land when their homes are dried up, but as a rule they remain in the lake,
pond, river, morass, or ditch in which they were born. Their favourite
resorts are the broad floating leaves of water-plants, for instance
water-lilies, or a prominent stone, a tussock of grass, or the banks of
their homes, where they sit motionless, basking for hours in a half-erect,
alert position, watching for insects and other small fry, which are secured
by a jump, and then lapped up. Sunshine is sure to bring them out, and on
our approach they make straight for the water, either by one tremendous
leap or with quick bounds, but without the slightest hesitation or stopping
on the way. With folded arms they take a header, swim, with the arms still
folded, for some distance under water, and conceal themselves in the mud,
between stones, or in the vegetation. We perhaps have not seen them at all,
whilst their watchful eyes and keen ears have noticed our approach, and the
pond might appear uninhabited if we had not heard the plumping noise. If we
{268}keep quite still, and they have not been disturbed previously, one
after another will wriggle out of the mud, rise slowly to the surface under
cover of the plants, and, without causing a ripple, rise just enough for
the prominent eyes and the nose to clear the surface. Then one scrambles
partly on to a leaf, but the sight of the huge human figure strikes him as
uncanny, as it certainly does not belong to the scenery, and he doubles
back, the broadly-webbed feet making a little splash. But another appears,
jumps on to a leaf in the middle, or at the farther end of the pond,
settles down, and utters a little croak, somewhat like "ooaar," and soon
the whole company appear one after another, each taking up its favourite
position. After all, their observing powers cannot be very great. If we
ourselves keep still we may wield a rod and fish for them. There is no need
of a hook, a piece of red cloth tied to the end of the line and skimmed
over the water causes a lively commotion. The new bait is noticed at once,
and arouses their curiosity; several jump at it, and the one which swallows
the bait can be lifted out before it has time to let go. However, this is
after all poor sport; the game is too eager. When a boy I have often caught
them with a noose of slender wire at the end of a long hazel rod. They do
not mind the rod at all, their attention being fixed on the person; they
allow the noose to be slipped over their heads, and a sudden jerk secures
the captive. In this way they can be singled out individually. Old frogs
are more wary and experienced than the younger members; they take up safer
positions, and by their sudden plunges give the alarm.
[Illustration: FIG. 51.–_Rana esculenta._ × 1. Three stages of the movement
of the tongue.]
The males are great musicians, singing for sheer enjoyment not only during
the pairing time, but throughout the months of June and July. Warm moonlit
nights are the favourite times {269}for the concert, which takes place in
the water, beginning at sunset, and continuing until the early dawn. A few
individuals here and there utter a single note, "gwarr, oo-arr," or
"coarx," but these are only preliminaries. The precentor–the country-folk
in North Germany firmly believe that in each pond one old male holds the
dignified position of choir-master–begins with a sharp-sounding "brekeke,"
and this is the signal for all the others to chime in with the same notes,
varied with all sorts of other sounds, bass, tenor, and alto, each
performer filling its resounding vocal sacs to bursting size, and these
bags then look as if they acted as floats. When there are several hundred
of these sociable creatures, the din is continuous, and may be heard more
than a mile off. There can be too much of this, just as there can be too
many nightingales; and a well-stocked pond in the neighbourhood may become
a perfect nuisance. There are accounts of servants having been employed in
the Middle Ages for the sole purpose of keeping the noise down by beating
the pond, throwing stones into the water, or otherwise disturbing the
frogs. Sometimes more vigorous and lasting measures seem to have been
taken; the monks exorcised them in order not to be disturbed in their
vigils. Near the former monastery of Chorin, in the province of
Brandenburg, the frogs have still the reputation of keeping very quiet on
account of some powerful abbot who threatened them with awful consequences
if they did not forego their concerts.
[Illustration: FIG. 52.–_Rana esculenta._ Male with inflated external vocal
sacs. × 1.]
The length of life which these frogs can attain is quite unknown. They do
not reach maturity until the fourth or fifth year, but this is long before
they stop growing, and it is no exaggeration to say that few, if any, frogs
die of old age, since they have so many enemies. The stork is their king in
the fable, and his daily visits to his realm strike dire distress amongst
his subjects, which soon learn to know his conspicuous white and black
garb, and seek imperfect safety at the bottom of shallow ponds and ditches,
not too deep for the long-legged and long-billed despot. Numbers are taken
by birds of prey; snakes and tortoises hunt them up in the water, and they
are good bait for pike and other voracious fishes. The specific {270}name
_esculenta_ needs no comment, and this species is as much a martyr to
science as the brown Grass-frog. The destroyers of tadpoles and young frogs
are unlimited. In their turn the frogs themselves, especially the old ones,
are very rapacious, and eat any living creature they can master,–insects,
worms and snails, other frogs, especially the brown kind, and the young
brood of fishes.
Recently caught Water-frogs are wild beyond description, much more so than
the Grass-frog, but even they calm down after some time, learn to know
their keeper, and allow him to handle them without trying to commit suicide
by jumping on to, into, and down anything. However, they do not thrive well
in captivity, and it is rare that they can be induced to breed, unless
their enforced new home affords them ample freedom, and plenty of water and
fresh air.
The Water-frogs appear in Germany rather late in the year, not before the
middle of April, first the younger, then the adult members. In Southern
Europe they show themselves earlier, and still further south they do not
hibernate at all. The breeding season begins in Germany towards the end of
May and continues well into June, the var. _ridibunda_ beginning mostly a
fortnight earlier. The male clasps the female under the arms, throwing its
own round her breast, the nuptial grey excrescences on his inner fingers
pressing against her skin, the palms being turned outwards. The embrace
does not last long, rarely extending over a few days. The eggs, to the
astonishing number of 5000 to 10,000 in full-grown specimens, are expelled
in several masses, which sink down and remain at the bottom. The eggs
measure only 1.5 mm. and are yellowish-grey above, pale yellow below; their
gelatinous cover swells to 7-8 mm. in width. The embryo escapes on the
fifth or sixth day as a very small larva, in which, however, the mouth,
eyes, and beginnings of the external gills are already discernible. At the
age of two weeks the gills have shrunk away, the left-sided "spiracle" is
completed, and the well-tailed tadpoles, olive brown above, yellowish white
below, still hang with their suckers on to plants and stones, or lie at the
bottom, nibbling away at any rotting animal matter or scraping off the
green algae.
It may here be mentioned that small tadpoles of any kind can with advantage
be used as cleaners of delicate and small {271}skeletons. The object is put
into a vessel, and the tadpoles will soon nibble and rasp away all the
edible portions, leaving the skeletal framework beautifully cleaned. But
they require attention lest they rasp away the cartilage.
The tadpole stage lasts three to four months; but cold, absence of
sunshine, and scarcity of food delay the metamorphosis well into the end of
summer, or force them to hibernate in the unfinished condition. They are
very gregarious, and when the tadpoles of several families combine, they
make imposing shows. By the time that their hind-limbs begin to sprout,
they frequently combine into large shoals, and instead of always feeding
they swim about in their tens of thousands, all moving in the same
direction, and making almost regular evolutions. Mill-ponds with steep
banks are good places for watching these peculiar habits. The tadpoles
reach a considerable size, the total length averaging 2½ inches, or some 60
mm. the tail taking up ⅔ of the whole length. Specimens which measure more
than 3 inches are rare. The baby-frogs hop on land while still provided
with a stumpy tail; when this is resorbed the little creature is scarcely
half-an-inch long, and for the rest of the available season leads a rather
more terrestrial life than ever after.
_Ex Africa semper aliquid novi!_ Quite recently Boulenger has received a
consignment of Anura from the French Congo, amongst which were several new,
remarkable genera, notably _Trichobatrachus_ and _Gampsosteonyx_. Both are
true Ranidae. Pupil vertical, with vomerine teeth. Omosternum with a bony
style. The outer metatarsals are bound together. In _Trichobatrachus
robustus_ the toes are webbed, and both sexes have the flanks and
corresponding portions of the thighs covered with numerous darkly
pigmented, filamentous, cutaneous excrescences; these are several
millimeters in length, giving the flanks and thighs a "hairy" appearance.
Mr. F. F. Laidlaw has examined these structures. Their most remarkable
feature is the presence in them of a great number of ordinary flask-shaped
cutaneous glands, whilst such glands are scarce on the surrounding skin.
They differ in no way from those seen in sections of the skin of the Common
Frog. The fibrous connective tissue is dense and vascular; the
pigment-cells are most plentiful at the base. Contrary to expectation no
nerve-endings were found in these filaments.
{272}_Gampsosteonyx_ has free toes. The terminal joints of the digits stand
out beyond the skin, and end in sharp, bony claws, like those of a cat.
SUB-FAM. 3. DENDROBATINAE.–About one dozen arboreal little frogs have been
separated from the Raninae proper on account of the entire absence of
teeth. This mere loss of teeth, and the geographical distribution suggest
that these frogs do not form a natural group, but have been developed
independently from other Ranidae, the Neotropical _Dendrobates_ from some
likewise Neotropical genus like _Prostherapis_, the Malagasy _Mantella_
from an African form like _Megalixalus_.
The sacral diapophyses are cylindrical. The omo- and meta-sternum are well
developed. The fingers and toes are free, their terminal phalanges are
T-shaped and carry regular, round, adhesive discs. The tympanum is
distinct, although sometimes, in _Dendrobates_, very small. The pupil is
horizontal.
_Dendrobates._–The tongue is elongate, entire and free behind. The
omosternum has a weak, semi-ossified style, but the metasternum remains
cartilaginous. The males have a subgular vocal sac. Seven closely-allied
species inhabit tropical America.
_D. tinctorius._–This pretty little species, scarcely 1½ inch in length, is
quite smooth, varies much in coloration, and forms local races to a certain
extent. Some are quite black, others are grey above, black on the sides and
under parts; or they are grey with large black patches. A fourth variety is
black above with several white or pink longitudinal stripes, while the
under parts are grey, spotted with black. In others, again, the
ground-colour is black, with white stripes and spots above, marbled below.
But this enumeration does not exhaust the list, since living specimens are
sometimes much more conspicuously coloured, some being black with large
patches of saturated yellow on the head and back, while the limbs are
orange red and black. This species has a wide range, from Panama to Ecuador
and to the mouth of the Amazon. It owes its specific name to the peculiar
use made by man of the strongly poisonous secretion of the tiny glands of
the otherwise smooth skin. Other species are doubtless employed in the same
way. The poison is mainly used for "dyeing" the green Amazon-parrots. This
is done as follows:–The green and blue feathers on the head and neck, or
other parts, according to the fancy of the {273}operator, are plucked out,
and these places are rubbed with the poison, often simply with the living
frog, certainly not with its blood, as is sometimes asserted. This
operation may be repeated when the new, young feathers begin to bud. The
result is that these appear yellow instead of green, and since the
Brazilians, and to a certain extent the Portuguese, are rather partial to
these artificially-produced freaks or "contrafeitos" as they call them, the
industry is kept up. That the poison is also used for arrows has been
mentioned on p. 38.
[Illustration: FIG. 53.–_Dendrobates tinctorius_, three colour-variations.
× 1.]
_D. trivittatus_, chiefly in Northern Brazil, has the first finger slightly
longer than the second. It likewise varies considerably in its coloration,
being either quite black, or spotted with white and brown, or with a
whitish forehead and several white patches on the back and hind-limbs. _D.
typographus_ of Central America is vermilion red, with small dark marks on
the back; the legs are black.
The various species of _Dendrobates_ take remarkable care of their young.
_D. braccatus_ lives in Brazil in "varzeas," _i.e._ moist but waterless
places, and carries its tadpoles on its back, to which they are attached by
a peculiar secretion. The same is said to be true of _D. trivittatus_,
which sits down in a drying-up puddle, lets the little tadpoles, when they
are only 6-7 mm. long, fasten themselves on, and conveys them to a safer
locality, where the water is calculated not to evaporate before the
metamorphosis is completed.
{274}_Mantella._–Both omo- and meta-sternum possess a bony style. The
tongue is free and distinctly mitred or cut out behind. The skin is very
granular. Several species, in Madagascar, were formerly put into the same
genus as the American forms, until Boulenger established the genus
_Mantella_ for them. The coloration is strikingly pretty. _M.
madagascariensis_ is a rare instance of difference in colour between the
two sexes. The male is bluish black, with light blue spots on the belly,
while the thighs and the inner sides of the legs are beautifully red. The
female is deep black, with a light green spot at the base and in front of
the limbs; the rest is coloured like the male.
_Cardioglossa gracilis_, quite recently discovered at the Gaboon, has
likewise to be added to the Dendrobatinae, on account of the absence of
teeth. It is a small, slender, arboreal frog, bearing an unmistakable
resemblance to the other genera by its general appearance and conspicuous,
contrasting coloration of black and white.
PART II
REPTILIA
"Cada uno es como Dios le hizo,
y aun peor muchas vezes."
"We are all as God made us
and many even worse."
SANCHO PANZA,
_Don Quixote_.
{277}CHAPTER VII
REPTILIA
DEFINITION AND CHARACTERS–POSITION OF THE CLASS REPTILIA IN THE PHYLUM
VERTEBRATA–CLASSIFICATION–SKULL AND VERTEBRAE.
The recent Reptiles comprise, broadly speaking, the Crocodiles, Tortoises,
Lizards, and Snakes. They are the only Vertebrates which are cold-blooded,
breathe by lungs, and have a median occipital condyle. Another equally
sufficient diagnosis is the following:–Tetrapoda, with a median occipital
condyle, with nucleated red blood-corpuscles, and with complete right and
left functional aortic arches. A still shorter diagnosis is:–Monocondylia
with a scaly skin.
