GENERAL ACCOUNT OF THE SKELETON IN REPTILES.
EXOSKELETON.
The exoskeleton both epidermal and dermal is exceedingly well
developed in reptiles.
EPIDERMAL EXOSKELETON.
This generally has the form of overlapping horny =scales= which invest
outgrowths of the dermis, and are found covering the whole body in
most Rhynchocephalia, Ophidia, and Lacertilia, and many Crocodilia. In
the Ophidia the ventral surface of the tail is commonly covered by a
double row of broad scales, while the ventral surface of the precaudal
part of the body is covered by a single row. In the burrowing snakes
(Typhlopidae) and some sea snakes (Hydrophidae) these broad scales do
not occur, the scales of the ventral surface being similar to those of
the dorsal.
In the Chelonia with the exception of _Dermochelys_, _Trionyx_ and
their allies there is a well-developed system of horny shields having
a regular arrangement which has been described in the account of the
Turtle's skeleton[91].
The =rattle of the rattlesnake= is an epidermal structure formed of
several loosely articulated horny rings, produced by the modification
of the epidermal covering of the end of the tail, which instead of
being cast off when the rest of the outer skin is shed is retained
loosely interlocked with the adjoining ring or joint. New rings are
thus periodically added to the base of the rattle, and in old animals
the terminal ones wear away and are lost.
Horny claws occur on the ends of some or all of the digits in most
living reptiles.
Owen's Chameleon bears three epidermal horns, one arising from the
nasal and two from the frontal region.
In the Chelonia, some of the Theromorpha such as _Udenodon_ and
_Dicynodon_, probably also in the Pterosauria and _Polyonax_ among the
Dinosaurs, the jaws are more or less cased in horny beaks. The horny
beaks of Chelonia are variable; sometimes they have cutting edges,
sometimes they are denticulated, sometimes they are adapted for
crushing.
DERMAL EXOSKELETON.
Nearly all Crocodilia, many Dinosauria, some Rhynchocephalia and
Pythonomorpha, and some Lacertilia such as _Tiliqua_, _Scincus_ and
_Anguis_ have a dermal exoskeleton of bony scutes, developed below and
corresponding in shape to the epidermal scales. Sometimes as in
_Caiman sclerops_, _Jacare_ and _Teleosaurus_, the scutes completely
invest the body, being so arranged as to form a dorsal and a ventral
shield, and a continuous series of rings round the tail. In
_Crocodilus_ they are confined to the dorsal surface, and in
_Alligator_ to the dorsal and ventral surfaces. The scutes of some
extinct forms articulate with one another by a peg and socket
arrangement as in some Ganoid fish.
The =carapace= of most Chelonia is a compound structure, being partly
endoskeletal and formed from the ribs and vertebrae, partly from
plates derived from the dermal exoskeleton. The common arrangement is
seen in fig. 36. All the surface plates are probably exoskeletal in
origin, but united with the ventral surfaces of the costal and neural
plates respectively are the expanded ribs and neural arches of the
vertebrae.
The =plastron= in the common genus _Chelone_ (fig. 37) includes nine
plates of bone, one unpaired and four pairs; they will be referred to
in connection with the ribs and pectoral girdle.
In the Leathery Turtle (_Dermochelys_) the carapace and plastron
differ completely from those of any other living form. The carapace
consists of a number of polygonal ossifications fitting closely
together and altogether distinct from the vertebrae and ribs. The
plastron is imperfectly ossified, and not united with the pelvis, and
the whole surface of both carapace and plastron is covered with a
tough leathery skin, without horny shields.
Some of the extinct Dinosauria have an enormously developed dermal
exoskeleton. Thus in _Stegosaurus_ and _Omosaurus_ the dorsal surface
is provided with flattened plates or with spines reaching a length of
upwards of two feet. In _Polacanthus_ the posterior part of the body
is protected by a bony shield somewhat recalling that of the little
armadillo _Chlamydophorus_. No exoskeleton is known in Ichthyosauria,
Sauropterygia, Pterosauria, many Dinosauria and Theromorpha, and some
Lacertilia, such as _Chamaeleon_ and _Amphisbaena_.
TEETH.
The teeth of reptiles are generally well developed, and in the great
majority of forms are simple conical structures, uniform in character,
generally somewhat recurved, and often with serrated edges. Another
common type of tooth is that with a laterally compressed triangular
crown provided with a double cutting edge which may or may not be
serrated. The teeth are mainly formed of dentine, with usually an
external layer of enamel, and often a coating of cement on the root.
