others the
Placodontia, an indeterminate group of extinct reptiles usually placed
with the Anomodontia. But their relationship with neither of these is
closer than with the crocodiles, dinosaurs, or pterodactyls. They are
the only reptiles that we know, besides the cotylosaurs, which have no
holes in the temporal roof of the skull, and as the cotylosaurs were
the most primitive and the oldest of reptiles, this fact incontestably
proves that the turtles had a very ancient origin, though we know
them no farther back than the later Triassic. They are the only order
of reptiles of which not a single member is known to have teeth, or
even vestiges of them. Until recently only a single specimen has been
known from the Trias, and of that only the casts of the shell; but
the shell was as fully developed and as complete as that of a modern
alligator snapper, which it resembled much in form and in size. And
doubtless the habits of this ancient _Proganochelys_ were similar
to those of the alligator snapper. The early cotylosaurian reptiles
were all littoral-or marsh-loving animals, and more or less aquatic,
and doubtless the early turtles continued in the same environments
and with the same habits after acquiring a shell for protection and
losing their teeth, which for some inexplicable reason they seemed no
longer to need. Until near the close of the Jurassic period probably
all turtles were amphibious animals of the marshes, spending much,
perhaps the larger part, of the time in the water, good swimmers, and
yet good crawlers. With the beginning of the Cretaceous, however, some
of them became ambitious for new and untried modes of life. Various
ones went down into the sea and became marine animals, reaching the
zenith of their prosperity and the maximum of size before the close
of the period, but continuing in diminished size and numbers to the
present time, if we may consider the leather-back turtle as really
their descendant. Others in the Cretaceous took to the rivers and
ponds, and became almost as thoroughly aquatic in their thin shape
and soft covering; and their lineal descendants still continue in the
rivers of the Northern Hemisphere. Still others, in the Age of Mammals,
took to the upland, and competed with the mammals in the open places
and prairies, reaching their maximum in Miocene-Pliocene times, when
for some unknown reason the giants among them were driven from the
mainlands to continue a precarious existence to the present time in
some of the larger islands.
[Illustration: FIG. 111]
[Illustration: FIG. 112
FIGS. 111 and 112.—_Graptemys._ (From Hay.)
FIG. 111.—Carapace: _cp1_, _cp2_, etc., costal plates; _cs1_, _cs2_,
etc., costal scutes, horny; _n1_, _n2_, etc., neural bones; _nup_,
nuchal bone; _nus_, nuchal scute; _py_, pygal bone; _spy_, suprapygal;
_spy 2_, second suprapygal, or postneural; _vs1_, _vs2_, etc.,
vertebral scutes; 1, 2, 3-12 on right side, marginal scutes; 1, 2, 3-12
on left side, peripheral bones.
FIG. 112.—Plastron: _ab_, abdominal scutes; _an_, anal scutes;
_ent_, entoplastron (interclavicle); _epi_, epiplastron (clavicle);
_fem_, femoral scute; _g_, gular scute; _hum_, humeral scute; _hyo_,
hypoplastron bone; _hypo_, hypoplastron; _in_, inguinal scute; _py_,
pygal bone; _xiph_, xiphiplastron.]
Were there no turtles living we should look upon the fossil forms
as among the strangest of all vertebrate animals—animals which had
developed the strange habit of concealing themselves inside of
their ribs, for that is literally what the turtles do. The box or
shell of an ordinary turtle is composed of the backbones and ribs,
to which are soldered a shell of bony skin plates above, with the
clavicles, interclavicle, and ventral ribs below. Except in the strange
leather-back turtle described farther on, these plates form definite
series. Ten of them cover the spines of the dorsal vertebrae, in the
mid-line, one over each, of which the turtles have the smallest number
of any known reptiles. There are eight on each side over the ribs,
united by suture with each other and with the middle series; and, in
addition, there are twenty-six bones surrounding them and attached to
them. All these bones compose what is called the carapace, which forms
a complete roof in the more terrestrial types, more or less imperfect,
with vacuities between the bones in the marine forms. On the under
side, in addition to the clavicles and the interclavicle, there are
three pairs of enlarged ventral ribs that go to form the plastron,
solid and complete in land turtles, with openings in the water forms.
And in the land forms the plastron is more or less firmly united with
the carapace.
[Illustration: FIG. 113.—_Toxochelys_; coracoid and scapula.]
In the skeleton contained within the box thus formed is the very
peculiar pectoral girdle, composed of scapula and coracoid, the scapula
so peculiar that the controversy as to its homologies is not yet quite
settled. Most authors, until recently, have believed that its peculiar
shape (Fig. 113) is due to the co-ossification of the procoracoid
with the scapula instead of as usual its loss or union with the true
coracoid, so called. We are now pretty sure that this is not true,
since in reality there is no such bone as the procoracoid, the bone
so called being the real or true coracoid; and because, in the second
place, the long anterior projection called the procoracoid is really
only an outgrowth of the scapula itself and not a fused, separate bone.
Hence the bone is properly called the scapula-proscapula, and not the
scapula procoracoid, as it usually has been. The coracoids are elongate
and flattened and without the usual supracoracoid foramen, so generally
present in reptiles. The only other reptiles having a similar structure
of the scapula are the plesiosaurs, and it has been because of this
apparent resemblance that some good paleontologists have thought the
turtles and plesiosaurs were allied. The sacrum is composed of two
vertebrae only, and the pelvis of the usual three bones, the ilium, the
ischium, and the pubis, all covered over by the shell.
[Illustration: FIG. 114.—Pelvis of _Chelone_, from below: _pu_, pubis:
_is_, ischium; _il_, ilium (in acetabulum).]
In every known turtle the neck is composed invariably of eight
vertebrae, but they are peculiar in many respects. In the earliest
known turtles the neck vertebrae were, as would be supposed, biconcave,
but they soon became very variable in all; in each neck some are
biconcave, some biconvex, some opisthocoelous, and some procoelous. And
Dr. Hay tells us that the neck has increased in length in the later
forms.
The skull also is very peculiar in that it has some very primitive
characters and others very aberrant. The temporal roof, as has been
said, has no holes through it, though it is often reduced by the
emargination of the borders, whether from below or behind, until in
some the whole temporal region is exposed, and not at all covered over.
There is no parietal foramen, so constantly present in all the early
reptiles and in the lizards and the tuatera of modern times. There
are no teeth or vestiges of teeth, but the jaws have usually a horny
cutting edge, which seems to be quite as serviceable; in the river
turtle the lips are fleshy. There is no transverse or transpalatine
bone. There is a single vomer only, not paired as in other reptiles,
whence comes the doubtful theory that the vomers of other reptiles
are not the real vomers originally so named in mammals, and hence
often called prevomers. The vomer of the turtles under this theory is
believed to be the real homologue of the mammalian bone. The palate is
always slightly, sometimes nearly wholly, underfloored, as in mammals,
carrying the internal nostrils far back in the mouth. In the occipital
region of the skull there is a separate bone on each side called the
paroccipital or opisthotic, which has been indistinguishably fused with
the exoccipital in all other reptiles except the ichthyosaurs since
Triassic times.