If our diagnosis is to include the fossil Reptiles we have not only to
discard the characters drawn from the soft parts as unavailable, but we are
forced to treat the condition of the occipital condyle with caution, since
there exist, or must have existed, transitional stages between Reptiles and
Amphibia and Mammals; and the winged class Pterosauria does not permit us
to use the wings as a differential character for the Birds. In fact, while
the Reptilia are sufficiently separated from the Amphibia by their
absolutely gastrocentrous vertebrae, it is difficult to distinguish them as
a class from the Birds; hence the term SAUROPSIDA, which is intended to
indicate the close relationship of the Reptiles to the Birds in opposition
to the Mammalia, and to the ICHTHYOPSIDA or Amphibia and Fishes. However,
the Reptilia take up a very central position in the evolution of the main
classes of the Vertebrata. On the one hand, there is not the slightest
doubt that they are evolved from some branch of {278}the Stegocephali,
whilst on the other hand the Reptiles, probably through some branch of the
Theromorpha, have given rise to the Mammals; some other Reptilian branch,
at present unknown, has blossomed out into the Birds.
PRINCIPAL CHARACTERS OF THE REPTILIA.
1. The vertebrae are gastrocentrous.
2. The skull articulates with the atlas by one condyle, which is formed
mainly by the basioccipital.
3. The mandible consists of many pieces and articulates with the cranium
through the quadrate bones.
4. There is an auditory columellar apparatus fitting into the fenestra
ovalis.
5. The limbs are of the tetrapodous, pentadactyle type.
6. There is an intracranial hypoglossal nerve.
7. The ribs form a true sternum.
8. The ilio-sacral connexion is post-acetabular.
9. The skin is covered (_a_) with scales, but (_b_) neither with feathers
nor with hairs; and there is a great paucity of glands.
10. Reptiles are poikilothermous.
11. The red blood-corpuscles are nucleated, biconvex, and oval.
12. The heart is divided into two atria and an imperfectly divided
ventricle. It has no conus, but semilunar valves exist at the base of the
tripartite aortic trunk.
13. The right and left aortic arch are complete and remain functional.
14. Respiration is effected by lungs; and gills are entirely absent, even
during embryonic life.
15. Lateral sense-organs are absent.
16. The kidneys have no nephrostomes. Each kidney has one separate
ureter.
17. There is always a typical cloaca.
18. The eggs are meroblastic.
19. Fertilisation is internal, and is effected, with the single exception
of _Sphenodon_, by means of male copulatory organs.
20. An amnion and an allantois are formed during development.
Numbers 1, 2, 6, 7, 8, 14, 16, 18, 20 separate the Reptiles from the
Amphibia. Cf. also pp. 4 and 5.
Numbers 9 (_b_), 10, 12, and 13 separate them from the Birds and
Mammals.
Numbers 3, 8, and 11 separate them from the Mammals.
The EVOLUTION OF THE CLASSIFICATION OF THE REPTILES has to a certain extent
been already treated on pp. 7-9. For a long time only Chelonia or
Tortoises, Ophidia or Snakes, and Saurii were recognised as their principal
divisions. Then the Crocodiles were separated from the Lizards; later the
Coeciliae were removed from the Snakes and referred to the Amphibia,
{279}and ultimately _Sphenodon_ was recognised as deserving a separate
position, equal in rank to the other groups. Stannius showed that the
Crocodiles and Tortoises are relatively near allies in opposition to the
likewise closely allied Lizards and Snakes (_Sphenodon_ was then unknown),
and he expressed this by the term Monimostylica, or creatures with fixed
quadrate bones, for the former, and Streptostylica, creatures with movable
quadrates, for the latter combination. The fossil Reptiles were hardly
allowed proper places in the system. In various zoological text-books they
were, or are even now, treated as inconvenient, outlying, or supernumerary
members. A long time elapsed before, thanks to the labours of H. von Meyer,
Owen, Huxley, Marsh, Cope, Zittel, and Seeley, it was recognised that the
extinct groups form the preponderant mass of Reptiles, and that it is the
recent groups which, in spite of the bewildering number of species of
Lizards and Snakes, are the comparatively few and much-reduced members of a
once flourishing class. With the exception of the Lizards and Snakes, which
are on the ascending branch, the modern _Sphenodon_, the Crocodiles and the
Tortoises are a mere fraction, comprising a few survivals of
richly-developed groups, while all the others, the overwhelming majority,
have died out.
The classification adopted in this volume is as follows:–
CLASS REPTILIA.
Sub-Class I. Proreptilia.
" II. Prosauria. Orders: Microsauri, Prosauri.
" III. Theromorpha. Orders: Pareiasauri, Theriodontia,
Anomodontia, Placodontia.
" IV. Chelonia. Orders: Athecae, Thecophora.
" V. Dinosauria. Orders: Sauropoda, Theropoda, Orthopoda,
Ceratopsia.
" VI. Crocodilia. Orders: Pseudosuchia, Parasuchia, Eusuchia.
" VII. Plesiosauria. Orders: Nothosauri, Plesiosauri.
" VIII. Ichthyosauria.
" IX. Pterosauria.
" X. Pythonomorpha. Orders: Dolichosauri, Mosasauri.
" XI. Sauria. Orders: Lacertilia, Ophidia.
The eleven principal groups are here called "sub-classes" to emphasise the
undeniable fact that these Reptilian groups are of much greater
morphological value than those which are most generally called "Orders" in
the Mammalia, that class which we consider as the standard or model of
classificatory units.
{280}[Illustration: FIG. 54.–Diagrams of skulls, showing especially the
composition of the bony arches of the orbito-temporal region.
A, C, D, E, THEROMORPHA. A, _Elginia_, p. 305; C, _Cynognathus_,
p. 306; D, _Gordonia_, p. 310; E, _Dicynodon_, p. 310.
B, G, PROSAURIA. B, _Sphenodon_, p. 294; G, _Palaeohatteria_, p. 291.
F, CROCODILIA, p. 434.
H, I, K, CHELONIA, p. 316. H, _Chelydra_, p. 338; I, _Chrysemys_,
p. 346; K, _Cistudo_, p. 361.
_E_, Epiotic; _F_, frontal; _IT_, infratemporal fossa; _J_, jugal, shaded
vertically; _L_, lacrymal; _M_, maxillary; _N_, nasal groove; _Na_, nasal
bone; _O_, orbit; _P_, parietal; _Po_, postorbital, dotted; _Pf_,
post-frontal; _Pm_, premaxillary; _Pr_, prefrontal; _Ptg_, pterygoid; _Q_,
quadrate; _Qj_, quadrato-jugal; _So_, supra-occipital; _Sq_, squamosal,
shaded obliquely; _St_ (in B-E), supratemporal fossa; _St_ (in A),
Supratemporal bone.]
{281}[Illustration: FIG. 55.–Diagrams of skulls, showing especially the
composition of the bony arches of the orbito-temporal region.
L, PYTHONOMORPHA. _Clidastes_, p. 490.
M, N, O, LACERTILIA, p. 496. M, _Varanus_, p. 543; N, _Uromastix_,
p. 524; O, _Lacerta_, p. 550.
P, ICHTHYOSAURIA, p. 479. _Ichthyosaurus_, p. 483.
Q, PTEROSAURIA, p. 484. _Dimorphodon_, p. 486.
R, AVES, generalised, for comparison.
S, MAMMALIA, generalised, for comparison.
T, OPHIDIA, p. 581.
_C_, Condyle of mandible; _Col_, columella cranii; _F_, frontal; _I_,
interparietal or pineal foramen; _I.A_, Inner angle of mandible; _J_,
jugal, shaded vertically; _L_, lacrymal; _M_, maxillary; _N_, nasal groove;
_Na_, nasal bone; _O_, orbit; _O_{1}_, preorbital fossa; _P_, parietal;
_Pf_, postfrontal; _Pm_, premaxillary; _Pr_, prefrontal; _Ptg_, pterygoid;
_Q_, quadrate; _Qj_, quadrato-jugal; _Sq_, squamosal, shaded obliquely;
_St_, supratemporal bone.]
{282}The families cannot well be changed, and terms like super-families and
super-orders are sometimes resorted to by those who do not like to look
stern facts in the face.
The sequence of the groups, although arranged as much as possible in
ascending order, is of necessity as unnatural as that of the maps in an
atlas. We cannot yet construct a satisfactory phyletic tree of the
Reptiles. The Proreptilia connect them with the Amphibia. Next follow the
Prosauria with _Sphenodon_ among the Prosauri as the key to most other
groups. Then follow the Theromorpha, and it is probable that from various
branches of these have arisen the Chelonia, Dinosauria, Crocodilia, and
Plesiosauria. The descent of the Ichthyosauria is very problematic. The
same applies to the Pterosauria and to the Pythonomorpha, but it is
possible that they, together with the Sauria, are connected with the
Prosauria.
With all reserve these hypothetical affinities may be expressed by the
following diagram:–
Lacertilia Ophidia
| /
Ichthyosauria Plesiosauria | /
\ \ | /
\ \ Crocodilia |/
? \ | Sauria
\ | |
Chelonia \| | Pterosauria
\ Dinosauria Sphenodon | /
\ | | | /
\ | | | / Pythonomorpha
\ | | |/ |
Theromorpha |–––––––––––––––
| |
Prosauria
|
Proreptilia
The eleven sub-classes of the Reptilia present so many important
differences that it is not advisable to give here a further general account
of their structure. The diagrammatic figures A to T on pp. 280, 281,
representing various types of skulls, are intended to explain their chief
modifications, all referable to Proreptilian and to certain Theromorphous
conditions. One of the most important features is that the mandible, which
is always composed of many pieces (cf. Fig. 142, p. 550), is invariably
carried by the quadrate bone. Diagrams of the generalised skulls of a Bird
and a Mammal have been added for comparison.
{283}[Illustration: FIG. 56.–Composition of vertebrae of Reptiles,
illustrated by the first and second cervical vertebrae. (1) Atlas (first
cervical) and axis (second) vertebra of _Crocodilus_. (2) Atlas and axis of
_Metriorhynchus_, a Jurassic Crocodile. (3) Analysis of the first two
cervical vertebrae of a Crocodile; 2, second basiventral complex or
"intercentrum" continued upwards into the meniscus or intervertebral pad.
(4) Diagram of the fundamental composition of a Reptilian vertebra; compare
this and (6) with Fig. 1 (8 and 9) on p. 13. (5) The first three cervical
vertebrae of _Sphenodon_. (6) Trunk-vertebrae of _Eryops_, a Permian
Proreptile; typically temnospondylous; _cp_, articular facet of the
capitulum of a rib. (7) The complete atlas of an adult _Trionyx hurum_; the
second basiventral (intercentrum) is attached to the posterior end of the
first centrum, which, not being fused with the second centrum, is not yet
an odontoid process. (8) The complete atlas of an adult _Trionyx
gangeticus_; still typically temnospondylous. (9) The first and second
cervical vertebrae of an adult _Platemys_. (10) The complete atlas of a
_Chelys fimbriata_. _Az_, Anterior zygapophysis; _B.D_, basidorsal; _B.V_,
basiventral; _C_{1}_, _C_{2}_, _C_{3}_, first, second, and third centra,
formed by the interventralia; _Cp^1_, _Cp^2_, articular facets of the
capitular portions of the first and second ribs; _I.V_, interventral;
_N_{1}_, _N_{2}_, _N_{3}_, first, second, and third neural arch, formed by
basidorsalia (_B.D_); _Od_, odontoid process = first centrum; _Pz_,
posterior zygapophysis; _R_{1}_, _R_{2}_, ribs; _Sp_, detached spinous
process of the first neural arch; _t_{1}_, _t_{2}_, tubercular attachments
of the first and second ribs; 1, 2, 3, 4, "intercentra" = basiventrals;
_I_, _II_, _III_, position of the exit of the first, second, and third
spinal nerves.]
As mentioned on p. 278 the vertebrae of the Reptilia and those of all other
Amniota are _gastrocentrous_; that is to say the centra or bodies of the
vertebrae are formed by the pairs of interventralia, while the
basiventralia are reduced, persisting either as so-called intercentra or
wedge-bones, or as intervertebral pads, or disappearing altogether. At the
earlier stages of development the gastrocentrous vertebrae behave in the
{284}same way as that described on p. 12 (Fig. 1), except that the
interdorsal elements are suppressed from the beginning. If the remaining
three pairs of constituent elements of each vertebra (the basidorsalia,
forming the neural arch; the interventralia, forming the body or centrum;
and the basiventralia) remain separate, the vertebrae are called
_temnospondylous_ (τέμνω, I cut, σπόνδυλος, a vertebra). If the neural
arches and the centra are suturally united or are fused with each other,
the vertebrae are called _stereospondylous_ (στερεός, solid). In many
Amniota the atlas or first vertebra remains in a relatively primitive,
embryonic condition, and is temnospondylous but for the usual modification
that its centrum becomes attached to that of the second vertebra, and forms
the odontoid process of the latter. The composition of gastrocentrous
vertebrae (cf. p. 282) is best illustrated by the first and second cervical
vertebrae of the Crocodile (Fig. 56, 3, p. 283).
Concerning GEOGRAPHICAL DISTRIBUTION, even a cursory study shows that the
sub-classes have come into existence at very different geological periods,
and have each followed their own lines of dispersal.