Vasodentine is found below the dentine in _Iguanodon_. The teeth of
reptiles never have the enamel deeply infolded, nor do they have
double roots.
Teeth may be present not only on the jaw-bones, but as in many
_Squamata_, also on the palatines, pterygoids or vomers. The method
by which they are attached to the bones varies much. Sometimes as in
_Iguana_ and some other lizards, they are pleurodont[1], sometimes
they are acrodont[92], as in the Rhynchocephalia, Pythonomorpha,
Ophidia and some Lacertilia such as _Agama_. Again they may be set in
a continuous groove as in the Ichthyosauria and young Crocodilia.
Finally the teeth may be _thecodont_ or placed in distinct sockets as
in the Theromorpha, Sauropterygia, adult Crocodilia, Sauropoda and
Theropoda. In _Iguanodon_ the teeth are set in shallow sockets in a
groove one side of which is higher than the other; the method of
attachment thus shows points of resemblance to the thecodont
condition, the pleurodont condition, and that met with in the
Ichthyosauria.
In _Ichthyosaurus_ the teeth are marked by a number of vertical
furrows, and it is from a furrow of this nature greatly enlarged and
converted into a tube that the channel down which flows the poison of
venomous snakes is derived.
In most reptiles the dentition is more or less homodont. The only
reptiles in which a definite heterodont dentition is known are the
extinct Theromorpha, and in them the teeth vary greatly. Thus
_Udenodon_ is toothless, the jaws having been probably cased in a
horny beak. In _Dicynodon_ the jaws are likewise toothless with the
exception of a pair of permanently growing tusks borne by the maxillae.
_Dicynodon_ is the only known reptile whose teeth have permanently
growing pulps. In _Pariasaurus_ the teeth are uniform and very
numerous, and though placed in distinct sockets are ankylosed to the
jaw. In _Galesaurus_ and _Cynognathus_ three kinds of teeth can be
distinguished, slender conical incisor-like teeth, large canine-like
teeth, and cheek teeth with two or three cusps. The teeth in
_Galesaurus_ are confined to the jaws, in _Placodus_ and its allies,
however, large flat crushing teeth are attached to the palatines as
well as to the jaw-bones, and in _Pariasaurus_ the vomer, palatine and
pterygoid all bear teeth as well as the jaw bones. The upper jaw of
_Sphenodon_ and other Rhynchocephalia is provided with two parallel
rows of teeth, one borne on the maxillae and one on the palatines, the
mandibular teeth bite in a groove between these two rows. The bone of
the jaws in _Sphenodon_ is so hard that when the teeth get worn away,
it can act as a substitute. In the young _Sphenodon_ the vomers bear
teeth, as they do also in _Proterosaurus_.
There is generally a continuous succession of teeth throughout life,
the new tooth coming up below, or partly at the side of the one in
use, and causing the absorption of part of its wall or base. In this
way the new tooth comes to lie in the pulp cavity of the old one. This
method of succession is well seen in the Crocodilia.
[Illustration FIG. 50. PREPARATION OF PART OF THE RIGHT MANDIBULAR
RAMUS OF _Crocodilus palustris_ × 1/2. (Brit. Mus.)
1. tooth in use.
2. fairly old germ of future
tooth.
3. symphysial surface of the
mandible.]
Teeth have been detected in embryos of _Trionyx_, but otherwise no
teeth are known to occur in Chelonia, or in _Pteranodon_
(Pterosauria), while the anterior part of the jaw is edentulous in
_Iguanodon_, _Polyonax_ and some other Dinosaurs, and in
_Rhamphorhynchus_.
ENDOSKELETON.
VERTEBRAL COLUMN.
The vertebral column is commonly divisible into the usual five
regions, but in the Ophidia, Ichthyosauria, and Amphisbaenidae among
Lacertilia, only into caudal and precaudal regions. In the Chelonia
there are no lumbar vertebrae.
The form of the vertebral centra is very variable. A large proportion
of extinct reptiles,--several entire orders,--and the earlier and more
primitive forms in some of the other groups have amphicoelous
vertebrae. Vertebrae of this type occur in the Theromorpha,
Ichthyosauria, most Sauropterygia and Rhynchocephalia, and many
Dinosauria, also in some of the early Crocodilia such as _Belodon_,
_Teleosaurus_ and _Goniopholis_, and the Geckonidae among Lacertilia.