[Illustration: FIG. 115]
[Illustration: FIG. 116
FIGS. 115 and 116.—_Trachemys._ (From Hay.)
FIG. 115.—Skull from above: _fr_, frontal; _ju_, jugal; _pa_, parietal;
_paoc_, paroccipital; _pfr_, prefrontal; _pof_, postfrontal; _pro_,
proötic; _qu_, quadrate; _sq_, squamosal; _soc_, supraoccipital.
FIG. 116.—Skull from below: _alv_, alveolar surface of maxilla; _boc_,
basioccipital; _bap_, basisphenoid; _exoc_, exoccipital; _mx_, maxilla;
_pal_, palatine; _paoc_, paroccipital; _pmx_, premaxilla; _pro_,
proötic; _pt_, pterygoid; _qu_, quadrate; _qj_, quadratojugal; _sq_,
squamosal; _vom_, vomer.]
In the feet the numbers of phalanges—that is, the bones of the free
digits—are like those of mammals, that is, two in the first and three
in each of the other four digits. The land tortoises have lost some of
these, while the river turtles have either gained one or two in the
fourth finger and fourth toe, or else have enjoyed an uninterrupted
descent from the primitive reptiles which normally possessed that
number. All other reptiles, save those phylogenetically allied to the
primitive mammals, that is, the Theriodontia and their allies, have
normally the phalangeal formula 2, 3, 4, 5, 4. It was partly because
of this similarity of the numbers of toe bones that the turtles have
been classed in the great group of reptiles that includes the ancestors
of the mammals; that is, under this theory, the turtles would enjoy a
nearer relationship to the mammals and to man himself than any other
living reptiles! But this classification has been shown to be quite
artificial.
[Illustration]
[Illustration: FIG. 117.—Limbs of _Colpochelys_, a recent sea-turtle:
_H_, humerus; _R_, radius; _U_, ulna; _r_, radiale; _i_, intermedium;
_u_, ulnare; _p_, pisiform; _c_, centrale; _T_, tibia; _F_, fibula;
_a_, astragalus; _m_, fifth metatarsal. (From Wieland.)]
From what has been said, it will be surmised that the Chelonia
represent in themselves one of the primary subdivisions of the class
Reptilia, and that, unlike most others, the order has enjoyed a most
remarkable longevity. And doubtless they are one of the primary
branches of the reptilian stock, which has remained distinct since
Permian times at least, if not since Carboniferous, isolated and
remarkably homogeneous, giving off no branches which departed far from
the main stock, and on the whole leading a singularly placid existence
for ten or more million years.
In most textbooks the order Chelonia is divided into three suborders,
the Pleurodira, the Cryptodira, and the Trionychoidea. In recent
years, however, the earlier members of the older group of Pleurodira
have been separated into a fourth suborder, the Amphichelydia, a group
characterized by some not very important differences in the plastron
and skull, and including those forms in which the cervical vertebrae
are amphicoelous. This group continued to Eocene times before it
became extinct, and consisted of archaic forms which persisted after
all the other suborders had come into existence. The Cryptodira,
especially characterized by the manner in which they withdraw the head
and neck within the shell by an =S=-like vertical flexure, are known
from the Lower Jurassic and are still the dominant group of today,
with more than one hundred and forty living species. The Pleurodira
in the narrower sense are first known from their remains in the Upper
Cretaceous of North America and are still represented by about forty
species, living in the Southern Hemisphere. They are distinguished from
the other groups by the manner in which they withdraw the neck and head
into the shell, by a horizontal, sidewise flexure. The third suborder,
the Trionychoidea, also began in Cretaceous times, so far as we know,
and are represented by about seventy living species, chiefly in the
Northern Hemisphere. They are especially characterized by the absence
of bony marginal plates and the soft epidermis.
With the exception of the land tortoises, all turtles from the
beginning of their career as an order to the present time have been
more or less at home in the water. In some, like the marine forms, the
adaptation to aquatic life has produced marked changes in structure: in
the loss of the horny dermal shields and in the loss of bone tissue;
in the flattening of the shell, and in the development of the front
legs into swimming flippers, with a loss of the claws. In the absence
of a guiding tail, which is always small in the marine turtles,
propulsion must of course be wholly by the aid of the limbs. As oar
propellers the marine turtles show some of the peculiar characters
of the plesiosaurs. With a like short and broad body, a more or less
elongated and flexible neck, there could be no sinuosity of the body in
swimming. As an oar-like organ the humerus became flattened, and its
muscular attachments, as in the plesiosaurs, descended far down the
shaft, giving greater mechanical advantage. Unlike all other aquatic
vertebrates, the turtles never developed real hyperphalangy. Only in
the river turtles is there a possibility of an increase in the bones of
the fourth digit.
To discuss in general the structure and habits of the living chelonians
would extend this chapter to an undue length, and would add nothing
to the many excellent works on natural history accessible to the
student. We have therefore contented ourselves with a brief outline of
the geological history of the order, with especial reference to their
aquatic habits.
SIDE-NECKED TURTLES. PLEURODIRA
The suborder of Chelonia, generally known as the snake-necked or
side-necked turtles or tortoises, comprises about forty living
species, confined to South America, Africa save the northernmost part,
Madagascar, New Guinea, and Australia. In Australia they are the only
members of the order known—another instance of the peculiar isolation
of the fauna of that region. In the past they lived in North America
during Upper Cretaceous times, the earliest known forms of the group
in its restricted sense, of which seven species are described by Hay.
In Eocene times they are also known from Europe and Asia, from both of
which regions they have long since disappeared.
The Pleurodira, as the term indicates, are easily distinguished from
all other turtles by the way in which they withdraw the head within the
shell. Instead of withdrawing it by an =S=-shaped flexure of the neck
between the shoulder-blades, as do other turtles, these bend the neck
laterally in a horizontal plane, bringing the head within the margins
of the shell in front of one or the other foreleg; and the margins of
the shell are produced here in an eave-like fashion for the greater
protection of the head. In the structure of the shell, which is always
fully developed into a box, these turtles do not differ very much from
the Cryptodira, though there may be some extra bones in the plastron,
as also in the skull. The nasal bones are always, the lacrimals
sometimes, well developed; the latter never, the former rarely, found
in other groups. The lower jaws articulate a little differently,
and the external ear is always fully surrounded by bone. Very
characteristic is the bony union of the pelvis with the plastron below,
which never occurs in other turtles, unless it be the Amphichelydia.
The side-necked turtles are all of fresh-water habit, similar to that
of the fresh-water tortoises spoken of farther on. The neck is often
very long and snake-like, which accounts for one of the names given
to these turtles; because it is withdrawn into the shell sidewise, it
has more distinctively ball-and-socket joints between the vertebrae,
with distinct transverse processes for the attachment of the necessary
side-moving muscles. The feet in all are more or less webbed and armed
with strong claws.
The largest and perhaps the best known of all living side-necked
turtles is the giant Amazon turtle of South America, which sometimes
has a shell nearly three feet in length. Its feet are broadly webbed,
and the shell is rather flat in the adult; it is an excellent swimmer
in the waters of the Orinoco and Amazon. Six or seven species of
the genus to which it belongs are known, all of them South American
except one that lives in Madagascar and one fossil found in the Eocene
of India. This remarkable distribution is but one more of the many
instances known in zoölogy and paleontology that seem to prove an early
land connection between India and South America. Had the migration
between the two continents occurred by way of Asia and Bering straits,
as did that of hosts of mammals, one would certainly expect to find
some evidence of it in the North American Tertiary rocks, which, so
far, is lacking.