{285}CHAPTER VIII
PROREPTILIA–PROSAURIA–THEROMORPHA
_SUB-CLASS I.–PROREPTILIA._
_Permian Temnospondylous Reptiles with well-developed limbs and girdles of
the terrestrial type._
The two genera _Eryops_ and _Cricotus_ of the North-American Permian
formation had until recently[120] been relegated to the Stegocephali. By
grouping them and their nearest allies together as Proreptilia it is
intended to indicate that they are the lowest known Reptiles and that they
probably link this class to the Amphibia. The superficial resemblance of
their tri- or bi-partite vertebrae, and their occurrence in the Lower
Permian, have caused the error of classing them with the Stegocephali, but
the composition of their typically gastrocentrous vertebrae leaves no doubt
as to their affinities. After all, we feel certain that Reptiles have
arisen from Stegocephalous Amphibia, and it is in the Lower Permian,
exactly where these debatable creatures lived side by side with
Stegocephali, undoubtedly likewise temnospondylous, that the change from
Amphibia into Reptiles seems to have taken place. Both are referable to
Amphibia with quadripartite vertebrae. The condition of the occipital
condyles determines nothing. This greatly exaggerated character has lost in
importance since we have known the condylar modifications of the
Theromorpha; moreover, _Cricotus_ itself seems to have possessed a single
condyle. We should even expect the Proreptilia to present many
Stegocephalous inheritances, for instance the condition of the skull roofed
in by dermal bones, a ventral dermal armour, a very complete pectoral arch
still without a sternum, and only one sacral vertebra.
{286}Until more genera are better known than they are now, it is premature
to divide the present sub-class into orders.
[Illustration: FIG. 57.–Trunk vertebrae of _Eryops_ (cf. Fig. 56, 4,
p. 283). _Cp_, Articular facet of the capitulum of a rib.]
_Eryops_, with several species in Texas and New Mexico. _E. megacephalus_
is the most abundant and the largest species, its broad and flattened skull
measuring more than 18 inches in length and 12 in width. With the exception
of the nostrils and the small orbits, the skull is entirely encased in
bone, with a rough, pitted surface, but without any distinguishable
sutures. The absence of mucous canals, so common in the Stegocephali, is
worthy of note. The quadrates extend obliquely outwards and backwards, so
that the joint with the mandible lies in a plane behind the occiput. The
mandibles are devoid of a projecting angular process. The teeth are
numerous, small, and pointed. The vertebrae are typically temnospondylous,
consisting each of three pairs of separately ossified pieces, which,
although closely packed together, are not suturally connected. The neural
arches possess high spinous processes, they articulate by short and broad
zygapophyses and are, with their triangular bases, wedged in between the
two ventral pieces, the posterior of which (the united interventralia) is
in broader contact with the neural arch and lies behind it; the anterior
piece (the united basiventrals) appear as typical, but large, intercentra,
and bear on their posterior, dorsal margin the facets for the ribs. The
latter are short, but are broad at their proximal ends, which are not
bifurcated; they extend their articulation from the "intercentra" upon the
short lateral processes of the neural arches. The tail is short and ends in
a pointed coccyx, owing to fusion of the last vertebrae.
The pubes and ischia are heavy, the former flattened and broadened out. The
limbs are of an almost ideal pentadactyloid type; strongly developed for
terrestrial locomotion. The ulna possesses a large olecranon. The carpus
consists of ten separate pieces, ulnare, intermedium, radiale, two
centralia and five distal carpalia. The latter support only four
metacarpals and fingers, the second finger being completely abolished, an
explanation suggested by Cope and corroborated by Emery.[121]
{287}_Cricotus_, with several species in Texas and Illinois. _C.
heteroclitus_ was perhaps 10 feet long and probably aquatic. The skull has
a long, narrow, depressed snout, the margins overhanging those of the lower
jaw; its surface is encased in dermal bones, most of which still show
sutures, so that for instance postfrontals, postorbitals, supratemporals
and squamosals can be distinguished; all these are in contact with the long
parietals and with the quadrato-jugal arch, covering the temporal region;
but the supratemporals have a free projecting border, like the squamosals
of the crocodiles. According to Cope's description the basioccipital is
connected with the first vertebra by an undivided discoid "intercentrum,"
probably the true centrum, while the first basiventral mass, which would
be, if independent, the first true intercentrum, is more probably connected
with the first neural arch, thus constituting the ring of the atlas.
The vertebrae are still temnospondylous, but no longer tripartite. The
neural arch is fused with the interventralia into one mass, which carries
the capitula and tubercula of the ribs, while the united basiventrals still
remain as separate intercentral wedges. In the tail these wedges carry
chevron-bones, and are enlarged into thick almost complete discs, or rather
rings, while the whole vertebral column is still perforated, as also in
_Eryops_, by the chorda dorsalis. The tail is long. The digits are devoid
of claws.
Remains of dermal armour exist on the throat in the shape of several large
gular plates, while the whole belly is covered with many closely packed
bony scales, which are arranged in chevron-shaped transverse rows.
Probably several other genera of American Permian and also of European
Permian strata will, when better known and critically examined, have to be
referred to the Proreptilia. Thus for instance the European _Melosaurus_
may have affinities with _Eryops_, while _Diplovertebron_ of Bohemia seems
to be allied to _Cricotus_. The difficulty of division will lie with those
Lower Permian Amphibia which, like _Archegosaurus_, _Euchirosaurus_,
_Actinodon_, possess tripartite vertebrae, which at first sight are
strikingly like those of _Eryops_. But the tail-vertebrae permit of no
mistake, and since these are quadripartite in _Archegosaurus_,
_Chelydosaurus_, and _Sphenosaurus_, these genera are safely to be classed
with the Amphibia, unless, indeed, for mere argument's {288}sake, it be
assumed that the intercentral discs of _Diplovertebron_ and _Cricotus_ are
formed by the fusion of Amphibian interdorsals with interventrals. Anyhow,
simply to state that the tripartite vertebrae of _Eryops_ are the same as
those of _Actinodon_, would be as convincing as saying that the English and
French flags are essentially the same, both containing the same colours,
but one is white, red, and blue, the other blue, white, and red. Tripartite
Amphibian vertebrae are composed of basidorsals + basiventrals +
interdorsals, those of Reptiles are made up of basidorsals + basiventrals +
interventrals. (Cf. Fig. 56, p. 283, and Fig. 1, p. 13.)
_SUB-CLASS II.–PROSAURIA._
_Mostly extinct Reptiles, with deeply amphicoelous but stereospondylous
vertebrae, with movable chevron-bones in the tail and frequently with
intercentra in the trunk._ Sphenodon, _the only recent genus, has no
copulatory organs_.
ORDER I. MICROSAURI.
_Extinct, small Reptiles, mostly Carboniferous and Permian, with dermal
armour on the dorsal and ventral side and with bifurcated ribs._
We retain this term of Dawson's for those small, newt-shaped, chiefly
Permian reptiles, which are allied to _Hylonomus_, after elimination of
contemporary forms like _Keraterpeton_ and _Urocordylus_, which belong to
the Branchiosaurian order of the Stegocephali. Until recently[122] all
these creatures had been classed with the Stegocephali. The Microsauri in
the present restricted sense reveal themselves, however, as reptiles by the
movable chevron-bones in their tail, their broad neurocentral sutures, the
possession of two sacral vertebrae (_Petrobates_), the bifurcated ribs
which always articulate with the centra (most clearly shown in
_Orthocosta_), and the possession of five fingers and toes.
Considering the age of these little creatures and their low position in the
reptilian scale–in fact, they stand almost as low {289}as the
Proreptilia–it is not to be wondered at that they still retain a number of
amphibian features. The skull is encased in dermal bones as in the
Stegocephali, and the dermal armour of the trunk and tail is composed of
many bony, sculptured scales, which cover back, sides, and under surface.
The middle rows on the back are the largest, while the scales on the belly
are arranged in transverse rows, which imbricate and converge obliquely
headwards. Special gular plates seem to be absent. The skull has an
interparietal foramen. The jaws and the palate are furnished with small,
simple teeth, and there is a large parasphenoid bone, an eminently
amphibian character. The occipital condylar articulation is supposed to be
double. The centra of the vertebrae are deeply amphicoelous, elongated, and
constricted in the middle, just like those of the Aistopoda and
Branchiosauri. The dorsal spinous processes are strongly developed, and
with the zygapophyses are very reptilian. Transverse processes are absent
or very short, the tubercular portions of the ribs articulating with the
centra, the capitula mostly intervertebrally, in any case close to the
anterior end of the centra. The tail-vertebrae possess very typical,
movable chevrons, placed intervertebrally, and bear an extraordinary
resemblance to those of Geckos. The ribs are long and slender, but there is
no sternum. The fore- and hind-limbs are pentadactyle, in opposition to the
invariably four-fingered Stegocephali. The shoulder-girdle consists of
scapulae, coracoids, clavicles, cleithra, and a T-shaped interclavicle. The
pelvis also resembles that of certain Stegocephali by the separately
ossified, somewhat disc-shaped, flat ischia and pubes, which seem to have
been joined together by cartilage into one broad mass.
_Hylonomus_, Dawson's type of Microsauri, was found in the Coal-measures of
Nova Scotia, within decayed tree-stumps. Closely allied, if not identical,
but much better known is _Hyloplesion_, e.g. _H. longicostatum_ of the
uppermost Permian of Nyrschan in Bohemia. Total length under 4 inches; eyes
with bony sclerotic rings; neck short. The truly Permian genera _Dawsonia_,
_Melanerpeton_, _Orthocosta_, and _Seeleya_ are allied forms, the last
scarcely one inch in length, but well preserved. _Petrobates_ of the
Triassic Lower Red Sandstone of Saxony has an arrangement of the ventral
dermal armour closely resembling abdominal ribs.
{290}ORDER II. PROSAURI
Mostly extinct, chiefly Permian and Triassic, terrestrial, unarmoured
reptiles with deeply biconcave vertebrae, numerous intercentra and
chevron-bones, fixed quadrates, complete pentadactyle limbs and
shoulder-girdle, entepicondylar foramina, acrodont teeth, and many small
abdominal ossifications.
The Prosauri differ from the Microsauri, with which they are closely
allied, by the more advanced solidification of the vertebrae, the reduction
of the tubercular portions of the ribs, the presence of an entepicondylar
foramen in the humerus, and the loss of the dermal ossifications on the
upper surface.
Their ancestors are the Microsauri, whilst they themselves seem to be very
near the root whence have sprung most, if not all, other main branches of
the reptiles, notably Crocodilia, Dinosauria, and Sauria. In fact the
Prosauri, although apparently few in number, seem to represent the central
stem of the reptilian tree. Only one of them is still surviving, the famous
_Sphenodon_, now represented by a single species in New Zealand.
SUB-ORDER 1. PROTOROSAURI.–The ventral half of the pelvis seems to have
formed one broad, continuous mass of cartilage in which the pubic bones are
represented by a pair of oval, rather disc-shaped ossifications, while the
ischia are more elongated. The pelvis consequently still bears a great
resemblance to that of the Microsauri, and thereby also to the
Stegocephalous condition, but the ilium seems to be attached to more than
two vertebrae. The vertebrae are deeply biconcave, perhaps even with a
persistent continuous chorda. The neural arches bear high, laterally
compressed spines, but no diapophysial or lateral processes, the ribs being
placed mostly intervertebrally and having lost their tubercular portions.
The ribs are continued to about the sixth caudal vertebra. Intercentral
wedges exist in an unbroken series between all the vertebrae from the atlas
to the tail, where they are represented by movable chevrons. A costal
sternum seems to be absent, unless it was quite cartilaginous. The
shoulder-girdle is complete, consisting of a long interclavicle, clavicles,
disc-shaped coracoids, and scapulae; but there are no cleithra, and no
indication of precoracoids or even notches in the coracoids. The fore- and
hind-limbs are complete and primitive, with five digits. The abdomen is
protected by numerous oat-shaped little {291}ossifications, which are
arranged in many transverse or rather chevron-shaped rows, still greatly
resembling the condition prevailing in the Microsauri, except that they
have sunk deeper into the skin, being no longer directly covered by the
scales. The skull, being no longer completely encased by bones, and
possessing now wide supra- and infra-temporal fossae, appears at first
sight much like that of a generalised lizard, except that it possesses
three very conspicuous and distinct arcades in the temporal region: namely,
the orbito-squamosal bridge across the temporal fossa, formed by the
postorbital and squamosal; the arch formed by the squamosal with the
postero-lateral buttress of the parietal; and the infratemporal arch or
jugal bridge. The jugal itself is long, connecting the quadrato-jugal with
the maxillary and lacrymal, and sending up an ascending process to the
postorbital bone, thus taking a considerable share in the formation of the
orbit. The quadrato-jugal is small, apparently fused with the quadrate,
which itself is firmly overlaid by the squamosal. The quadrates are further
fixed by being buttressed by the pterygoids, which rest upon short
basisphenoid processes and extend far forwards, meeting the vomers and
separating the palatines. The premaxillae are short, the nares small and
terminal, the nasal bones are large. There is a small interparietal
foramen. The teeth are acrodont and pointed, forming unbroken series on the
premaxillaries, maxillaries, palatines and dentaries, and there are
scattered little teeth on the vomers.
_Palaeohatteria longicaudata_ from the Lower Red Sandstone of Saxony. Total
length about 18 inches, with six cervical, twenty trunk, three or four
sacral, and about fifty caudal vertebrae. The teeth are ankylosed with the
supporting bones. The five fingers have 2, 3, 4, 5, 3 phalanges
respectively. For the skull see Fig. 54, G, p. 280. _Telerpeton elginense_
from the Triassic sandstone of Scotland, and perhaps _Saurosternon_ of the
South African Karroo sandstone seem to be allied.
_Protorosaurus_ (πρῶτος = first, ὤρα = spring, or dawn, not
_Proterosaurus_) apparently several species, e.g. _P. lincki_ in the Upper
Permian (marl-slate and magnesian limestone) of Thuringia and Durham. About
4 or 5 feet long, and in its general appearance rather like a
Monitor-lizard, with about eight cervical vertebrae, most of which carry
slender backwardly-pointing ribs, sixteen long-ribbed trunk-vertebrae,
followed by three or four {292}sacrals and more than thirty caudals, some
of which have bifurcated spinous processes.