The majority of living reptiles have procoelous vertebrae. Thus they
occur in the Lacertilia (excluding the Geckos), the Ophidia, and the
Crocodilia, also among extinct forms in the Pterosauria and many
Dinosauria. On the other hand some Dinosauria such as _Iguanodon_ have
opisthocoelous cervical vertebrae, while others have opisthocoelous
thoracic vertebrae. The vertebrae of the Ceratopsidae and some
Sauropterygia, the thoracic vertebrae of _Iguanodon_, and the sacral
vertebrae of Crocodilia have flat centra. The first caudal vertebra of
modern Crocodilia is biconvex, and in the Chelonia all types of
vertebral centra are found. The cervical vertebrae of _Sphenodon_ are
noticeable for the occurrence of a small pro-atlas, which may
represent the neural arch of a vertebra in front of the atlas.
In most reptiles the vertebrae are fully ossified, but in some of the
more primitive forms the notochord persists in the centre of the
vertebra (i.e. intervertebrally), this is the case for instance in
many of the Theromorpha and Rhynchocephalia, and also in the Geckos.
In other reptiles it persists longest intravertebrally.
The centrum of each of the caudal vertebrae of most Lacertilia is
traversed by an unossified septum along which it readily breaks.
Chevron bones occur below the caudal vertebrae in Lacertilia,
Chelonia, Ichthyosauria, many Dinosauria, and _Sphenodon_,
articulating with quite the posterior part of the centrum which bears
them. In Lacertilia and Crocodilia (fig. 41, 3) the axis has a
well-marked odontoid process. The ventral portions of the
intervertebral discs are sometimes ossified, forming wedge-shaped
inter centra, as in Geckos, and the cervical vertebrae of _Sphenodon_.
In snakes, Theropod Dinosaurs, and the iguanas among lizards, the
neural arches are provided with _zygosphenes_, and _zygantra_.
The neural arches are usually firmly ankylosed to the centra, but in
the Crocodilia and some Chelonia, Sauropterygia, and Dinosauria, the
suture between the centrum and neural arch persists at any rate till
late in life. In the Ichthyosauria the neural arches were united to
the centra by cartilage only.
The thoracic vertebrae of some of the Theromorpha (_Dimetrodon_) are
remarkable for the extraordinary development of the neural spine, and
those of Chelonia for the absence of transverse processes.
In living reptiles the number of sacral vertebrae is nearly always
two, but in the Theromorpha, Dinosauria, and Pterosauria, as many as
five or six bones may be ankylosed together in the sacral region. In
Crocodiles the two halves of the pelvis sometimes articulate with
different vertebrae. The vertebrae of some of the great Sauropoda are
remarkably hollowed out, having a large vacuity on each side of the
centrum communicating with a series of internal cavities. The whole
structure of these vertebrae shows a combination of great strength
with lightness.
THE SKULL.
The reptilian skull is well ossified and the bones are noticeable for
their density. The true cranium is often largely concealed by a
secondary or false roof of membrane bones, which is best seen in the
Ichthyosauria and some of the Chelonia. In other reptiles the false
roof is more or less broken up by vacuities exposing the true cranial
walls. The ethmoidal region is the only one in which much of the
primordial cartilaginous cranium remains. The lateral parts of the
sphenoidal region are also as a rule not well ossified.
In some reptiles, such as most Lacertilia and Chelonia, the orbits are
separated only by the imperfect interorbital septum, while in others,
such as the Ophidia, Crocodilia and Amphisbaenidae, the cranial cavity
extends forwards between the orbits.
In the occipital region all four bones are ossified. The great
majority of reptiles have a single convex occipital condyle, but some
of the Theromorpha such as _Cynognathus_ have two distinct condyles as
in mammals. Sometimes, as in Chelonia, Ophidia and Lacertilia, the
exoccipitals, as well as the basi-occipital, take part in the
formation of the single condyle; sometimes, as in Crocodiles, it is
formed by the basi-occipital alone, as in birds. The relations of the
bones to the foramen magnum vary considerably, in Chelonia the
basi-occipital generally takes no part in bounding it, and in the
Theromorpha, Crocodilia, and Ophidia, the supra-occipital is excluded.
The parietals are paired in Geckos and Chelonia alone among living
forms, and in the extinct Ichthyosauria and some Theromorpha; in all
other reptiles they are united.