CRYPTODIRA
The chief families of the Cryptodira turtles are the Chelydridae,
or snappers; the Emydidae, or marsh tortoises; the Testudinidae, or
land tortoises; the Chelonidae, or sea-turtles; the Protostegidae, or
ancient sea-turtles; and the Dermochelydidae, or leather-backs. Other
doubtful or smaller groups, both living and extinct, may be omitted, or
incidentally mentioned.
SNAPPING TURTLES
The family of snapping turtles, the Chelydridae, are of interest
because of their peculiar geographical distribution at the present
time. Only four species are known, three of them from North America,
the fourth from New Guinea. The family is one of the most primitive of
living turtles, though no members of it are known with certainty from
earlier rocks than the Oligocene. In all probability, also, they have
retained, more than have any other group of turtles, unless it be some
of the fresh-water tortoises, the primitive habits of the earlier or
earliest turtles, though of course there have been modifications, both
in structure and in habits. The three species of the United States
include two of the snapping turtles proper and the alligator turtles of
the southern states, which sometimes reach a length of three feet. All
the species are largely aquatic in habit, powerful and active swimmers,
with webbed feet and strong claws, and both on the land and in the
water they are bold and fierce. They have a relatively large head and
very strong jaws. Agassiz saw one bite off a piece of a plank an inch
in thickness, and they can usually be raised from the ground by any
object which they seize. The carapace and plastron are much reduced,
and are rather loosely united. The shell is not large enough for the
complete withdrawal of the head and legs within it, and the tail is
unusually large and strong. The common snapping turtle, _Chelydra
serpentina_, is found from Canada to Ecuador, and its remains have been
found with those of the mammoth and mastodon in Pleistocene deposits;
and related species of the same genus have been reported from the
Miocene of England.
FRESH-WATER OR MARSH TORTOISES
The family of turtles or tortoises (Emydidae) represented at the
present time by the common terrapin, painted tortoise, and box tortoise
of the United States, and commonly called fresh-water turtles or
tortoises, comprises the largest group of living chelonians—nearly a
third of all existing members of the order. They are widely distributed
over all parts of the earth except Australia, and are of very varied
habits. Some are almost exclusively aquatic; others, like the painted
tortoise, are partially so, while others, especially the common box
tortoise, are almost as exclusively terrestrial as the true land
tortoises, dying even, if forced to live long in water.
The shell in the more aquatic forms is depressed or flattened, but
in the terrestrial kinds may be as highly arched as in the true land
tortoises. The feet are adapted primarily for walking, but nearly
always have the toes webbed, and the digits are longer than are those
of the land tortoises. Only the two or three middle toes have claws.
Some species have developed hinges in the plastron, whereby they may be
completely closed up after the head and legs are withdrawn within the
shell. Most of the species are carnivorous in habit, but a few, like
the box tortoise, are strictly vegetarian.
Geologically the fresh-water tortoises have a not very ancient history,
going back no farther than do the land tortoises, that is, to the
beginning of the Cenozoic or Age of Mammals. Fully fifty species
are known from the Tertiary rocks of North America, or more than
three-fourths as many as are now living upon the earth.
The family at most can be said to be only amphibious in habit, and has
no striking aquatic adaptations, since the shell is well developed and
is covered with horny shields. The flattened shell of the more aquatic
forms is characteristic, as is also the greater degree of webbing
between the toes.
LAND TORTOISES
Perhaps the last of the more noteworthy specializations of
the Chelonia, and indeed among the last of the more important
specializations of the Reptilia, are the upland tortoises, of which
the common “gopher” of the southern states is almost the only remnant
in North America. They formed a part of the great hegira of forest and
marsh animals to the open prairies, away from the lowlands and water
which the turtles had inhabited almost exclusively for millions of
years.
[Illustration: FIG. 118.—_Testudo sumeirei_, a giant upland tortoise.
(From Hay, after Rothschild.)]
They began their career, Dr. Hay thinks, at about the beginning of
the Cenozoic, that is, with the great development of the mammals, and
reached the maximum of their development in the Miocene; and they
have been on the decline ever since. In the Northern Hemisphere, at
least, the slowly cooling climate throughout the Eocene, and a decided
decrease in moisture, brought about the prairies and prairie plants
before its close. Just as the horses, rhinoceroses, camels, and other
herbivorous mammals took to these open places for the better and more
abundant food found therein, so also the lowland tortoises found better
food and fewer enemies there, for they are all strictly herbivorous in
habit. The mammals became more conspicuous to their enemies when they
went into the open, and it was only by the development of speed, more
sober coloration, and perhaps greater cunning that they found safety
from them. The tortoises were handicapped by low intelligence, and they
could not develop speed, for they were not constructed to that end. But
they did find protection in their bony shell, which became thicker,
higher, and more convex, and with smaller openings. To quote Dr. Hay:
“We may suppose that it would be much more difficult for a carnivorous
animal to effect an entrance into such a shell than into one depressed,
and whose borders may be spanned by the jaws of their enemies.” Perhaps
also the highly arched form of the shell gave greater capacity for the
lungs, and the tortoises in general, it is said, do have better lung
capacity than the more aquatic or lowland types of turtles. Possibly,
also, the heavier shell lessened the evaporation of the body fluids,
and made the tortoises less dependent upon the vicinity of water.
Certain it is that the common box tortoise, of like form and habits,
occurs not rarely on the arid plains, far from water.
The neck and legs became fully retractile within the shell; the digits
were shortened up, without a vestige of webbing membrane between them;
the phalanges were reduced in number to two in each toe, and nearly all
the toes have well formed claws. The feet are placed squarely upon the
ground, and the body is elevated in walking. They can swim, when by
accident they are thrown into the water, only as any terrestrial mammal
can.
About forty species of land tortoises are known throughout the world
at the present time, though North America, the probable original home
of the tribe, has but three, all small. The larger species are all now
denizens of islands, especially the Galapagos Islands, where the giant
tortoises have long been famous. And many of our living forms have
changed but little since Eocene times. In the Oligocene and Miocene
they inhabited western North America in enormous numbers. In the Bad
Lands of South Dakota one can often see the remains of a dozen or more
of these giant tortoises at one time, specimens varying from one to
three feet in length of shell. In river deposits, those of the late
Miocene or early Pliocene, the writer has seen areas of an acre or more
literally strewn with their remains, as though droves of them had been
overwhelmed and perished together. About fifty species of these land
tortoises are known from the American Tertiary, thirty-two of them
belonging to the modern genus _Testudo_, which comprises the giant
tortoises of the Galapagos. The largest known species of the group is
one of _Testudo_ from the Pliocene of India, which had a shell six feet
in length. Why the larger species became extinct in Pliocene times on
the mainland to survive only in the islands is not known; possibly
their carnivorous enemies became too cunning and too numerous.