SUB-ORDER 2. RHYNCHOCEPHALI.–The ventral pelvic bones resemble those of
lizards and enclose a wide pubo-ischiadic foramen. There are only two
sacral vertebrae. The abdominal ribs are closely packed, each transverse
set consisting of only three rod-shaped pieces instead of many small
oat-shaped nodules. The intercentra are sometimes suppressed in the
trunk-region.
_Rhynchosaurus_ from the Upper Trias of Warwickshire and Shropshire, and
_Hyperodapedon_ of the same age, found at Elgin, in Warwickshire, and also
in Central India, are rather large, _H. gordoni_ measuring 6 feet in
length. Both have a short, broad, and stout cranium, and curved down,
toothless premaxillae, hence the name Rhynchocephali; the nares are
confluent; the teeth are numerous and small, and are liable to be worn down
so that the animals ultimately bite with the edges of the jaws, to which
the teeth are ankylosed. The premaxillaries of _Rhynchosaurus_ are curved
downwards over a slightly upcurved, likewise toothless process of the
mandibles, which form a strong symphysis. All the teeth are very small,
absent, or minute on the mandibles, forming one series on the maxillae,
several rows on the vomers and especially on the palatines, which latter
remain separated from each other. _Hyperodapedon_ seems to have lost the
intercentra; its vertebrae are solid, those of the neck are opisthocoelous.
The interparietal foramen is likewise abolished. The hook-shaped end of the
curved-down premaxillae fits into a bifurcation of the mandibles in front
of their stout symphysis. The teeth are similar to those of the other
genus. Whilst these, the earliest known genera of Rhynchocephali, are
already in various ways rather specialised, _e.g._ the hooked beak and the
loss of the intercentra, the two following fossil genera, although of much
later date, namely Upper Triassic, are more closely allied to the recent
_Sphenodon_.
_Homoeosaurus pulchellus_ and other species in Germany are only 6 to 8
inches long. The vertebral column consists of twenty-three presacral and
many caudal vertebrae. The first five cervicals are devoid of ribs.
Intercentra are restricted to the neck and the anterior portion of the
tail. The mandibles are not fused together. The nares are divided by a bony
septum. Each premaxillary has one rather broad tooth.
{293}[Illustration: FIG. 58.–_Sphenodon punctatum._ × ⅓.]
{294}The teeth of the maxillaries and mandibles are triangular, much worn
down in front. The ribs are devoid of uncinate processes. Closely allied
but larger is _Sauranodon_ of France, which has lost the upper teeth and
uses the sharp margins of the jaws instead.
_Pleurosaurus_ of Germany and France, about 5 feet in length, is remarkable
for the shortness of its still pentadactyle extremities, for its short
neck, and very long tail;–an interesting parallel to what has happened in
many genera of recent lizards.
_Sphenodon_ s. _Hatteria_ is the sole surviving member of the whole group
of Prosauria, and is represented by one species only, _S. punctatum_, in
New Zealand. As the last living witness of bygone ages this primitive,
almost ideally generalised type of reptiles, this "living fossil," deserves
a detailed description.
Total length of very large male specimens up to two feet and a half; in
general appearance like many a stoutly built lizard. The general colour of
the skin is dark olive-green with small white or yellowish specks on the
sides. A series of slightly erectile spines of yellowish colour extends
from the top of the head to the end of the tail, but is interrupted on the
neck; they are cutaneous, covered with a thin sheath of horn. The under
surface is covered with numerous scales, arranged in transverse rows; the
rest of the body is rather granular. The tail is thick, slightly compressed
laterally. The eye is large, dark brown, with a vertical pupil.
Those who are satisfied with superficial resemblances still group this
creature with the lizards, but it reveals itself as a primitive reptile or
Prosaurian by the following characters, every one of which distinguishes it
from the lizards:–The temporal region is bridged by three bony arcades. The
large vomers, palatines, and pterygoids form a broad bony roof to the
mouth; the large quadrates are firmly fixed by the pterygoids, squamosals,
lateral occipital bones, and by the jugal bridge. The vertebrae possess an
unbroken series of intercentral wedge-bones. There is an elaborate system
of abdominal ribs. The humerus has an entepicondylar foramen, and there is
also, in contradistinction to the fossil Rhynchocephalia, an ectepicondylar
foramen for the passage of the radial nerve. The carpus still has the
primitive number of ten bones, all of which remain separate, including the
intermedium. Of soft parts are to be mentioned above all the entire absence
of external copulatory organs, _Sphenodon_ being the {295}only recent
reptile which is devoid of them; a most primitive condition, sufficient by
itself to separate this creature from all the other living reptiles.
[Illustration: FIG. 59.–A, Dorsal; B, ventral; C, left-sided view of the
skull of _Sphenodon_. × 3/2. _Col_, columella auris; _Cond_, occipital
condyle; _E.P_, ectopterygoid; _F_, frontal; _Jug_, jugal; _Max_,
maxillary; _Na_, nasal; _No_, anterior nasal opening; _Pal_, palatine;
_Par_, parietal; _Pmx_, premaxillary; _Prf_, prefrontal; _Pt.f_,
postfrontal and postorbital; _Ptg_, pterygoid or endopterygoid; _Q_,
quadrate and quadrato-jugal; _Sq_, squamosal; _Vo_, vomer. See also Fig.
54, B, p. 280.]
The supratemporal bridge is formed by the squamosal and postorbital (Fig.
59, C, _Pt.f_), the latter being continued forwards and fused with the
postfrontal (A, _Pt.f_). The postorbital joins the ascending branch of the
jugal, both together forming the hinder border of the orbit; this is
bordered below chiefly by {296}the maxillary, which is long, while the
anterior process of the jugal is much reduced. There is no pre-orbital
fossa. The nares are terminal and lateral, well separated by the
premaxillaries. The posterior temporal bridge is formed by the squamosal
and parietal, the bridge extending laterally over the quadrate and
enclosing a wide space between itself and the buttress-like expansion of
the lateral occipital bone. The space enclosed between this occipital
buttress, the quadrate, and the pterygoid support of the latter is likewise
very large; it is of course the cavity of the middle ear, and as such is
crossed by the columellar chain of the ear. The infratemporal bridge or
jugal arch is formed by the jugal, which joins the descending process of
the squamosal, and by the quadrato-jugal, which is small and fused with the
quadrate. The latter is consequently very firmly fixed.
The teeth are acrodont, ankylosed in one series with the supporting bones,
triangular and much worn down in older specimens. Originally there seem to
be several in the premaxilla, but the adult bite with the somewhat
curved-down portions of the premaxillaries themselves, or with what remains
of the fused bases of the original teeth, which then, together with the
bone, look like one pair of large chisel-shaped incisors. The lateral edges
of the palatines likewise carry teeth, those of the mandibles fit into the
long slit-like space between the palatine and the maxillary teeth. Young
specimens have a few small teeth on the vomers, which are large, and
separate the long choanae from each other. The pterygoids form an anterior
symphysis, posteriorly they rest upon short processes of the basisphenoid
and send short flanges to the quadrates.
The vertebral column is very primitive. The atlas is still typically
temnospondylous. The first intercentrum or fused pair of basiventrals is
broad and thick, and forms the ventral half of the atlas-ring, which
articulates with the first centrum and with the second intercentrum. The
irregularly shaped neural arches remain separate from each other and from
the centrum; they carry on the dorsal side a pair of disconnected
supradorsals, the so-called pro-atlas. The second intercentrum is fused
with the first and second centrum. The second to ninth intercentra have low
median ridges or knobs, and are as a rule more firmly attached to the
cranial ends of the centra. Those of the trunk are small. From the third or
fourth caudal vertebra {297}backwards they appear as chevrons, articulating
more with the vertebra in front than with the one behind. The bases of the
right and left chevrons are frequently fused across so that the caudal
canal is completely surrounded by bone, a feature common in Dinosaurs.
Every intercentrum, be it a pair of chevrons, or an unpaired nodule, or
crescent, extends dorsalwards into a fibro-cartilaginous ring which
surrounds the chorda. The centra of the vertebrae are deeply amphicoelous,
the cavity being filled throughout life by the chorda; but the middle of
the centra is solid. Most of the caudal vertebrae are transversely divided
into two parts, the posterior of which carries the greater share of the
arches; they resemble in this respect those of lizards, and the lost tail
is likewise reproduced. The first three ribs are represented by bands of
connective tissue. The first is attached to the side of the first
intercentrum; the second arises from the second intercentrum, and forms a
small tubercle on the side of the second centrum; the third behaves
similarly. The vertebral arteries and lateral strands of the sympathetic
nerve-chain pass through these double basal attachments of the reduced
ribs. The other ribs are osseous; they possess short capitula which retain
their partly intercentral attachment, while the short tubercula are carried
by low processes of the centra, not of the neural arches. Already in the
thoracic region both capitulum and tuberculum merge into one facet, at
first dumb-bell shaped, further towards the tail oval, gradually shifted
backwards and dorsalwards upon the middle of the centrum, until the facet
reaches and ultimately lies right across the neuro-central suture. The
first few caudal vertebrae also possess ribs, which are however very short
and fuse with the diapophyses, immediately below which lies the
neuro-central suture.
[Illustration: FIG. 60.–The first three cervical vertebrae of _Sphenodon_.
1, 2, 3, 4, Intercentra; _C_{1}-C_{3}_, centra; _N_{1}-N_{3}_ neural
arches.]
The whole column consists of twenty-five presacral, two sacral, and about
thirty caudal vertebrae. Some of the thoracic ribs have cartilaginous
uncinate processes. Three or four pairs of ribs join a typical sternum,
into the antero-lateral portion of which are let in the coracoids. The
sternum is raised into a low median crest which fuses with the posterior
branch of the {298}T-shaped interclavicle, while the lateral branches of
the latter fuse with the clavicles. The coracoids are broad and entire,
still without fenestrae or notches indicative of precoracoids. The
parasternum is very elaborate; it extends from the sternum to the pubic
bones, and consists of about twenty-four transverse rows, each of which is
composed of a median and two lateral splint-bones. They are irregularly
shaped, partly with imbricating hooks, and are firmly attached to, in fact
still connected with, the deeper portions of the cutaneous scales of the
belly. The three pairs of pelvic bones are fused together at the
acetabulum. Pubes and ischia each form one symphysis, and these are
connected with each other by partly ossified cartilage and ligaments, so
that the originally heart-shaped space is divided into a pair of ovals. The
lateral processes of the pubes are thick, but very short. The ischia have
postero-lateral processes. There is also a mostly cartilaginous, unpaired
hypo-ischium.
The fore- and hind-limbs are still primitive in structure; both
pentadactyle. The carpus consists of ten, sometimes eleven pieces,
according to the single or double nature of the central element. The
proximal series is formed by the radiale, intermedian, and ulnare, with a
pisiform. The ulna and radius remain separate. The humerus has the usual
ectepicondylar in addition to the entepicondylar foramen common to all the
Prosauri and Theromorpha. The hind-limbs are typically plantigrade.
The tail is capable of regeneration, as in many lizards.
The development of this reptile has recently been studied and described by
Howes,[123] who quotes the literature bearing upon the whole subject.
A good account of the occurrence and habits of the "Tuatera" has been given
by Newman.[124] The Maoris call it "ruatara," "tuatete," or "tuatara," the
latter meaning "having spines." Formerly common on the main islands of New
Zealand, they are now apparently restricted to some of the islets in the
Bay of Plenty, North Island. Bush-fires, wild pigs, dogs and cats,
reptile-eating Maori tribes, and the advance of civilisation, have swept
them away except on some of the small uninhabited islands, difficult of
access, where they dig burrows, into which they retreat at the slightest
sign of danger. They sleep during the {299}greater part of the day, are
very fond of lying in the water, and they can remain below for hours
without breathing. They live strictly upon animals, but these are only
taken when alive and moving about. The kind of food seems to vary according
to the custom or fancy of the individuals. Sir W. L. Buller observed that
some of his captives stubbornly refused to eat until one day, rather
accidentally, minnows were offered. Others eat insects and worms; those
which live near the seashore not improbably eat also crustaceans. From
November to January they lay about ten eggs–white, hard-shelled, long and
oval–about 28 mm. long, in holes in the sand, where they can be warmed by
the sun. They are as a rule lazy in their movements. The usual pace is a
slow crawl, the belly and tail trailing on the ground, but when chasing
prey they lift the whole trunk off the ground. After running, or rather
"wobbling" three or four yards, they grow weary and stop. They cannot jump
the smallest obstacle.
Von Haast[125] has carefully examined their habitations on the Chicken
Islands. The Tuatara excavates its own hole, and this is shared sociably by
various kinds of Petrels. The entrance to the chamber is generally 4 or 5
inches in diameter, and the passage leading into the inner chamber is 2 to
3 feet long, first descending and then ascending again. The chamber itself
is one foot and a half long, by one foot wide and 6 inches high, lined with
grass and leaves. The petrel lives usually on the left side, the Tuatara on
the right side of the inner chamber. Whilst very tolerant of the bird with
its egg and young, it does not allow another of its own kind to live in the
same hole, which it is ready to defend by lying in such a manner that the
head is placed where the passage widens out into the chamber. On putting
one's hand or a stick into the burrow the Tuatara bites at them furiously.
They can run very fast, and defend themselves with great pluck against dog
or man by biting or scratching. As soon as the sun has set they leave their
holes to seek food. During the night, and especially during the pairing
season, they croak or grunt.
The eggs, having been deposited during the Southern summer, from November
to January or February, in holes on a sunny and sandy spot, contain nearly
ripe embryos in the following August. They are, however, not hatched until
about thirteen {300}months old. In the meantime they seem to undergo a kind
of aestivation. The nasal chambers become blocked with proliferating
epithelium, which is resorbed shortly before hatching.