The frontals are paired in Ichthyosauria (fig. 32, 5), Chelonia,
Ophidia, _Sphenodon_ (fig. 52, B, 4) and some extinct crocodiles, such
as _Belodon_. They are completely fused in living Crocodilia and some
Lacertilia and Dinosauria. In the gigantic _Polyonax_ they are drawn
out into a pair of enormous horns, and the parietals and squamosals
are greatly expanded behind.
An interparietal foramen occurs in the Theromorpha, the Ichthyosauria
(fig. 32, 10), _Sphenodon_, the Sauropterygia and most Lacertilia.
The posterior part of the skull is curiously modified in some
Chamaeleons, the parietals and supra-occipitals being drawn out into a
backwardly-projecting sagittal crest which unites with the two
prolongations from the squamosals. In other Chamaeleons (_C. bifidus_)
prolongations of the prefrontals and maxillae form large
forwardly-projecting bony processes.
The roof of the skull is characterised by the development of
prefrontals and postfrontals, which lie respectively near the anterior
and posterior extremity of the orbit. In Theromorpha, Squamata,
Crocodilia, and some Dinosauria lachrymals are developed. There is a
ring of bones in the sclerotic in the Ichthyosauria (fig. 32, 15), the
Metriorhynchidae among Crocodiles and some Rhynchocephalia,
Dinosauria, and Pterosauria.
The pro-otic lies in front of the exoccipital and together with the
opisthotic forms the hind border of the fenestra ovalis. In Chelonia
the opisthotic remains separate, in all other living reptiles it fuses
with the exoccipital. The epi-otic fuses with the supra-occipital.
The parasphenoid, so important in Ichthyopsids, has very often
disappeared completely; it is present, however, in the Ichthyosauria,
the Plesiosauridae, and a number of Squamata, in many Ophidia its
anterior part forming the base of the interorbital septum.
In the Plesiosauridae and most Lacertilia, but not in the
Amphisbaenidae, a slender bone, the epipterygoid, occurs uniting the
parietal or the anterior end of the pro-otic with the pterygoid. A
homologous arrangement occurs in the Ichthyosauria and some Chelonia.
In most reptiles a transpalatine occurs, connecting the maxillae with
the pterygoid, but this is absent in the Chelonia, and some
Dinosauria, and in the Typhlopidae among snakes.
The quadrate is always well developed, and except in the Squamata is
firmly fixed to the surrounding bones. The Chamaeleons also, among the
Squamata, have a fixed quadrate, and in them too the quadratojugal is
absent. Separate nasal bones do not occur in any living Chelonia.
The vomers are generally paired as in Squamata, sometimes unpaired as
in Chelonia.
[Illustration FIG. 51. DORSAL (TO THE LEFT) AND VENTRAL (TO THE RIGHT)
VIEWS OF THE SKULL OF THE COMMON SNAKE (_Tropidinotus natrix_). (After
PARKER.)
1. premaxillae (fused).
2. anterior nares.
3. nasal.
4. prefrontal.
5. frontal.
6. parietal.
7. maxillae.
8. transpalatine.
9. palatine.
10. pterygoid.
11. pro-otic.
12. exoccipital.
13. supra-occipital.
14. opisthotic.
15. epi-otic.
16. quadrate.
17. parasphenoid.
18. basisphenoid.
19. basi-occipital.
20. occipital condyle.
21. splenial.
22. dentary.
23. angular.
24. articular.
25. supra-angular.
26. coronoid.
27. vomer.
28. squamosal.
IX, X foramina for the ninth
and tenth cranial nerves.]
The disposition of the bones of the jaws is subject to much
modification in the Ophidia in order to adapt them for swallowing very
large prey. The arrangements again differ greatly in the venomous and
non-venomous snakes. In the non-venomous snakes, such as _Python_ and
_Tropidonotus_, the palatine is large and is fixed to the pterygoid
which extends outwards (fig. 51, 10) so as to be united to the
quadrate, and is at the same time firmly connected by the
transpalatine with the maxillae. The quadrate is united to the
squamosal, which is loosely attached to the cranium. The premaxillae is
moderately developed and bears teeth, and the maxillae forms a long bar
loosely connected with the rest of the skull. The rami of the mandible
are united only by an extremely elastic ligament. It is as regards the
maxillae and premaxillae that the skulls of venomous and non-venomous
snakes differ most. In the rattlesnake (_Crotalus_) and other venomous
snakes the premaxillae is extremely small and toothless. The maxillae is
small and subcylindrical, and is movably articulated to the lachrymal,
which also is capable of a certain amount of motion on the frontal.