SEA-TURTLES. CHELONIDAE
The sea-turtles, or Chelonidae comprise five or six living species,
inhabitants for the most part of tropical and subtropical oceans, of
which the green or edible turtle (_Chelone_), the hawksbill turtle
(_Caretta_), and the loggerhead (_Eretmochelys_ [Fig. 119]) are the
best known. They are all thoroughly aquatic in habit, and of large
size, from three to five feet in length. The carapace is heart-shaped,
and reduced, that is, with large openings between the ribs; the
plastron also is reduced and loosely united to the carapace. The neck
is short and the head is not retractile within the shell. The temporal
region of the skull is roofed over. The four legs form large and
powerful flippers, and the hind legs are relatively small. The body is
flattened and the tail is small. The aquatic characters of the limbs
are seen especially in the broad and strong humerus, with the radial
crest for the attachment of powerful muscles situated far down on
the shaft; in the relative shortness of the radius and ulna, and the
large size of the latter bone; in the flattened carpal bones; and in
the great elongation of the digits and the absence of all but one or
two of the claws. Unlike the leather-back turtle and the Cretaceous
sea-turtles, the carapace and plastron are completely covered with
horny shields, from which indeed the tortoise shell of commerce is
derived. Except the green turtle, all members of the family are
carnivorous.
[Illustration: FIG. 119.—_Eretmochelys_, loggerhead turtle. (By
permission of the New York Zoölogical Society.)]
Extinct members of the family are known from scanty remains in Cenozoic
and late Cretaceous rocks. From the earlier Cretaceous deposits of the
plains more primitive allied forms occur, often classed in distinct
families of which _Toxochelys_ (Fig. 120) and _Desmatochelys_ are the
more noteworthy. The latter genus, especially, might well have been an
ancestor of all the modern forms. About three feet in length, it had
all the essential characteristics of the sea-turtles, in its thin form,
roofed-over skull, reduced carapace, loose plastron, and flipper-like
limbs. The single known specimen, preserved in the museum of the
University of Kansas, came from the lower rocks of the Upper Cretaceous
of Nebraska. Yet earlier, at the close of the Jurassic, there were
shore turtles of considerable size that had begun to develop a fondness
for the open seas; to acquire a depressed form and lightened shell, the
limbs still retaining, however, more of the terrestrial or crawling
form. They are grouped as a separate family, the Thalassemydae,
and include the first of the Chelonia to depart from the marsh and
fresh-water habits which for long ages, perhaps, had limited the
activities and evolution of the turtles.
[Illustration: FIG. 120.—Carapace of _Toxochelys bauri_, an Upper
Cretaceous sea-turtle: _ep_, epineural. (After Wieland.)]
ANCIENT SEA-TURTLES. PROTOSTEGIDAE
[Illustration: FIG. 121.—_Toxochelys latiremis_; front leg: _hum_,
humerus; _rad_, radius; _ul_, ulna; _int_, intermedium; _uln_, ulnare;
_p_, pisiform; _cen_, centrale. (From Wieland.)]
[Illustration: FIG. 122.—_Desmatochelys lowii_; skull from above and
below.]
Forty-four years ago the late Professor E. D. Cope, one of the
greatest naturalists America has ever produced, in almost the earliest
exploration of the great Cretaceous fossil deposits of western
Kansas, discovered and collected a remarkable specimen of one of
the most extraordinary turtles that is known even yet. By an error
somewhat natural for those times, when the theory of evolution was
just beginning to attain acceptance by naturalists, he thought that
the specimen, notwithstanding its monstrous size, represented a very
primitive kind of turtle, and gave to it the name _Protostega gigas_,
meaning gigantic first roof! The late Professor George Baur, to whom
paleontology owes so much, showed that, far from being a primitive
turtle, _Protostega_ was really one of the most specialized types of
the order. Professor Cope’s account of the discovery of the specimen is
of so much interest that it may be quoted here:
“In the very young tortoise or turtle the ribs are separate, as in
other animals. As they grow older they begin to expand at the upper
side of the upper end, and with increased age the expansion extends
throughout the length. The ribs first come in contact where the
process commences, and in the land tortoise they are united at the
end. In the sea-turtles the union ceases a little above the ends. The
fragments of the _Protostega_ were seen by one of the men projecting
from a ledge of a low bluff. After several square feet of rock had
been removed, we cleared up the floor and found ourselves well repaid.
Many long, slender pieces of two inches in width lay upon the ledge.
They were evidently ribs, with the usual heads, but behind each head
was a plate-like the flattened bowl of a huge spoon, placed crosswise.
Beneath these stretched two broad plates, two feet in width, and no
thicker than binder’s board. The edges were fingered and the surface
was hard and smooth. All this was quite new, among fully grown animals.
Some bones of a large paddle were recognized, and a leg bone. The
shoulder-blade of a huge tortoise came next, and further examination
showed that we had stumbled on the burial place of the largest species
of sea-turtle yet known. But the ribs were those of an ordinary turtle
just hatched, and the great plates represented the bony deposit in the
skin, which, commencing independently in modern turtles, unite with
each other at an early day. But it was incredible that the largest
of known turtles should be but just hatched, and for this and other
reasons it has been concluded that this ‘ancient mariner’ is one of
those forms, not uncommon in old days, whose incompleteness in some
respects points to the truth of the belief that animals have assumed
their modern perfection by a process of growth from more simple
beginnings.”
Later studies by Doctors G. Baur, E. C. Case, O. P. Hay, and especially
G. R. Wieland, of the abundant and excellent material, preserved in the
museums of Yale and Kansas universities and the Carnegie Institution,
and especially the discovery by Wieland in 1895 of an allied and yet
larger form which he called _Archelon_, have determined practically
every detail of the structure of this remarkable group of sea-turtles.
A surprisingly complete specimen of _Archelon_ is mounted in the museum
of Yale University.
[Illustration: FIG. 123.—_Archelon ischyros_; skeleton from above: _n_,
nuchal, _r_, _r_, _r_, ribs; _m_, _m_, peripheral bones; _h_, humerus;
_r_, radius; _u_, ulna; _t_, tibia; _fi_, fibula. (From Wieland.)]
About a half-dozen species and two genera of the family have so far
been described, all coming from the Upper Cretaceous deposit of Kansas
and South Dakota, the genus _Archelon_ from later rocks than those
which have yielded _Protostega_.
The general form and structure of _Archelon_ will best be understood
from the accompanying figures after Wieland (Figs. 123, 124, 125) and
the restoration of the living animal as interpreted by the writer
(Fig. 126). If the leather-back turtle, described farther on, is really
the descendant of these or allied turtles, as many authors believe, it
of course represents the very highest aquatic specialization of all
Chelonians. If, on the other hand, as some believe, the leather-back
is the end of a long and independent line of descent, then _Archelon_
represents the highest aquatic specialization of all other turtles.
In size, at least, _Archelon_ attained the maximum of the order,
reaching a length of more than twelve feet, and a weight of more than
three tons. Except that the shell was not heart-shaped or elongated as
in all modern sea-turtles, but nearly circular in outline, it had all
the aquatic adaptations of the sea-turtle in a yet higher degree.