I have kept half-a-dozen specimens in a green-house for several years, and
have come to the conclusion that they are dull, not companionable
creatures, in spite of their imposing, rather noble appearance when, with
their heads erect, they calmly look about with their large, quiet eyes.
Each dug its own hole in the hard ground underneath and between large
stones. At dusk they sat in front of the holes or walked leisurely to the
pan with the earthworms which formed their principal food. Meat they did
not touch, but they killed and chewed up lizards and blind-worms. Sometimes
they soaked themselves for many hours in the shallow, warm water. The skin
is shed in flakes. I never found them basking in the sun, and the pineal
eye, still so well developed in these strange creatures, caused them no
distress when bright light was thrown upon it. They grew tame enough not to
run away when found roaming about at night, but they did not like being
handled, and they inflicted the most painful bites when taken up
carelessly. The biggest, a male, was rather quarrelsome, grunted much, and
worried the others.
_SUB-CLASS III.–THEROMORPHA._
The Theromorpha comprise a great number of extraordinary, extinct reptiles,
which as a group had a wide range in space and time. The earliest known
occur in the Lower Red Sandstone of Thuringia and Bohemia, and in the
middle Permian strata of Russia. The majority have been found in strata
transitional between the Permian and the Triassic age, notably in the
Karroo sandstone of South Africa and in corresponding levels of North
America. Closely allied to them are those of the Triassic sandstone of
Elgin in Scotland, and of India. They seem to have died out with the
Muschelkalk or Middle Trias.
The various genera exhibit such a diversity of structure, shape, and size,
and many are still so imperfectly known, that {301}any diagnosis is liable
to be faulty, even assuming that they are a homogeneous group. To avoid
confusion, we characterise the Theromorpha as _Reptiles with a firmly fixed
quadrate, a single temporal arch, an interparietal foramen, and a pelvis in
which the pubes and ischia form one stout, ventral symphysis_.
The dentition is most abnormal, and permits the division of the Theromorpha
into two or three main groups. In the Pareiasauri the teeth of the upper
and lower jaws form rather even series of nearly equal size; smaller teeth
are carried by the palatal bones. In the Theriodontia the teeth are
differentiated in a truly Mammalian fashion into incisors, prominent
canines, and multicuspid or tubercular molars. Each tooth, and this applies
to all Theromorpha, is implanted in a separate alveolus; _Tritylodon_ only
seems to have double-rooted molars. The lower canines cross in front of the
upper, just as in Mammals. In _Placodus_, which probably belongs to this
assembly, the teeth are few in numbers, very broad and flat, especially
those of the palate. In _Dicynodon_ and _Gordonia_ the teeth are restricted
to a pair of conical, sometimes very large, tusk-like upper canines, and in
_Oudenodon_ the whole mouth is toothless.
The configuration of the skull shows two main types. In the Pareiasauri it
is completely roofed in by dermal bones, the only holes on the surface
being the nostrils, orbits, and the interparietal foramen.
The most striking feature of the second type of skull is the tendency to
form an almost Mammalian zygomatic arch by the junction of the much
elongated squamosal with the jugal bone, both abutting against a downward
process of the postfrontal bone. The skull shows a pair of wide
supratemporal foramina bordered by the parietals, squamosals, and
postfrontals. The composition of the temporal arch varies considerably in
detail, and in _Cynognathus crateronotus_ at least there is a small hole
within the arch, between the squamosal and jugal, probably the last remnant
of the otherwise absent infratemporal foramen. Except in the roofed-in
skulls of _Pareiasaurus_ and _Elginia_ there is no separate quadrato-jugal
element. The quadrate is firmly fixed by the overlapping squamosal, and the
whole pedicle for the support of the mandible is rather elongated, and
either stands vertically or slants forwards. The mandible itself is
compound. The pterygoids extend backwards so as to approach or reach the
{302}distal portion of the quadrate; separate ectopterygoids do not seem to
be developed. The shoulder-girdle consists on either side of a large
scapula, which is mostly directed obliquely backwards, and is fused with
the coracoid; a precoracoid is present or at least indicated by a notch or
foramen; it is usually fused with the other bones. At least some genera
possess a T-shaped interclavicle and clavicles; _Pareiasaurus_ possesses
also a pair of cleithra.
The pelvis is in every respect constructed upon the Mammalian plan. The
three constituent parts meet at the acetabulum, and the ventral bones,
pubes and ischia, form one broad symphysis, leaving two, sometimes very
small, obturator-foramina. The ilium is attached to one to five sacral
vertebrae, and since the whole pelvis slants obliquely downwards and
backwards, this sacral attachment is distinctly pre-acetabular, perhaps
most markedly so in _Dicynodon_. The limbs are mostly stout, humerus and
femur with strong crests; the feet are thoroughly plantigrade, with five
fingers and toes. The details of the carpus and tarsus are not well enough
known to permit of generalisation, but there is a tendency to form a heel,
and to develop the cruro-tarsal joint into the chief joint of the hind
feet. The vertebrae are amphicoelous, sometimes with rather thin-walled
centra, so that in these cases the chorda was continuous. Intercentral
wedges, or basiventral elements, are frequent in the cervical and caudal
regions. Most of the ribs, especially those of the neck, have a tuberculum
attached to the neural arch, and a distinct capitulum which articulates
either with the centrum or with the intercentrum, or lastly, if the latter
is absent, between two centra. The axis and atlas vertebrae are united.
The occipital condyle exhibits every stage between the single median knob
(_Pareiasaurus_) formed almost entirely by the basioccipital bone, a triple
condyle (_Dicynodon_) to which both lateral and the basioccipital bones
contribute, and a kidney-shaped or double condyle (_Cynognathus_) from
which the middle or basioccipital portion is more or less withdrawn.
Dermal bony armour reached an extraordinary development on the head of
_Pareisaurus_ and _Elginia_; whether other parts of the body were protected
is doubtful, but the flattened tops of the neural spines of _Pareiasaurus_
suggest that they carried bony scutes. Abdominal protective ossifications
are unknown. {303}Many of the Theromorpha[126] reached a considerable size,
massive skulls of one foot in length being not uncommon. The tail was
comparatively short.
The many resemblances of these strange creatures to Mammals have naturally
suggested that the Mammalia have sprung from some such Theromorpha or
"beast-shaped" animals. The resemblances are chiefly the dentition, the
zygomatic arch, the pelvis, the cruro-tarsal joint, the scapula which is
sometimes possessed of a spine, and the occasionally double occipital
condyle. The general shape of the skull of _Cynognathus_ is indeed
strikingly like that of a Carnivorous Mammal, and the shape of the whole
body suggests rather a Mammal than a reptile; and when we have to deal with
the fragmentary skulls of _Tritylodon_ (cf. p. 309) it is, indeed,
difficult to decide to which of the two classes such a creature belongs.
But the Theromorpha possess a number of important characters by which they
reveal themselves at once as reptiles: (1) the large and fixed quadrate
bone, which is still the sole support of the lower jaw; (2) the compound
mandible, which is composed of at least an articular, dentary, angular,
supra-angular, and splenial element; (3) the interparietal foramen; (4) the
possession of prefrontal and postfrontal bones, sometimes also postorbital,
supratemporal, and quadrato-jugal bones. Of course, any of these ancestral
bones may be lost, and the interparietal hole may be closed as in tortoises
and crocodiles. We can also imagine that the quadrate may be relieved of
its jaw-bearing function and become loosened, but this is not easy,
considering the strong development of the squamoso-quadrate pedicle. Those
Theromorpha in which the quadrate itself is small, whilst the squamosal
reaches down, or at least approaches the mandible, as in _Dicynodon_ and
_Gordonia_, are so hopelessly pledged, or specialised in other directions,
that it is impossible to connect them ancestrally with Mammals.
However, it is beyond reasonable question that the Mammals have sprung from
some reptilian stock (the attempts to derive {304}them from Amphibia,
without the intervention of Reptiles, are as gratuitous as they have proved
futile), and the Theromorpha undoubtedly comprise creatures which of all
animals approach nearest to Mammals, and coincide with them in most
important features. But we have not yet found a single Theromorph which can
claim to be a direct ancestor of Mammals. Since the latter occur already in
the Trias, we have to look for their reptilian forefathers at least in the
Lower Permian, and this naturally excludes all the known forms. The filling
up of this gap is but a question of time.
The ancestry of the Theromorpha themselves is also shrouded in mystery.
Attempts have been made to connect them with the Permian _Protorosaurus_,
_Palaeohatteria_, and _Eryops_. On the other hand, some retain various
Stegocephalous reminiscences (_e.g._ the roofed-in condition of the skull
by membrane-bones, amongst which, besides others, supratemporals and
postorbitals can be recognised; occurrence of cleithra in _Pareiasaurus_;
distinct epiotic bones in _Elginia_). Although they have died out as a
group, they have perhaps given rise to several side-branches, one of which
(leaving aside the question of Mammalian origin) seems to have flourished
as the Dinosauria.
We divide the Theromorpha into four orders, which are, however, liable to
run into each other, and it is reasonably to be hoped that many forms may
be discovered which will connect not only these provisional orders with
each other, but also with other sub-classes.
ORDER I. PAREIASAURI
Cranium completely roofed in by membrane-bones. The only foramina are the
nostrils, orbits, and the interparietal foramen. The teeth are
comparatively small, and stand in even series in both jaws.
_Pareiasaurus_, several species from the Karroo sandstone of South Africa.
_P. baini_ was an extremely clumsy brute, of most uncouth appearance,
standing between 2 and 3 feet high, and measuring with the short tail
nearly 8 feet in length. The skull is very massive, 18 inches long and
slightly broader, with a rugose, deeply pitted surface. The teeth are
thickly enamelled, serrated at the margin, with many pointed cusps; those
{305}of the vomer, palatines, and pterygoids are recurved and arranged in
several longitudinal rows. There is a small incisive foramen in the
premaxilla; the choanae lie within the pterygoids. The palate has a pair of
large lateral vacuities. Between the squamosal and quadrate is a small
foramen, as in _Belodon_ and _Sphenodon_. The nares are terminal, bordered
behind by the nasals, and divided by the premaxillaries. The occipital
condyle is a single knob, but the lateral occipital bones also partake in
its formation. The shoulder-girdle is strong. The scapula slants backwards,
is broad, and possesses a longitudinal spine, an almost exclusively
Mammalian character. The scapula, coracoid and precoracoid are fused
together, and are united ventrally with those of the other side. There is a
T-shaped interclavicle, a pair of clavicles, and a pair of slender, long
cleithra, which extend along the upper anterior margin of the scapulae. The
humerus possesses enormous crests. The broad ilium is attached to two, or
perhaps three, sacral ribs. The acetabulum is closed. The pubes and ischia
are united into one broad mass of bone, and the obturator-foramina seem to
be just large enough to permit of the passage of the nerve. Both fore- and
hind-limbs are plantigrade and five-toed. The tibia articulates with one
large bone, which is supposed to represent the united astragalus and
calcaneum, the latter being without an indication of a prominent heel,
although there is a tendency to develop the crurotarsal into the chief
joint. The number of vertebrae amounts to eighteen presacrals, eight to ten
of which are cervicals. There are two or three sacral and about twenty-four
mostly shortened caudal vertebrae. The latter possess intercentral wedges
and chevron-bones; wedges occur also between the cervical and some thoracic
vertebrae. Some of the posterior cervical ribs are very peculiar–straight,
broadened out, turned backwards, partly overlapped by one another, and 18
inches long, recalling the first two ribs of the crocodiles. Sternum and
abdominal ribs are unknown.
_Elginia mirabilis._–The skull (Fig. 54, A, p. 280)–nothing else is
known–indicates one of the most remarkable reptiles hitherto found on this
side of the Atlantic. It was discovered in the Red Sandstone of Elgin
(Lower Trias). The skull reminds us in its general shape and by its spikes
and horns of the little American Iguanoid lizard, _Phrynosoma_. The length
of the cranium is about 6 inches, the distance between the tips of the two
largest {306}horns measures 9 inches. The teeth are small and resemble
those of an _Iguana_ in their shape and finely serrated edges, indicating
herbivorous habits, but there are also several rows of smaller teeth on the
palate, the configuration of which is not unlike that of _Sphenodon_. The
top and sides of the skull, except the interparietal foramen, the orbits,
and nostrils, are completely encased by rugose, pitted, dermal bones, most
of them with strange, horn-like spikes. In the encasement of the temporal
region can be discerned a postfrontal, parietal and squamosal, a conically
projecting epiotic, a postorbital and supratemporal, a jugal and a
quadrato-jugal, which latter almost completely covers the quadrate bone.
The interparietal foramen lies far forwards, almost on a level with the
orbits. The nostrils are terminal, surrounded by the short nasals, the
maxillaries and the premaxillaries, which latter divide them.
ORDER II. THERIODONTIA.
The cranium is not roofed in, but shows a pair of large supratemporal
fossae, bordered below by the zygoma, which is formed mainly by the
squamoso-jugal bridge, and is shut off from the orbit by the postfrontal
joining the bridge. The teeth are differentiated into incisors, canines,
and molars (Fig. 54, C, p. 280). The lower canines close in front of the
upper.
_Cynognathus_, Karroo formation of South Africa. _C. crateronotus_ has a
skull about 16 inches long, looking like that of a ferocious Carnivore;
there are four incisors, huge canines, and nine molars, the latter with
serrated edges and anterior and posterior cusps. The wide supratemporal
fossa is bordered and closed behind by the broad lateral extension of the
parietal, which joins a similar extension of the squamosal bone. The latter
is very long, extending to the postfrontal and to a bone which, bordering
the orbit posteriorly, is either an upward branch of the jugal, or a
postorbital bone; the latter interpretation is made probable by the
occurrence of a suture with the jugal in _C. platyceps_. The jugal bone is
very long, beginning at the quadrate, running along the squamosal, and
forming the lower border of the orbit.