The maxillae is connected by means of the transpalatine with the
pterygoid, which in its turn is united to the quadrate. When the mouth
is shut the quadrate is directed backwards, and carrying back the
pterygoid and transpalatine pulls at the maxillae and causes its
palatal face, to which the poison teeth are attached, to lie back
along the roof of the mouth. When the mouth opens the distal end of
the quadrate is thrust forward, and this necessitates the pushing
forward of the pterygoid and transpalatine, causing the tooth-bearing
surface of the maxillae to look downwards and the tooth to come into
the position for striking.
The Ophidian skull is also noticeable for the absence of the jugals
and quadratojugals. In poisonous snakes the place of the jugal is
taken by the zygomatic ligament which connects the quadrate and
maxillae.
The extent to which the palate is closed in reptiles varies much. In
many reptiles, such as the Squamata and Ichthyosauria, the palate is
not complete, both palatines and pterygoids being widely separated in
the middle line. In others, such as the Crocodilia, Sauropterygia, and
Chelonia, there is a more or less complete bony palate. In many
Chelonia this is chiefly formed of the vomer, palatines, and
pterygoids, the posterior nares being mainly bounded by the palatines.
In living Crocodilia, however, outgrowths are formed from the
pterygoids and palatines which arch round and meet one another
ventrally, forming a secondary palate (fig. 43, A), which completely
shuts off the true sphenoidal floor of the skull, and causes the
posterior nares which are bounded by the pterygoids to open very far
back. Though this feature is common to all postsecondary crocodiles,
it is interesting to notice that it is not found in the earlier forms,
but that its gradual evolution can be traced. In the Triassic
_Belodon_, for instance, the posterior nares open far forwards, and
are not surrounded by either the palatines or pterygoids. In the
Jurassic crocodile, _Teleosaurus_, the posterior nares lie further
back, being surrounded by the palatines, but the pterygoids do not
meet them. Finally, in the Tertiary forms the arrangements are as in
living crocodiles.
A short secondary hard palate is found also in the Theriodontia. The
palatines of _Ichthyosaurus_ are noticeable for their transverse
position, which recalls that in the Frog.
The various =fossae= or =vacuities= in the false roof of the skull are
important, and their relations may best be understood by a description
of their mode of occurrence in _Sphenodon_, a form in which they are
very completely developed.
[Illustration FIG. 52. SKULL OF HATTERIA. (_Sphenodon punctatus_). A,
lateral; B, dorsal; C, ventral; D, posterior. (After VON ZITTEL.)
1. premaxillae.
2. nasal.
3. prefrontal.
4. frontal.
5. postfrontal.
6. parietal.
7. squamosal.
8. quadratojugal.
9. quadrate.
10. postorbital.
11. jugal.
12. maxillae.
13. vomer.
14. palatine.
15. pterygoid.
16. transpalatine.
17. exoccipital.
18. epipterygoid.
19. basisphenoid.
20. supratemporal fossa.
21. infratemporal or lateral temporal
fossa.
22. orbit.
23. post-temporal fossa.
24. foramen magnum.
25. anterior nares.
26. interparietal foramen.
27. dentary.
28. supra-angular.
29. articular.]
In _Sphenodon_, then, on the dorsal surface of the skull, are the
large =supratemporal fossae= (fig. 52, 20). Their inner margins are
separated from one another by the parietal walls of the cranium, while
externally each is bounded by a bony arch, the =supratemporal arcade=,
formed of the postfrontal, postorbital, and squamosal. Posteriorly the
boundary is formed by a =post-temporal bar=, formed by the parietal
and squamosal. Below the supratemporal arcade is another large
vacuity, the =infratemporal= or =lateral temporal fossa= (fig. 52,
21). This is bounded above by the supratemporal arcade, and is
separated from the orbit in front by the =postorbital bar=, formed by
the union of outgrowths from the jugals and postorbitals. Behind it is
bounded by a continuation of the post-temporal bar formed of the
squamosal and quadratojugal, and below by an =infratemporal arcade=,
which is chiefly composed of the quadratojugal and jugal.
Below the post-temporal bar is a third vacuity, the =post-temporal
fossa= (fig. 52, D, 23), bounded above by the post-temporal bar and
below by the exoccipital and opisthotic.