[Illustration: FIG. 124.—_Archelon_ from below, without plastron: _h_,
humerus; _r_, radius; _u_, ulna; _sc_, scapula; _c_, coracoid; _p_,
pubis; _i_, ischium. (From Wieland.)]
[Illustration: FIG. 125.—_Archelon_; skeleton from below: _hp_,
hyoplastron; _hpp_, hypoplastron. (From Wieland.)]
The shell was depressed; the dermal plates covering the ribs had almost
entirely disappeared, remnants only of their upper ends remaining; the
skull (Fig. 127) had the temporal region wholly roofed over; the neck
was short and not retractile. The front legs were strong flippers,
the humerus was long and stout, with the crest for the attachment
of muscles far down on the shaft; the digits were greatly elongated
and clawless, etc. The plastron only was less reduced than in the
case of the modern sea-turtles. No traces of horny shields have been
discovered. As to the nature of the covering and the general appearance
of the turtle when alive, Dr. Wieland has kindly given the writer his
views, as follows:
“After direct study or fairly close examination of all the fossil
material of importance thus far collected representing the
Protostegidae, it seems certain that in all the members of the group
an external leathery layer was well developed. In no instance is there
the slightest trace of horny shield sulci, or grooves; though it seems
probable that there was some gradation toward a thin and perhaps even
slightly horny hide. In _Archelon ischyros_ the reduced condition
of the carapace and the presence of the continuous row of large,
median, supraneural elements render it quite certain that there was a
development of leathery hide comparable to that of _Dermochelys_. The
same may be said of _Protostega gigas_. But _Archelon Marshii_ had a
less reduced carapace, and the leathery skin was probably less well
developed; and _Protostega Copei_, in which no trace of supraneurals
remains, must have made some approach to the horn-shield condition.
A more distinct suggestion of transition from the leathery to the
horny shield covering may be seen in the very different contemporary
Cretaceous form, _Toxochelys Bauri_, where ossified epi-or supraneurals
occupy quite exactly the nodal relation of the five vertebral horn
shields of later turtles, like _Lytoloma_, though there are not the
slightest traces of sulci.
[Illustration: FIG. 126.—_Archelon ischyros_, a gigantic sea-turtle
from the Upper Cretaceous of South Dakota.]
[Illustration: FIG. 127.—Skull of _Archelon ischyros_: _pa_, parietal;
_f_, frontal; _pm_, premaxilla; _pf_, prefrontal; _ptf_, postfrontal;
_m_, maxilla; _j_, jugal; _qj_, quadratojugal; _sur_, surangular; _d_,
dentary; _an_, angular. (After Wieland.)]
“From a purely anatomical standpoint I have suggested that _Archelon_
had seven dorsal keels corresponding to those of _Dermochelys_. There
is much excellent reason for regarding dermogene ossification as
essentially double-layered throughout the Reptilia.
“In any restoration of _Archelon ischyros_ only the mid-line should
be accentuated as a series of rather sharp supraneural crests. These
are shown to have been present by the characteristic groove-like
median pits with radiating striae, which are such a prominent feature
of epineurals. It is reasonable to believe that the pits mark the
attachment of horny crests developed in the leathery hide. Such
were doubtless projected, more or less keel-like, to a height of
one or two inches, and thus gave to the mid-line of the carapace,
when seen laterally, a distinctly sinuous outline not unlike that of
_Toxochelys_.”
As regards the habits of these ancient sea-turtles, we may offer
tolerably certain conjectures. In the opinion of the writer, the
less reduced plastron indicates a bottom-feeding habit, a view that
is strengthened by the more rounded form of the shell, like that of
the river turtle. All in all it would seem that _Protostega_ and
_Archelon_ lived habitually on the soft bottoms of the shallower
seas, feeding upon the hordes of large shell-fish, for which their
powerful parrot-like beak was admirably adapted. That the species of
_Protostega_ did not commonly frequent the deeper oceans is indicated
by the general absence of their remains in the deeper water deposits.
The writer, in a long collecting experience, always found their remains
associated with those of the smaller _Toxochelys_, toothed birds,
pterodactyls, and the smaller mosasaurs.
Perhaps no one can speak more authoritatively as to the habits of these
gigantic sea-turtles of the Cretaceous than Dr. Wieland:
“With regard to the general habits and appearance of _Archelon_ much
might doubtless be said if the present-day sea-turtles were more
familiar objects. Dr. Hay thought that _Archelon ischyros_ was a clumsy
or even a sluggish, mainly littoral animal, moving slowly about the
bottom of quiet inlets in quest of shell-fish; I, on the contrary,
much struck by the powerful flippers, and especially by the flattening
of the humerus, with its low radial crest and obviously strong
musculature, have held that unusual swimming power and adaptation to a
strictly marine life were indicated. Perhaps, as usual where experts
differ, it is probable that both views are in part correct, and that
_Archelon_ was only a moderately good swimmer. It may be noted that,
notwithstanding the almost circular body, the femoral notch, that for
the hind leg, lies far back, so that it is not necessary, on the score
of bulk, to assume slowness of motion, or the inability to pursue a
sea-going life. Furthermore, it is now known that the development of
the digits fell little short of that seen in _Colpochelys_ (Fig. 117)
or _Eretmochelys_, truly marine turtles.
“Therefore, while there can be no doubt that _Archelon_ was strictly
carnivorous in habit, and well able to navigate the open seas, it
is not likely that it fed on other than relatively slow-moving prey.
Lydekker looked upon the broad mandibles and broad palate of _Lytoloma_
as specializations for a mussel diet; and very similarly in _Archelon_,
while the decurved beak would easily transform him into a most
formidable enemy, the heavy premaxillaries and vomer, and the flat but
deep lower jaw, suggest an adept crusher of crustaceans. The presence
of vast quantities of _Nautilus dekayi_, which I found accompanying one
of the specimens, was doubtless accidental, but it plainly suggests
that this cephalopod was one of the teeming sources of food in the
_Archelon_ environment.
“The huge bulk of the mature _Archelon_ might account for the shearing
off and loss of the flippers of younger forms caught between the shells
of the ‘elder boatmen of the Cretaceous seas,’ as Cope has called them,
during any sudden rush while herding on the shores. But probably the
young turtles did not much frequent the shores at either egg-laying or
other times. Whence it is much more likely that it was a mosasaur or
some of the gigantic fishes like _Portheus_ which bit off the right
hind flipper in the type-specimen of _Archelon ischyros_, well above
the heel, as I have described it. That this happened rather early in
life is shown by the arrested growth of the right femur and remaining
portions of the tibia and fibula, which are all uniformly 10 per cent
smaller than the corresponding bones of the left flipper.”
While there were many small fishes in the Niobrara seas which the
Protostegas inhabited, the most striking thing in the fauna is the
great abundance of molluscal shells, especially _Ostrea congesta_.
And with them were great hordes of larger pelycypod mollusks, some of
them of enormous size. Some of the largest reach a diameter of nearly
four feet, with shells so thin that one can hardly understand how they
could have supported such large, oyster-like creatures. One can imagine
that such shell-fish might have afforded an almost inexhaustible
source of food for the large turtles; and several times the writer has
found remains of _Protostega_ associated with such shells. From all
of which evidence it seems very probable indeed that Dr. Wieland is
right in imputing to these gigantic turtles a shell-feeding habit, a
habit which required neither speed nor great prowess; and perhaps the
formidable beak was used more in social quarrels than for food-getting.