The number of vertebrae is large, there being as many as twenty-nine
presacrals, six of which belong to the cervical region. {307}The atlas is
fused with the axis; most of the thoracic ribs articulate partly upon the
intercentra. The lumbar ribs are very peculiar; they are much expanded
horizontally, and overlap each other, forming thereby intercostal foramina.
The broad ilium is attached to three or four sacral ribs. The acetabulum is
closed. The ventral side of the pelvis shows a broad symphysis and has a
pair of obturator-foramina. The scapula is large, directed backwards, and
shows a distinct, very Mammalian spine; it is fused with the coracoid and
precoracoid.
The occipital condyle of _C. platyceps_ is kidney-shaped, with the
concavity directed upwards; in _C. berryi_ it is separated into two
distinct knobs, the middle, basioccipital portion being apparently wanting.
The mandible possesses a long coronoid process which ascends obliquely into
the temporal fossa.
_Aelurosaurus_, _Lycosaurus_, _Galesaurus_, and many others, likewise of
the Karroo formation. In the first genus the splenial bones help to form
the symphysis of the lower jaw; teeth are also found on the palate, in
opposition to _Lycosaurus_. This has a skull 6 inches in length; the dental
formula on either side is _i._ 4/3, _c._ 1/1, _m._ 5/5; the molars are
slender, conical, and recurved. _Galesaurus_ seems to have been rather
small, the low, triangular skull measuring only 2 to 3 inches in length,
with four or five sharply pointed incisors, prominent canines and four or
five small multicuspid or deeply serrated little molars.
_Endothiodon_, with several species from the Karroo formation, is of
uncertain systematic position, only imperfect skulls being known. The
animals must have been large and bulky, the skulls being very massive and
at least one foot in length. The premaxillaries and the maxillaries are
toothless, their alveolar borders forming cutting, prominent edges. The
same applies to the very strong lower jaw; but there is a pair of
tooth-like stout projections in the upper and lower jaws in the place of
canine teeth. True, enamelled, small, apparently conical or low and perhaps
blunt teeth occur on either side in one or three longitudinal series upon
the palate, and in corresponding positions on the inner sides of the two
halves of the lower jaw. It is doubtful if the upper teeth are carried by
the palatines or by the broadened inner flanges of the maxillaries. The
choanae seem to lie between the pterygoids and the palatines, incompletely
roofed in by ventral extensions of the latter towards the middle line.
{308}Direct affinity of _Endothiodon_ (ἐνδοθί, within) with _Placodus_ is
unlikely; the same applies to the Dicynodontia, although the restriction of
the teeth to the palate seems to point as much to the former genus as do
the toothless cutting edges of the jaws to the forms like _Oudenodon_.
Other Theriodont reptiles have been described from the upper Permian of
Russia, for instance _Deuterosaurus_ and _Brithopus_, but the determination
rests upon insufficient fragments. North America has yielded many strange
Theromorphous fossils, some of which may belong to the Theriodont order,
while others seem to be intermediate between this and the other orders.
_Diadectes_ of Texas, for instance, seems to be a Theriodont creature;
while in _Empedias molaris_, with a skull about 8 inches in length, the
teeth form an uninterrupted series without distinct canine tusks, and the
incisors are distinguished from the molars only by the transversely
broadened shape of the latter. Very small teeth are arranged along the
median line of the vomer and united palatine bones. In _Clepsydrops_,
_Dimetrodon_, and _Naosaurus_ of Texas the teeth are differentiated into
incisors, canines, and molars, although not so regularly as in the typical
Theriodont forms described above, one or more pairs of teeth being enlarged
into canine-like tusks. In the latter two genera the spinous processes of
the thoracic vertebrae are enormously elongated, standing up vertically to
a height of 2 feet, while the centra of the vertebrae measure only one inch
in diameter. In _Naosaurus claviger_ these upright spines carry on either
side half a dozen transverse projections. _Stereorhachis_ of the Permian of
France is typically Theriodont in the structure of its shoulder-girdle,
humerus, and pelvis, but the dentition is composed of 3/3 incisors, no
canines, and 6/10 pointed molars.
The following genera have been placed by Seeley in the family
Gomphognathidae. _Microgomphodon_, with broader and less prominently
multicuspid teeth than those of the typical Theriodonts, seems to lead to
_Gomphognathus_, which has the following dentition: _i._ 3/3, _c._ 1/1,
_m._ 12/12, with a long diastema between the canines and molars, some of
which latter are nearly as broad as they are long, and have comparatively
low tubercles on the crowns. The skull is remarkably like that of a
Carnivorous Mammal. There are incisive foramina behind the premaxilla. The
maxillaries and palatines form a united palatal roof, and behind them open
the {309}choanae. The occipital condyle is kidney-shaped. The mandible is
most extraordinary, approaching that of the Mammalian, especially the
Marsupial type, except that it is still composed of several pieces. The
articular facet for the mandible is borne by an outward or lateral
projection, while the bulk of the posterior half of the jaw projects
inwards like a broad flange, undoubtedly recalling the so-called inner
inverted angle of the Marsupial jaw. The coronoid process is large and
extends far into the temporal fossa. Nearly the whole skeleton of
_Microgomphodon_ is known; the lumbar ribs are broadened and overlap as in
_Cynognathus_, and the mandible is typically compound, so that there is no
doubt about the affinities of this genus with the Theriodontia. It throws
light upon _Gomphognathus_ and the three likewise South African genera
_Diademodon_, _Trirachiodon_ and _Tritylodon_, which are all known from
imperfect skulls only. Their teeth are restricted to the jaws, the molars
have flat, multitubercular crowns and bear an extraordinary resemblance to
those of Mammals. Some of the molars of _Tritylodon_ are said even to
possess two roots, but this point, absolutely unique in Reptiles, but
common in Mammals, is not certain. The few upper incisors of _Tritylodon_
are rather large, chisel-shaped, and extend like those of the Rodent-type
back into the maxillaries; canines are absent, leaving a diastema.
_Trirachiodon_ has prominent canines, the five upper molars are
multitubercular, rather flat, and much broader transversely than in the
longitudinal direction. Still, even these creatures, with skulls of the
size of that of a small fox, possessed distinct prefrontal and postfrontal
bones, and are, at least in this respect, typical Reptiles.
ORDER III. ANOMODONTIA.
The cranium is not roofed in. The pedicle for the suspension of the lower
jaw is much elongated, slants slightly forwards, and is composed of the
long quadrate, which is laterally overgrown by the squamosal bone. The
teeth are restricted to a pair of strong, tusk-like canines, or they are
altogether absent. The margins of the upper and especially those of the
lower jaw are trenchant, and were possibly furnished with a thick horny
armature like those of tortoises.
{310}_Dicynodon_, with many species from the Karroo formation of South
Africa, reached formidable dimensions. The thick, curved skull is in size
and outline not unlike that of a large lion, hence _D. leoniceps_, _D.
tigriceps_, etc. The zygomatic arch is almost mammalian, except that the
posterior boundary of the orbit is formed by a distinct postfrontal bone.
The nostrils are lateral. The canine tusks (Fig. 54, E, p. 280) are very
large. The choanae open behind the rhomboid vomer and between the separated
palatine bones, which are posteriorly confluent with the medially united
pterygoids. The latter send out flat extensions, along the lateral side of
the palatines; these extensions reach the maxillaries and probably
represent the ectopterygoids. The occipital condyle is distinctly triple,
being equally composed of the basi- and latero-occipital bones.
The three bones of the shoulder-girdle meet at the glenoid fossa; the
scapula has the indication of a spine. The pelvis is stout, attached to
four or five vertebrae, converting the latter into a very Mammalian-like
sacrum, the position of which lies distinctly in front of the acetabulum.
The latter is closed, composed by the three pelvic bones. The pubes and
ischia are fused together, leaving only a very small obturator-foramen. The
limbs are plantigrade and pentadactyle, very stout; the humerus and femur
have enormous crests.
_Oudenodon_, of which several species have been described, is so much like
_Dicynodon_, except for the complete absence of teeth, that it has been
suggested that these skulls belong to females of this genus. This view is
strengthened by the fact that tusk-like canines exist, or are absent in
some of the species which have been described as _Cistecephalus_, a genus
closely allied to _Dicynodon_. The latter, which, like _Oudenodon_ and
_Cistecephalus_, occurred in Africa, extended also into India, _D.
orientalis_ having been found in the Panchet formation of Bengal, of
transitional age between the Permian and Triassic epochs. _Oudenodon
rugosus_, on the other hand, has been described from the Ural.
_Gordonia_ and _Geikia_, of the New Red Sandstone of Elgin, are known from
their skulls only, but these are so well preserved that there is no doubt
about their close relationship to the typical South African Dicynodontia.
The skull of _Gordonia_ is about 7 inches long and 4 inches high. The
canines (Fig. 54, D, p. 280) are reduced to short, but thick, conical
tusks. The most {311}remarkable feature is the very elongated
squamoso-jugal arch, which arises moreover from the dorsal end of the long
squamoso-quadrate pedicle. The two wide and long temporal fossae are
dorsally divided by narrow parietal crests. There is a distinct
interparietal bone, and the usual interparietal foramen. The choanae are
united and lie within the palatines, which themselves are united; the large
lateral palatal foramina are otherwise enclosed by the pterygoids,
quadrates, and laterally by the squamoso-jugal arch.
ORDER IV. PLACODONTIA.
These are the latest and last members of the Theromorpha, unfortunately
known from skulls only, from the Muschelkalk or Middle Trias of Germany and
Russia. The skull of _Placodus gigas_ is about one foot long, rather high
and triangular owing to the lateral expansion of the temporal arches, which
diverge posteriorly. The squamoso-jugal arch is very broad, and most of the
posterior border of the orbit is formed by the large postorbital bone. The
maxillary bone seems to extend back to beyond the level of the orbits. The
choanae lie behind the premaxillaries. The palatines and pterygoids are
fused in the middle line, forming a broad bony palate, which, owing to the
broad, posteriorly extended wings of the pterygoids, much resembles that of
the crocodiles. The teeth are very remarkable. There are two or three
stout, conical, or chisel-like teeth in each premaxillary bone, and three
to five broad and flat maxillary teeth; three pairs of huge, broad, and
quite flat teeth are crowded together and fill up the whole vomerine and
palatine portion of the palate. These crushing teeth indicate that
_Placodus_ probably lived upon hard-shelled molluscs, and this would be in
conformity with its occurrence in the Muschelkalk, which is a strictly
marine deposit and full of shells. Another closely allied genus is
_Cyamodus_, one species of which is known from Russia. The teeth are fewer
in number and not so large as those of _Placodus_.
{312}CHAPTER IX
CHELONIA–ATHECAE–THECOPHORA
_SUB-CLASS IV.–CHELONIA._
There is no mistaking a tortoise. The shell and the horn-covered toothless
jaws separate them from all other four-footed creatures.
They may be described as terrestrial or aquatic, pentadactyle reptiles,
with walking limbs or with paddles; ribs with capitular portions only, two
sacral vertebrae, humerus with entepicondylar foramen, pubes and ischia
forming symphyses, quadrate bones fixed, jaws without teeth, but with
cutting horny sheaths. Trunk encased in a bony shell, composed of numerous
dorsal and ventral dermal bones, forming a carapace and a plastron, which
may or may not be covered with horny shields. Copulatory organ unpaired,
cloacal opening more longitudinal than round, never transverse. Oviparous.
It is customary to distinguish the marine, paddle-limbed kinds as
_Turtles_, the others as _Land-_ and _Water-tortoises_.
Tortoises occur already in the Trias. They reached their greatest
development towards the end of the Mesozoic and in the earlier Tertiary
periods. They are now comparatively reduced in the number of families and
genera, although they are still represented by about 200 species. The
sub-class as a whole is cosmopolitan, but does not occur in the colder
regions.
Their origin is quite unknown. Of recent groups only the Crocodilia and the
Rhynchocephalia come into consideration. Combination of these groups with
the Chelonia leads to some unknown forms whence also the Theromorpha have
arisen. Palaeontology does not help us, all the leading, main groups of
Chelonia having been in existence in the earlier Mesozoic ages, {313}and
Palaeozoic Chelonia are still unknown. We can, however, to a certain
extent, reconstruct an ideal primordial Chelonian by assigning to it all
the ancestral characters actually observed in recent and fossil kinds, and
by reducing to simpler conditions those features which we know to be more
or less exaggerated specialisations. It is reasonable to assume that
originally each metamere, except those of the anterior half of the neck and
the posterior half of the tail, carried a transverse series of dermal
plates, covered with horny shields, while the trunk, according to the
greater bulk of the body, increased in size, converging towards the root of
the neck and tail. By concentration, reduction of the number, and increase
in the size of some of the remaining plates and shields, the skull assumed
its characteristic box-like shape, the neck and tail becoming at the same
time free. Chelonia are without doubt descendants of terrestrial, or at
least semi-aquatic reptiles, and the marine paddled forms subsequently
developed from terrestrial kinds.
CLASSIFICATION OF CHELONIA.–After many vicissitudes it was recognised that
the Chelonia cannot naturally be divided according to the modification of
their feet. The TRIONYCHOIDEA were clearly separated from the rest by
Stannius in 1854. Cope, in 1870, was the first to emphasise the important
character of the mode in which the neck is either bent sidewards
(PLEURODIRA) or withdrawn in an S-shaped curve in a vertical plane
(CRYPTODIRA); and he also separated _Sphargis_ as ATHECAE from all the
other Chelonians, for which Dollo in 1886 proposed the term THECOPHORA. The
division of the latter into recognisable families, based upon reliable,
chiefly internal, skeletal, characters, has been effected by
Boulenger;[127] and his classification has been adopted in the present
volume, after intercalation of the more important fossil forms. The
relationships between these various families may perhaps be indicated as
follows:–
{ ATHECAE Sphargidae
{
{ { _Pleurodira_ { Pelomedusidae
{ { { Chelydidae–Carettochelydidae
{ {
CHELONIA { THECOPHORA { _Cryptodira_ { Chelydridae–Dermatemydidae–
{ { { Cinosternidae
{ { { Platysternidae
{ { { Testudinidae–Chelonidae
{ {
{ { _Trionychoidea_ Trionychidae
{314}The guiding taxonomic characters are fully mentioned at the head of
the different families, and are mostly internal. The following "key,"
adapted from Boulenger, and based upon external characters, is preferable
for practical purposes.
_For the position and names of the horny shields_ see Fig. 61 on p. 315.
Shell covered with horny shields.
Digits distinct, with 5 or 4 claws.
Pectoral shields separated from the marginals by inframarginals.
Tail long and crested. Plastron small and cruciform.
North America _Chelydridae_, p. 338.
Tail long, covered with rings of shields.
Plastron large. Indo-China _Platysternidae_, p. 345.
Tail short. North and { _Dermatemydidae_, p. 341.
Central America { _Cinosternidae_, p. 342.
Pectoral shields in contact with the marginals.
Plastral shields 11 or 12, without an intergular.
Neck retractile in an p. S-shaped vertical curve
_Testudinidae_, p. 345.
Plastral shields 13, an intergular being present.
Neck bending sideways under the shell
{ _Chelydidae_, p. 399.
{ _Pelomedusidae_, p. 390.
Limbs paddle-shaped, with one or two claws _Chelonidae_, p. 378.
Shell without horny shields, covered with soft, leathery skin.
Digits distinct, broadly webbed, but with only
three claws _Trionychoidea_, p. 404.
Limbs paddle-shaped.
Shell composed of regular series of
bony plates. Two claws _Carettochelydidae_, p. 404.
Shell composed of very many small plates arranged like
mosaic. No claws _Sphargidae_, p. 333.
The VERTEBRAE are, sometimes in the various regions of the same individual,
amphi-, opistho- or pro-coelous, or even biconvex. Traces of the chorda
remain longest in the middle of the centra. Intercentra occur regularly on
the first two or three cervicals, and then again in the tail as paired or
unpaired nodules, or as short chevrons. The latter occasionally fuse with
the caudal end of their centra. Intercentral discs of fibrous cartilage
occur regularly in the neck and tail. The ribs develop originally in the
same transverse level with these discs, and frequently the anterior
thoracic vertebrae retain this intercentral or intervertebral position
throughout life. Farther back they often show a gradual change from the
intercentral to a more central and ultimately {315}remarkable to a purely
neural attachment. In all the Chelonia the ribs are devoid of the
tubercular portion.
[Illustration: FIG. 61.–Various plastra and their horny shields. 1,
_Testudo ibera_; 2, _Macroclemmys temmincki_; 3, _Cinosternum odoratum_; 4,
_Sternothaerus nigricans_; 5, _Chelodina longicollis_; 6, _Chelone mydas_.
_a_ or _an_, Anal shield; _abd_, abdominal shield; _f_ or _fem_, femoral;
_g_ or _gul_, gular, unpaired in Fig. 3; _h_ or _hum_, humeral shield; _i_
or. _int.g_, intergular; _im_, infra-marginals; _m_, marginals; _p_ or
_pect_, pectoral; _x_, in Fig. 1, inguinal shield constituting, with the
axillary _xx_, the last trace of infra-marginals.]
The cervical vertebrae have no ribs, except mere traces in the shape of
small nodules. On the tail the ribs are often large, and, when fused with
their neural supports, look like transverse processes; the whole
arrangement exactly resembles that of Crocodilia. The first pair of
thoracic ribs, those borne by the ninth vertebra, are peculiar. They arise
from the anterior portion of the centrum, are much reduced, sometimes to
mere threads of bone, and lean against the anterior rim of the second pair
of ribs, in many cases without reaching the carapace. The next following
ribs, those of the tenth to the sixteenth vertebra, are intimately involved
in the formation of the first to seventh costal plates. The ribs of the two
sacral vertebrae sometimes remain quite distinct throughout life, just
touching the upper {316}ends of the iliac bones; but since these find a
much more effective support in the shell, the distal ends of the sacral
vertebrae fuse with the eighth, or so-called last, pair of costal plates.
The neural arch of the ninth vertebra rests upon its centrum; but the
neural arches of the other trunk-vertebrae, although long, rest upon two
centra; retaining, like the ribs, their original intercentral position; and
in most cases the neuro-central sutures remain throughout life. The atlas
and the last cervical vertebra deserve special attention. In many
tortoises, _e.g._ _Trionyx_, _Clemmys_, _Testudo_, the three constituent
parts of the atlas, namely, the neural arch, the centrum, and the
intercentrum or first pair of united basiventralia, do not ankylose, but
remain loosely connected; and the first centrum, instead of forming an
odontoid process, remains movably attached to the second centrum, although
it sometimes carries, and fuses with, the second intercentral piece. In
other tortoises, _e.g._ _Platemys_ and _Chelys_, however, all the parts of
the atlas co-ossify and form a complete, solid vertebra which articulates
by a concavo-convex joint with the centrum of the second vertebra. The
normal number of cervical vertebrae is eight in all Chelonians. The first
spinal nerve issues between occiput and atlas, all the others behind the
neural arches of their vertebrae. The last, or eighth cervical, owing to
the retractility of the neck, forms elaborate joints; its centre fits with
a knob into a cup of the ninth, and its post-zygapophyses form broad,
curved articulating concave facets for the reception of the anterior
zygapophyses of the fixed ninth vertebra. In the Trionychidae the
zygapophyses are most elaborate, and they alone articulate with the ninth
vertebra, while the centra do not join, but remain, or rather become,
separated by partial resorption. In the Chelonidae, in conformity with the
non-retractile and short neck, all the cervical joints are much reduced.
[Illustration: FIG. 62.–1, The complete atlas of an adult _Trionyx hurum_.
The second basiventral (white) is attached to the posterior end of the
first centrum, which, not being fused with the second centrum, is not yet
an odontoid process. 2, The complete atlas of an adult _Trionyx
gangeticus_, still typically temnospondylous. 3, The first and second
cervical vertebrae of an adult _Platemys_. 4, The complete atlas of a
_Chelys fimbriata_.]
{317}[Illustration: FIG. 63.–Skull of _Chelone mydas_. A, from the left
side; in B, the postfrontal and squamosal bones have been removed, and the
broad expansions of the jugal, quadrato-jugal, parietal, and quadrate bones
have been reduced in order to reduce the skull to more primitive
conditions. _F_, Frontal; _J_, jugal; _L.o_, lateral occipital; _Mx_,
maxillary; _Op_, opisthotic; _Pal_, palatine; _Par_, parietal; _Prf_,
prefrontal; _Pro_, pro-otic; _Pt.f_, postfrontal; _Ptg_, pterygoid; _Q_,
quadrate; _Qj_, quadrato-jugal; _S.o_, supra-occipital; _Sq_, squamosal.]
The SKULL (cf. Fig. 54, H, I, K, p. 280) agrees fundamentally with that of
_Sphenodon_ and of the _Crocodilia_, but it is characterised by several
special features. There are no ectopterygoids or ossa transversa; no
lacrymal bones, no interparietal or pineal foramen; the vomer is unpaired
and the nasal bones are mostly absent, unless they are fused with the
prefrontals. The premaxillae are very small. The single vomer forms a
septum between the choanae; and these are, except in _Sphargis_, ventrally
roofed over by wings sent out by the palatines. The latter form a
continuous bony roof to the mouth with the pterygoids, and these diverge
posteriorly, being connected suturally with the quadrates, lateral and
basi-occipital bones, and with the unpaired basi-sphenoid, which appears
between the basi-occipital and the diverging pterygoids, but is in most
cases to a great extent overlapped by the latter. The occipital condyle is
distinctly triple; the basi-occipital sometimes helps to border the foramen
magnum. The supra-occipital sends out a long vertical blade, directed
backwards and generally projecting far over the neck, for the attachment of
the powerful cranio-cervical muscles. The quadrate is very peculiar. Firmly
attached, and hemmed in on nearly all sides by the neighbouring bones, it
stands nearly vertically and forms a broad articulating surface for the
mandible. Its posterior side shows either a transverse, horizontal groove,
in which lies the columella auris, or the groove is transformed into a more
or less closed canal. Moreover, the hinder lateral margin of the quadrate
forms most of the tympanic frame; its margins being curved backwards,
leaving in the Cryptodira, however, a {318}wide notch behind; in the
Pleurodira this part of the quadrate is transformed into a trumpet, the
wide rim of which, forming a complete ring, carries the tympanic membrane.
The tympanic cavity thus formed often leads into a deep recess which
extends beneath the squamosal towards the opisthotic and bears some
resemblance to the intricate tympanic recesses which pervade that region of
the Crocodilian skull.
Dorsally the quadrate is broadly overlaid by the squamosal, which
frequently forms an arch with the parietal. Anteriorly the quadrate is
connected through a variably sized quadrato-jugal with the jugal; and this,
by joining the maxilla and postfrontal, helps normally to form the
posterior rim of the orbit. All the bones which border the temporal fossa
vary much in extent in the different groups of Chelonia. The extremes are
represented by _Cistudo_ and _Geoemyda_, in which the bony infratemporal
arch is absent, owing to the loss of the quadrato-jugal; and on the other
hand by the Chelonidae and by _Sphargis_, in which the whole temporal
region is covered over by an additional "false cranial" roof. This roof is
produced chiefly by lateral wing-like expansions of the parietal and
postfrontal bones, which meet the likewise much expanded jugal,
quadrato-jugal, and squamosal bones. In the lower diagram of Fig. 63
(_Chelone mydas_) the squamosal has been removed, and the other bones have
been reduced to their normal, or rather primitive condition, for comparison
with the external view of the complete skull of the same animal. The lower
diagram shows also the connexion of the pterygoid with a descending process
of the parietal; this column, paired of course, usually contains a separate
bone, the epipterygoid, the portion between _Ptg_ and _Par_.
The hyoidean apparatus is well developed, and sometimes assumes large
dimensions, especially in _Chelys_. The two pairs of "horns" are the first
and second branchial arches, whilst the hyoid arches are reduced to a pair
of small, frequently only cartilaginous, nodules attached near the anterior
corners of the basis linguae, which generally fuses with the os entoglossum
in the tip of the tongue.
The PECTORAL ARCH consists of a pair of long coracoids sloping obliquely
backwards, the distal cartilages of which scarcely touch each other in the
middle line, and the scapulae. The upper end of the scapula frequently
touches the inside of the {319}first costal plate, protected by a
cartilaginous pad. Near the glenoid cavity arises a long process (PC in
Fig. 65), placed transversely and approaching its fellow. The distal end is
connected with that of the coracoid by a fibro-cartilaginous band. The
homology of this scapular process is not quite clear. The band just
mentioned favours the idea that the process represents the precoracoid, but
its being an outgrowth from the scapula suggests that it is merely the much
enlarged acromion. It certainly does not represent the clavicle, which
forms part of the plastron: and this is not in contact with the
shoulder-girdle at all.
[Illustration: FIG. 64.–Diagram of the skeleton of _Testudo elephantopus_,
after removal of the left half of the carapace. The plastron is roughly
indicated by a section through the middle line. _Fe_, Femur, foreshortened;
_Fi_, fibula; _H_, humerus; _Il_, ilium; _Is_, ischium; _P.P._, pubis; _R_,
radius; _Scap_, scapula; _Tb_, tibia; _u_, ulna; 3, third cervical
vertebra; 1, 3, 5, first, third, and fifth fingers; _XIII_, thirteenth
(fifth thoracic) vertebra.]
The PELVIS is strong. Ilium, pubis, and ischium meet at the acetabulum. The
dorsal end of the ilium is generally broadened, and is attached to one or
both sacral vertebrae, but it is also in contact with the superimposed last
costal plate. This additional connexion often becomes predominant and the
sacral vertebrae are partly or completely relieved of the iliac support,
fusing in this case more or less with the costal plates. The pubes have
strong lateral processes, directed obliquely forwards and downwards. The
pubes and the ischia, which latter are much smaller, form broad symphyses,
and these are connected with each other by a longitudinal cartilaginous
band (_Chelone, Trionyx_); or the connecting bridge is broad and quite
ossified (_Testudo_), forming in the latter case two roundish
obturator-foramina. Cartilage frequently remains at the anterior end of
{320}the pubic symphysis, and a smaller, longer, and narrow piece of
cartilage extends sometimes backwards from the ischiadic symphysis, as the
so-called hypo-ischium. In the Pleurodira the ends of the ilia, and those
of the lateral processes of the pubes, are much broadened and firmly
ankylosed with the posterior costal plates and with the xiphiplastron
respectively.
[Illustration: FIG. 65.–Ventral view of the bony shell of _Chelone mydas_,
the Green Turtle, after removal of the plastron (Fig. 66). The costal
plates are marked by cross lines to distinguish them from the ribs. _C_,
coracoid; _Fe_, femur; _Fi_, fibula; _H_, humerus; _Ma._1-_Ma._12, marginal
plates, some of which are fused together; _Nu_, nuchal plate; _PC_,
"precoracoid"; _R_, radius; _Sc_, scapula; _I_, _V_, first and fifth
digits; _IX_, Ninth vertebra or first thoracic.]