That these marine turtles departed from the usual reptilian habit of
laying their eggs upon land is improbable. The tortoise shell turtles
of the Bahamas lay three or four hundred eggs in a hollow scooped out
in the sand and then leave the young to their own devices; certainly
many a one is gobbled up by birds of prey or other enemies on their way
to the water. Perhaps the young _Archelon_ lost its hind leg in some
such mishap.
LEATHER-BACK MARINE TURTLES
The most remarkable member of the Chelonia now living is _Dermochelys
coriacea_ (Fig. 128), the great leathery or leather-back turtle of
the warmer parts of the Atlantic, Indian, and Pacific oceans, the
sole member of the family Dermochelydidae. It is the largest of
all living turtles and the most thoroughly aquatic of all, whether
living or extinct. It sometimes reaches a length of six feet, or
half that of the largest known extinct forms, and weighs a thousand
or more pounds. Agassiz saw a specimen that he said weighed a ton.
Unlike other turtles, it has a carapace quite peculiar to itself,
composed of a layer of thin, irregularly polygonal bones forming a
mosaic, completely hidden in the thick skin, and entirely free from
the skeletal bones beneath them. The larger of these skin bones form
seven rows above, which appear in the living animal as sharp keels
running the whole length of the shell. On the under side there are five
rows of smaller-sized bones, under which there are vestiges of bones
representing the normal plastron of turtles. The limbs are powerful,
flattened paddles, not unlike those of _Eretmochelys_, but wholly
destitute of claws. The front paddles are much larger than the hind
ones; the humerus is long and flattened, and the digits are elongated.
The leather-back is a powerful and effective swimmer, going long
distances. Its habits are not well known; its food is chiefly fish,
crustaceans, and mollusks.
[Illustration: FIG. 128.—_Dermochelys coriacea._ (From Brehm.)]
So very different is the structure of its shell that some excellent
naturalists regard _Dermochelys_ as the equivalent in rank of all other
turtles combined, the sole representative of the suborder Athecae, as
distinguished from the Thecophora. Dr. Hay, whose authority on fossil
turtles is of the highest, believes that its line of ancestry has been
distinct from that of all other turtles from Triassic times at least.
Others believe that the leather-back is merely a highly specialized
form derived from the ordinary shelled type, a descendant of some of
the marine turtles of Cretaceous times. In support of the primitive
ancestry of the leather-back Dr. Hay offers the following:
“The writer holds the view that the earliest turtles possessed
practically two kinds of shell, one purely dermal, consisting probably
of a mosaic of small bones arranged in at least twelve longitudinal
zones. Each zone probably consisted of a row of larger bones bordered
on each side by smaller bones. Each of these bones was covered by a
horny scute. The nearest approach to such a dermal shell is in our days
seen in _Dermochelys_. Beneath the skin there seems to have existed a
carapace more or less complete, which consisted of a nuchal, a median
row of neurals, eight pairs of costals, a pygal, probably one or more
supraneurals, and about eleven peripherals on each side. To what extent
the neurals and the costal plates had become anchylosed to the neural
spines and the ribs respectively, it is now impossible to determine.
Nor can we say to what extent the various elements of the carapace had
become connected with one another. There was a subdermal plastron which
was composed of at least eleven bones.
“According to the author’s view, as time went on the external,
mosaic-like shell disappeared in most turtles, while a more efficient
armor was developed out of the subdermal elements. In the ancestors of
_Dermochelys_, however, the dermal armor was retained, while the more
deeply seated one disappeared, with the exception of the nuchal bone.”
Such a hypothesis as the foregoing satisfactorily explains the
extraordinary mosaic shell of the leather-back, and is perhaps an
acceptable explanation of the rather strange fact that the horny
shields of turtles do not correspond with the bones below them, as
might be expected. Unfortunately this hypothesis lacks sufficient
proof. About the only evidence that is offered in its support is the
existence of a row of bones along the middle line in the Cretaceous
_Toxochelys_, and notably in _Archelon_, both aquatic forms. It is
urged that these bones, the epineurals of Wieland, are really the
remains of an external layer that persisted in these turtles. However,
they might have been new ossifications, such as we know did occur in
not a few of the land tortoises later over the tail and limbs. Aside
from _Proganochelys_ in the vast interval of time from the Triassic to
the Eocene no other evidence of such an external dermal layer has been
discovered. The chief argument against such divergent ancestry of the
turtles in two chief lines of descent is the fact that in its other
structure _Dermochelys_ shows great resemblance to other sea-turtles of
the Cretaceous times—so much resemblance that it seems impossible that
the ancestors of the leather-back should have paralleled them in almost
everything except the shell.
On the other hand, those who disagree with this view believe that the
modern leather-back is the descendant of such Cretaceous marine turtles
as _Protostega_ or _Archelon_, some of which had lost nearly all of
the costal plates and had the neurals and marginals reduced. It is
urged that some of these early marine forms, after they had practically
lost the ordinary bones of the carapace, for some reason or other found
a bony shell again necessary for their welfare. Possibly they had
become littoral in habit; possibly they again became subject to new and
dangerous enemies in their unprotected condition, notwithstanding their
great size; perhaps the zeuglodons were among their enemies. Now, as we
have seen, an animal never takes a back track and recovers a thing it
has once lost. It was impossible for the ancestors of the leather-back
again to acquire an orthodox shell, and they forthwith proceeded to
acquire quite another kind that would serve the same purpose.
Possibly the truth lies between the variant views, in the theory
recently expressed by Versluys: “The shell of tortoises and turtles is
formed by a combination of two layers of dermal ossifications, a thecal
layer and a more superficial epithecal layer, the latter generally
represented by the marginals only. The leather-back is a member of the
Cryptodira, and is allied to the other marine turtles. The problem of
the origin of the aberrant shell of the leather-back seems to find its
solution in the hypothesis that it is a secondary proliferation of the
marginals and such other epithecal elements as were present in its
thecophorous ancestors.”
In other words, Versluys believes that Hay’s and Wieland’s views
of the primitive double layer of exoskeletal bones is essentially
correct, but that _Dermochelys_ was derived from later forms in which
some of them, only, as in _Archelon_, had remained. Baur’s contention
that “_Dermochelys_ is not the least, but the most specialized marine
turtle” seems to have been fully justified.
RIVER TURTLES. TRIONYCHOIDEA
No reptile is more familiar or more exasperating to the river fisherman
than the turtle, variously known as the river, soft-shelled, or mud
turtle. It lives, often in great numbers, in most of the rivers, ponds,
and bayous of the interior east of the Rocky Mountains, and especially
in those of sluggish current and muddy bottoms. It is voraciously
carnivorous in habit, feeding upon the smaller fish, mussels, and such
other living food as it can capture. With its long, sinuous neck and
snake-like head, and soft, mottled skin, it is repulsive enough to most
persons, but is especially annoying to the fisherman, since it devours
with impunity his bait so long as he feeds it, and can seldom be caught
on the hook because of its hard and bony mouth, in which only by good
luck will the hook catch. And the luckless string of fish that the
fisherman leaves in the water may be almost completely devoured in a
few hours by these fiercely predaceous feeders. However, if so annoying
while seeking for better game, it in part makes up for the annoyance it
causes by furnishing in its own body a not unpalatable food for those
who like to eat reptiles.
[Illustration: FIG. 129.—_Trionyx_, river turtle. (By permission of the
New York Zoölogical Society.)]
The river turtles will be readily recognized from the accompanying
illustration (Fig. 129). They are very flat, covered with a soft,
smooth skin, with a long, sinuous neck and a small, snake-like and
vicious-looking head which has a protuberant snout with the external
nasal orifice at its end. Their feet are webbed and somewhat
paddle-like, but always with three stout claws—whence comes the name of
the group—on the anterior digits, which are used for burrowing in the
mud and excavating holes for their eggs. These turtles burrow more or
less in the mud, with the long neck free, lying in wait for their prey,
and coming to the surface from time to time to breathe. As the shape
of the body and the paddle-like feet would suggest, they are active
swimmers and purely aquatic in habit, never leaving the water unless
compelled to. They bury their hard-shelled eggs on the shores only a
few feet from the water, and leave them to their fate. If the pools
in which they live dry up, they burrow deeply in the mud and await
the rains and floods. In captivity they feed upon all kinds of food,
vegetable as well as animal, and are active and aggressive.
[Illustration: FIG. 130.—_Aspideretes_, a trionychoid turtle from the
Basal Eocene of New Mexico; skull from above. (From Hay.)]
Because of certain peculiarities, they are usually classed in a
separate suborder all their own, the Trionychoidea, especially
distinguished from the Cryptodira, which in general they resemble in
most respects, aside from the absence of the usual horny dermal plates,
in the lack of a marginal row of bony plates around the carapace—not a
very important distinction. Less than thirty living species are known,
all of them exclusively, or chiefly of fresh-water habit. Six species
are known from North America; the remainder inhabit Africa, south of
the Sahara Desert, southern India, and most of the East Indian islands;
none is known from Australia. No species lives in South America and
none is known to have lived there in past times. During Eocene,
Oligocene, and Miocene times these fresh-water turtles lived in the
region of Europe in great numbers, but for some inexplicable reason
they became extinct there and never returned. Nearly seventy species of
the Trionychoidea, belonging in two families, are described by Dr. Hay
from the Tertiary rocks of North America, more than twice the number
now living throughout the world. Some of these were of relatively
large size, measuring fully two feet in the length of the shell. And in
some places they must have been very abundant. The writer has seen, in
the Bad Lands of the Continental Divide, their weathered-out remains so
numerous that they might be raked into windrows miles in length along
the sloping bluffs, all in small fragments, for their bones, like those
of most turtles, are only loosely united by sutures and readily drop
apart before fossilization. Their shells may be readily distinguished
from those of all other turtles by the granulated, pitted, or
sculptured exterior surface, that was covered by the skin in life;
other turtles have the surface smooth below the horny shields, the
margins of which are marked on the bones by grooves or sulci; the few
marine turtles of the past that were probably covered with a soft skin
instead o£ horny shields had the shell smooth and much less completely
ossified.
[Illustration: FIG. 131.—_Aspideretes_, a trionychoid turtle from the
Eocene of New Mexico; front leg. (From Hay.)]
As to the origin of the soft-shelled turtles there has been not a
little difference of opinion. The earliest ones known in geological
history date back only to about the middle of the Cretaceous; perhaps
they branched off from the horny-shelled turtles somewhat earlier, but
probably not much. There are some, however, who think that this group
of turtles was very primitive, perhaps the most primitive, but the
writer agrees with Dr. Hay in rejecting this view. Unlike those of all
other turtles, the fourth digit in front and hind feet has one or two
more phalanges than have other turtles. We have seen that the oldest
known reptiles had the digital formula 2, 3, 4, 5, 3 or 4. Most other
turtles have the same numbers of bones in the digits that mammals have,
that is, two phalanges in the thumb and big toe and three in each of
the other digits. The river turtles have a larger number in the fourth
digit, either four or five. It seems to be a law that evolution is
irreversible, and if so could the river turtles have been descended
from forms with a less number of phalanges? But, the skeleton of the
Trionychoidea resembles the more specialized turtles in so many ways
that one can hardly believe they were all accidental or parallel.
We may then assume that at about the time that the ordinary marsh
turtles took to the sea to become marine, others took advantage of
the fresh-water ponds and rivers, and in doing so, like the marine
turtles, lost their horny epidermal shields, and became thinner in
shape, thereby reducing the resistance to the water. Instead, however,
of reducing the costal plates over the ribs, they retained them intact
and complete for some reason or other, but lost instead the marginal
row of bones, unlike the marine turtles which retained them even after
they had lost nearly all of the costal plates. Possibly also they
regained additional bones in the fourth digit, a sort of hyperphalangy
like that of the more strictly aquatic reptiles. Or, possibly, they may
have descended from some branch of the turtles which had not yet lost
these bones, retaining them because they were still serviceable for
swimming. We know nothing yet about the structure of the feet of the
early turtles, and it is possible that not all had acquired the reduced
phalangeal formula.
In the development of aquatic habits the river turtles do not show
the same degree of specialization in the limbs that the strictly
marine forms do. The humerus (Fig. 131) is a slender bone, with the
tuberosities for the attachment of the muscles situated near the
proximal end. The radius and ulna are relatively short, and the foot is
long. The hind legs, as would be supposed, are less highly specialized
as swimming paddles, and are relatively smaller. Nevertheless the
Trionychoidea present an interesting type of adaptation to water
habits, both in body and in limbs.
INDEX
Adaptation to aquatic life, 59.
Aetosauria, 187.
Aigialosaurs, 146.
Alligator, _197_.
Amblyrhynchus, _142_.
Andrews, C. W., 75.
Angistorhinus, 190.
Anomodontia, 102.
Aquatic reptiles, adaptation of, 59.
Araeoscelis, _133_, 138.
Archelon, 234, _236_, _238_, 239.
Archosauria, 33.
Aspideretes, _246_, _247_.
Baptanodon, _113_, _114_, _117_.
Baur, George, 119, 120, 185, 234.
Belodon, 185, 189, _191_.
Belodontia, 185.
Bêche, De la, 73.
Bogalobou, 75.
Broom, Robert, 5, 102, 103, 129.
Brown, Barnum, 93, 179.
Buckland, Dean, 75, 77.
Cacops, _35_.
Camper, Adrian, 149, 166.
Camper, Peter, 149, 166.
Captorhinus, _49_.
Caretta, 230.
Carpus, 38.
Casea, _55_.
Case, E. C., 234.
Champsosaurus, _179_, _180_, _181_, _182_.
Chelone, _220_, 229.
Chevrons, 32.
Choristodera, 178.
Classification of reptiles, 13.
Clavicles, 37.
Cleithrum, 36.
Clidastes, _147_, _154_, _155_, _157_, 166.
Collection of fossils, 10.
Colpochelys, _222_.
Conybeare, Rev., 73, 110, 149.