The LIMBS are typically pentadactyle and complete, and are most primitive
in water-tortoises, e.g. _Chelydra_ and _Emys_, in which the carpus
consists of the typical ten separate elements, including the pisiform. In
_Testudo_ the centrale is fused with the intermedium, and the first three
distal carpals are also fused together. In the marine turtles the limbs are
transformed into paddles, but all the bones retain their independence; the
pisiform {321}and the first metacarpal are enlarged and flattened, thereby
giving additional width to the paddle. The tarsus remains less primitive;
the centrale and the proximal elements have a tendency to fuse together,
most completely in land-tortoises; the fifth distal carpal is enlarged, and
stands out hook-like from the rest. The number of the phalanges of the
fingers and toes varies slightly. It is noteworthy that none of the
Chelonia possess more than three phalanges. The three middle fingers and
toes have mostly three phalanges; the pollex and hallux have always two;
the number of phalanges of the fifth finger varies from three to one, of
the fifth toe from two to none. The greatest reduction occurs in _Testudo_
and its allied genera of typical land-tortoises, _Homopus_, _Pyxis_, and
_Cinixys_, the formula for the fingers being 2, 2, 2, 2, 2 or 1, and 2, 2,
2, 2, 0 for the toes. In _Pelomedusa_ all the fingers possess two phalanges
only, owing to fusion of the first and second phalanges with each other.
[Illustration: FIG. 66.–The bones composing the plastron of _Chelone
mydas_. On the right side the position of the covering horny shields[128]
is indicated by dotted lines. _a_, Anal horny shield; _ab_, abdominal; _f_,
femoral; _g_, gular; _h_, humeral; _ig_, intergular; _im_, infra-marginals;
_p_, pectoral.]
The SHELL, which is the most characteristic feature of the Chelonia,
consists of the dorsal "carapace" and the ventral "plastron." Each is
composed of a considerable number of bony plates which arise as
ossifications of nearly the whole thickness of the cutis, only a thin layer
of subcutaneous connective tissue remaining soft and lining the inside of
the shell. We restrict ourselves to a description of the shell of the
Thecophora, leaving the discussion of the peculiar shell of _Sphargis_ to
p. 336 f. Very young tortoises are still soft, and the plates which are
beginning to ossify are not yet suturally united. The plastron (Figs. 66
and 67) consists of the paired epi-, hyo-, hypo-, and xiphi-plastral
plates, and the unpaired endo-plastral plate.
{322}[Illustration: FIG. 67.–Bony shell of _Testudo ibera_. A, Ventral; B,
dorsal; C, left-side view. In B, and on the right half of A, the position
of the horny shields is indicated by dotted lines. The underlying bony
plates are marked by strong lines. In B the 1st neural and costal plates,
the 4th neural, costal, and 6th marginal plates, and the 7th neural plate
are shaded. 1, 4, 6, First, fourth, and sixth neural plate; _M_, in C,
fifth left marginal plate; _Nu_, nuchal plate.]
The latter is homologous with the interclavicle, the epi-plastra are
homologous with the clavicles of other Reptiles, while the other pieces are
genetically derived from, and are further modifications of, the so-called
abdominal ribs of the Crocodilia and Prosauria, These plastral plates are
never in direct contact with the shoulder-girdle or with any other parts of
the internal skeleton. In the young of all tortoises, and in the adult of
the Chelonidae and Trionychidae, the several plastral plates enclose large,
irregularly-shaped fontanelles. These are more or less filled up in the
other groups; and in the Testudinidae especially the whole plastron forms
one continuous mass. The navel is situated between the hyo- and
hypo-plastrals. Both these pairs are broader than the others, and are
connected with the carapace by {323}means of several marginals. The
connecting region is called the bridge. In several tortoises, e.g. _Emys_,
the connexion with the marginals is formed by ligaments only and remains
movable. In others, transverse, more or less perfect hinges are formed
across the plastron. A rather imperfect joint between the hypo- and
xiphi-plastrals develops with age in _Testudo ibera_. In _Cistudo_ and
_Cyclemys_ a very effective hinge lies below the hyo- and hypo-plastrals,
just in front of the bridge; and the anterior and posterior lobes of the
plastron can be closed against the inner rim of the box, fitting tightly in
_Cistudo_. In _Pyxis_ the front lobe only is movable.
[Illustration: FIG. 68.–A, Diagrammatic transverse section through the
shell of _Testudo_. On the right side the horny shields have been removed,
on the left are shown the neural, costal, marginal, and pectoral shields.
The bony dermal plates are dotted. _Cap_, Capitular portion of rib; _Sp.C_,
position of spinal cord. B, Vertical section through part of the shell,
magnified and diagrammatic. B, Bony layer of the cutis; L, leathery layer
of the cutis; M, cells of the Malpighian layer; P, star-shaped
pigment-cells; SC, stratum corneum, composing the horny shields.]
The carapace is composed of one median series, a right and left lateral
series of costal plates, and a series of marginals which surround the
whole. The median series consists of one large nuchal plate, normally eight
neurals and one to three supracaudal plates. The characteristic feature of
the neural plates is that they are firmly fused with the broadened neural
spinous processes of the underlying vertebrae. The nuchal plate lies in
front of the first thoracic or ninth vertebra; it overlies the last
cervical vertebrae, with the eighth of which it is connected by ligament
only; but the posterior corner of the plate often fuses with the spine of
the ninth vertebra. In the Chelydridae, and still {324}more in the
Trionychidae, the nuchal sends out a pair of long rib-like processes, which
either extend to below some of the neighbouring marginals, or their ends
overlap those of the ribs of the second thoracic vertebra (e.g. _Trionyx_),
or, lastly, they are in turn overlapped by the first costal plates (e.g.
_Cyclanorbis_). Such rib-like processes are also present, well developed in
the young, shorter in the adult, in the Dermatemydidae and Cinosternidae.
It is possible that the nuchal plate represents the fused neural of the
eighth and the costal plates of the ninth vertebrae. An indication of the
compound nature of the nuchal may be found in the fact that two nuchals
have been described in _Chelydropsis carinata_, a Miocene relation of
_Chelydra_. Somewhat similar modifications have taken place in the
post-sacral region. The one to three supracaudal plates are, namely,
neurals which have lost their connexion with, or perhaps have never been
fused with, the spinous processes of the movable tail-vertebrae. The number
of neural plates is mostly eight, but there are sometimes individually nine
or ten, the gradual suppression taking place first in the sacral region.
When such a plate is suppressed the neighbouring costal plates usually
close up and meet in the median line. In _Cistudo_, for instance, there are
only seven normal neurals, the eighth pair of costals meet, and the
original eighth neural is transformed into a supracaudal. In _Cinosternum_
the sixth to eighth costals meet, separating the one supracaudal widely
from the remaining five neurals. The meeting of the last pair of costals,
with co-ordinate reduction of the neurals to seven, is almost universal in
the Pleurodira; and this tendency is carried out to an extreme in the
Brazilian _Platemys_ and in the Australian _Chelodina_ and its allies, in
which all the costals meet in the middle line, and the neurals are
completely suppressed. Every stage intermediate between complete neurals
(_Sternothaerus_) and interrupted, vestigial, and vanished neurals, is
still represented by some genus. This process takes place independently,
both in America and in Australia, and is one of the most recently
introduced modifications.
The costal plates arise, like the neurals, independently in the cutis, but
they soon come into contact with the underlying cartilage of the ribs,
which are long enough to reach the marginals. The ribs flatten, become
surrounded by the growing membrane-bone of the plates, and the cartilage of
the ribs, {325}instead of ossifying, undergoes a process of calcification.
Ultimately this is more or less absorbed, its place is taken by the dermal
bone, which forms so to speak a cast of the rib, preserving in many cases
the shape of the vanished rib, only the capitular portions of which remain
unaffected. The number of costal plates is very constant, namely eight on
each side, but some fossils have nine or ten, and there are still
individual variations in recent forms, indicative of that number. In a
large _Chrysemys concinna_ I find the last pair of costals clearly composed
of at least two pairs, and this same specimen has nine distinct neural
plates.
The marginal plates are originally paired, almost always eleven pairs, very
rarely ten or twelve; an unpaired posterior plate, the pygal, is always
present, and is probably the result of fusion. In the Chelonidae large
fenestrae remain between the costal and marginal plates, only covered by
leathery unossified cutis, and of course by the horny shields. In the
Indian fresh-water genus _Batagur_ similar windows are gradually filled up
with age, and the horny shields become extremely thin and almost confluent.
On the other hand, in _Testudo polyphemus_, the bony shell, always very
thin, becomes still thinner with age and finally fenestrated by absorption.
Great reduction has taken place in the carapace of the Trionychidae. The
American species of _Trionyx_ have only seven pairs of costal plates; in
_Cyclanorbis_ the neurals are reduced to two. The whole dorsal shell is
much smaller than the body, and marginal plates are absent or merely
vestigial. It is doubtful if the ossifications in the posterior half of the
marginal flap of some genera are homologous with true marginals.
Externally the whole shell is covered, except in the _Trionychidae_, in
_Sphargis_ and _Carettochelys_ with horny, epidermal shields. These are
phylogenetically older than the dermal plates, and they do not correspond
with them either in numbers or in position, although there exists a general
resemblance in their arrangement. On the plastron we distinguish an
unpaired or paired gular, and a pair of gular, humeral, pectoral,
abdominal, femoral, and anal shields (Fig. 66). Sometimes there are also
intergulars, paired in _Macroclemmys_ and _Chelys_, unpaired in _Chelone_;
in many of the Pleurodira an unpaired intergular lies behind the gulars.
{326}The carapace of most Chelonians is covered with five neural, four
pairs of costal and twelve pairs of marginal _shields_, the last of which
often forms an unpaired pygal. In front of the first neural lies the nuchal
shield, very variable in size, often absent. The Chelydridae,
Dermatemydidae, Platysternidae, and Cinosternidae possess moreover several
inframarginals, intercalated on the bridge between the marginal and some of
the plastral shields. In many of the other families these inframarginals
are restricted to the anterior and posterior corners of the bridge, as the
so-called axillaries and inguinals, mostly small and variable. Lastly,
_Macroclemmys_ has several small supramarginals.
There are consequently eleven longitudinal rows of shields in all; by
elimination of the supra- and infra-marginals they are reduced to seven
rows. It is absolutely certain that the number of transverse rows also was
originally much greater than it is now. The mode of reduction of the number
of the neural and costal shields has been studied in _Thalassochelys
caretta_ (cf. p. 388.) The accompanying illustration (Fig. 69) shows some
of the main stages actually observed in the reduction of these shields. The
chief point is that certain shields are squeezed out, or suppressed by
their enlarging neighbours. The ultimate result is the formation of fewer,
but larger shields.
Each shield grows individually as follows. Every year, or rather during
every periodically recurring period of growth, the area of the Malpighian
layer belonging to each shield increases peripherally in size, and at the
same time produces a new layer of horn. The original little shield, with
which the tortoise is born, remains for years, often throughout life, as
the so-called "areola;" it increases in thickness owing to the new layer of
horn added from below, and peripherally the increase in size is indicated
by the overlapping concentric rings. Each ring represents a year's growth,
at least in tortoises which live in temperate zones, where hibernation
means a complete suspension of growth. It is not known if the same applies
to tropical species, which grow either throughout the year, or which
undergo one or more periods of rest. The areola does not remain central;
the growth is uneven. With age the oldest layers of the areola are
frequently rubbed off, and the areola then appears enlarged.
{327}[Illustration: FIG. 69.–Diagrams illustrating the progressive
reduction of the horny shields in various Chelonians. The shields, the fate
of which it is desired to follow, are indicated by distinctive shading. I.
Hypothetical, primitive stage. Eight neural (including the nuchal) and
eight costal shields. Both neurals and costals lie in the same transverse
planes. II.-VII. Successive stages in the reduction and suppression of
various shields, observed in specimens of _Thalassochelys_, the normal
condition of which is represented by VII. VIII. Six neurals and only four
costals. The normal condition of _Chelone_. IX. The nuchal shield has
become very small and the resulting gap has been filled up by an
enlargement of the first pair of marginals. This is the normal condition of
most Cryptodirous tortoises. X. The first marginals meet in front and the
nuchal is either suppressed (X_a_[X^a in diag]), _e.g._ in several species
of _Testudo_, or it is surrounded by the marginals (X_b_), _e.g._ in
_Sternothaerus_. (From Willey's _Zool. Results_, 1899.)]
{328}For the first dozen years or so the annual rings can be easily
followed, but when the creature approaches maturity each shield adds very
little to its growth, and the rings become very fine, crowded and
irregular. Only by careful counting and comparison of the rings on the
costals, marginals, and plastrals, can a reliable average be arrived at. In
some tortoises, e.g. _Chrysemys_, the whole outer layer of the shields
peels off periodically; only a thin smooth layer like mica or tracing-paper
remains, of course without any indication of rings. The pigment is formed
in the Malpighian layer, but it frequently diffuses into the horny shields
themselves, notably in _Chelone imbricata_, which yields the beautiful
"tortoise-shell." The colour of the pigment is either black, yellow, or
red, with resulting combinations. The green colour, often so beautiful in
baby-specimens of _Chrysemys_, is optical, produced, according to Agassiz,
by a network of black pigment, spread over a layer of yellow oil.
Horny scales, sometimes forming spines, and covering a nodule of dermal
ossification, are also common on other parts of the skin, especially on the
limbs of land-tortoises, and also on the tail of _Chelydra_. Sometimes the
end of the tail is protected by a claw-like nail, for instance in _Pyxis_.
In some of the gigantic land-tortoises, and in _Chelone mydas_, this nail
assumes large dimensions, and several of the terminal caudal vertebrae are
fused together into a regular urostyle. In some subfossil specimens of
Mauritian tortoises, these ankylosed complexes are 12 cm. long and more
than 5 cm. broad!