Cope, E. D., 126, 176, 178, 185, 231, 233.
Coracoid, 36.
Cotylosauria, 16.
Cryptodira, 226.
Cretaceous of Kansas, _8_.
Crocodiles, ancient, 204;
marine, 207;
modern, 195.
Crocodilia, 15, 194.
Crocodilus, _195_.
Cuvier, Georges, 73, 97, 107, 110, 149.
Darwin, Charles, 142.
De Fond, St. Faujas, 148.
Dermochelys, 241, _242_.
Desmatochelys, 231, _232_.
Dimetrodon, _36_, _51_.
Dinosauria, 18.
Dolichosaurs, 145.
Dolichobrachium, 54.
Dollo, Louis, 167, 179.
Edaphosaurus, _23_.
Elasmosaurus, _79_, _84_, _86_, _87_.
Enaliosauria, 75.
Eosauravus, 52.
Episcoposaurus, 190.
Eretmochelys, _230_.
Eryops, _31_, _47_.
Eusuchia, 195.
Exoskeleton, 43.
Extinct reptiles of North America, 52.
Femur, 41.
Fibula, 41.
Foot, 42.
Fraas, Eberhard, 74, 75, 99, 111, 115, 185, 213.
Gastroliths, _92_, 200.
Gavial, Borneo, 201;
Gangetic, _198_, _199_, _202_.
Gavialidae, 203.
Geological Ages, 46.
Geosaurus, _208_, _210_, _211_, _212_, 214.
Gervais, Professor, 126.
Gilmore, Charles, 113, 167.
Globidens, _167_.
Goldfuss, August, 151.
Graptemys, _218_.
Hadrosaurus, 56.
Hand, 38.
Hauff, B., 122.
Hay, Oliver P., 220, 227, 228, 234, 242.
Hofmann, Dr., 148.
Holops, 207.
Home, Everard, 110.
Homo diluvii testis, 108.
Huene, Friedrich von, 131, 185.
Humerus, 38.
Huxley, Thomas, 185, 207.
Hydrus, _169_.
Hyperphalangy, 118.
Hypocentrum, 36.
Ichthyosauria, 17, 107.
Ichthyosaurus, _108_, _112_, 119, 121, _122_.
Iguana, _140_.
Ilium, 39.
Interclavicle, 37.
Intercentrum, 30.
Ischium, 39.
Jaeger, George, 184.
Jaekel, 185.
Karoo beds, 102.
Koenig, 110.
Labidosaurus, _22_, _26_, _50_.
Laecertilia, 140.
Lariosaurus, _99_, _100_.
Leather-back turtles, 241.
Leidy, Joseph, 151.
Limnoscelis, _20_, _47_.
Lizards, 140;
flat-headed, 144;
Galapagos, 142.
Lull, Richard, 54.
Lystrosaurus, 103, _104_.
Lortet, M., 135.
McGregor, J. H., 126, 129, 131, 185, 186, 189.
Mandible, 25.
Mantell, Dr., 75.
Marsh, O. C., 54, 185.
Merriam, J. C., 112, 120, 171, 174.
Merriamia, _118_.
Mesosaurus, 126, _127_, _128_.
Mesosuchia, 185, 204.
Meyer, Hermann von, 75, 87, 132, 134, 184.
Mixosaurus, 119.
Monitor lizards, 144.
Mosasauria, 148, 166.
Mosasaurus, 148;
hofmanni, 149;
maximiliani, 151.
Münster, Georg von, 97.
Mudge, B. F., 93.
Mystriosuchus, 185, _188_, _189_, _191_.
Nares, 23.
Nectosaurus, 175.
Nothosauria, 95.
Nothosaurus, _96_, _97_, _98_.
O’Fallen, Major, 151.
Ophiacodon, _30_, _32_, _37_, _38_, _40_, _42_.
Ophidia, 168.
Ophthalmosaurus, _118_, 125.
Orders of reptiles, 16.
Osborn, H. F., 126.
Ostodolepis, _33_.
Owen, Richard, 75, 103, 110, 111, 185.
Paleorhinus, 190.
Paliguana, 5.
Parasuchia, 18, 184.
Parietal foramen, 23.
Pectoral girdle, 34.
Pelvic girdle, 39.
Pelvis, 39.
Pelycosauria, 186.
Pelycosimia, 187.
Phytosauria, 18, 187.
Phytosaurus, 186, 187.
Pineal foramen, 23.
Platecarpus, _151_, _155_, _156_, _158_, _159_, 166.
Platynota, 148.
Plesiosauria, 77.
Plesiosaurus, _74_.
Pleurodira, 224.
Pleurosaurus, _134_, _136_, 177.
Polycotylus, 80.
Proatlas, 32.
Proganochelys, 217.
Proganosauria, 17.
Proteosaurus, 110.
Protorosauria, 17, 132.
Protorosaurus, 132.
Protostega, 233, 234.
Protostegidae, 231.
Pseudosuchia, 185.
Pterosauria, 18.
Pubis, 39.
Pythonomorpha (Mosasauria), 166.
Range of Reptilia, _45_.
Rhachitomous vertebrae, 17.
Rhynchocephalia, 17, 176.
Rhytidodon, 190.
Ribs, 33.
River turtles, 244.
Rutiodon, _190_.
Sacrum, 32.
Sapheosaurus, _178_.
Sauranodon (Baptanodon), 125.
Sauropterygia, 17, 73.
Scheuchzer, 107.
Seeley, H. G., 75, 126, 132.
Seymouria, _21_, _48_.
Shoulder-girdle, 34.
Simoedosaurus, 179.
Skeleton of reptiles, 19.
Skull of reptiles, 21.
Snakes, 168.
Spener, 132.
Sphenodon, _24_, 132, _176_.
Squamata, 17, 138.
Stegocephalia, 48.
Stereosternum, 126.
Sternum, 37.
Stomach-stones, 200.
Tarsus, 42.
Teeth, 21, 25.
Teleosaurus, _205_.
Temporal openings, 23.
Testudo, _228_.
Thalattosauria, 17, 171.
Thalattosaurus, _172_, _173_, _174_.
Thalattosuchia, 207.
Therapsida, 16.
Theromorpha, 16.
Thaumatosaurus, _74_.
Thoracosaurus, 207.
Tibia, 41.
Tomistomidae, 201.
Tortoises, 216;
fresh-water, 226;
land, 227;
marsh, 226.
Toxochelys, _219_, _231_, _232_.
Trachemys, _221_.
Trimerorhachis, _25_.
Trinacromerum, _77_, _81_, _83_, _85_, _88_, _89_.
Trionychoidea, 244.
Trionyx, _245_.
Tuatera, 176. _See_ Sphenodon.
Turtles, 216;
river, 244;
sea, 229;
ancient sea, 231;
side-necked, 224.
Tylosaurus, _153_, _155_, _157_, _160_, _165_, 166.
Varanus, _144_.
Varanops, _53_.
Ventral ribs, 34.
Vertebrae, 28;
notochordal, 29;
rhachitomous, 31.
Versluys, 244.
Watson, D. M. S., 103.
Wieland, G. R., 234, 236, 239, 243.
Woodward, A. S., 126.
Zittel, Carl von, 185.