_Sphenodon_ and the Crocodilia are the only living reptiles with
complete supratemporal and infratemporal arcades, but they are both
present in the extinct Pterosauria and some Dinosauria.
Supratemporal fossae, bounded below by supratemporal arcades, occur in
all reptiles except some Chelonia, the Ophidia, the Geckonidae among
Lacertilia, and the Pariasauria and others among Theromorpha; they are
specially large in _Nothosaurus_ among the Sauropterygia, _Dicynodon_
among the Theromorpha, and many Crocodilia and Pterosauria. In some
Dinosaurs, such as _Ceratosaurus_, they are very small, while the
infratemporal fossae are correspondingly large.
In _Elginia_[93] (Theromorpha) and some Chelonia, such as _Chelone_,
there are no fossae on the surface of the skull, a complete false roof
being developed; in other Chelonia, such as _Trionyx_, the true
cranium is freely visible, the only part of the false roof developed
being the infratemporal arcade.
In many reptiles large =pre-orbital vacuities= occur; they are
specially large in the Pterosauria and in some of the Crocodilia and
Dinosauria (fig. 35, 3). In some Pterosauria they are confluent with
the orbits.
The premaxillae are usually separate, but sometimes, as in some Ophidia
(fig. 51, 1), Chelonia, Lacertilia (Agamidae), and Dinosaurs
(Ceratopsia) they are united. In the Dinosaur _Hadrosaurus_ they are
exceedingly large and spatulate. In the Rhynchocephalian
_Hyperodapedon_ they are drawn out into a strongly curved beak.
As regards the mandible, sometimes, as in most Rhynchocephalia,
Ophidia and Pythonomorpha, the rami have only a ligamental union;
sometimes, as in Crocodilia, the Rhynchosauridae and the majority of
Lacertilia, they are suturally united. In Chelonia (fig. 28, B, 12),
and apparently in Pterosauria, the two dentaries are completely fused
together. The sutures between the various bones of the lower jaw
usually persist, but in Ophidia those between the angular,
supra-angular, articular and coronoid are obliterated. There are
sometimes large vacuities in the mandible, as in Theromorpha,
Crocodilia, and some Dinosauria. In _Iguanodon_, _Polyonax_,
_Hypsilophodon_ and _Hadrosaurus_ among Dinosaurs the mandible has a
predentary or mento-meckelian bone which, in some cases at any rate,
was probably sheathed in a horny beak.
The principal part of the auditory ossicular chain is formed by a
rod-like columella. The development of the hyoid apparatus varies, and
it often happens that the first branchial arch is better developed
than is the hyoid arch. In the Crocodilia and Chelonia there is a
large basilingual plate or body of the hyoid (fig. 53, 1); but while
in the Crocodilia the first branchial forms the only well-developed
arch, in the Chelonia the first and second branchials are both
strongly developed, and the hyoid is often fairly large.
THE RIBS.
[Illustration FIG. 53. HYOIDS OF AN ALLIGATOR (_Caiman latirostris_)
(TO THE LEFT) AND OF A GREEN TURTLE (_Chelone midas_) (TO THE RIGHT) ×
5/8. (Brit. Mus.)
The cartilaginous portions are dotted.
1. basilingual plate or body of
the hyoid.
2. hyoid arch.
3. first branchial arch (anterior
cornu).
4. second branchial arch (posterior
cornu).]
Ribs are always present, and may be attached to any of the precaudal
vertebrae. In most reptiles the posterior cervical vertebrae bear
ribs, while the atlas and axis are ribless; in Crocodiles and Geckos,
however, ribs are borne even by the atlas and axis. On the other hand,
in the Chelonia none of the cervical vertebrae bear obvious ribs. In
the following groups the thoracic ribs have both capitula and
tubercula--Theromorpha, Ichthyosauria, Crocodilia, Dinosauria,
Pterosauria. In the other groups each rib articulates by a single
head, and the position of the facet is subject to a considerable
amount of variation, thus in the Squamata it lies on the centrum, and
in the Sauropterygia on the neural arch, while in the Chelonia the rib
articulates with the contiguous parts of two centra instead of
directly with one.
In most reptiles a greater or smaller number of ribs are united
ventrally with a sternum; but in snakes a continuous series of similar
ribs, all articulating freely with the vertebral column, extends from
the third cervical vertebra to the end of the trunk. The number of
ribs connected with the sternum varies from three or four in Lizards
to eight or nine in Crocodiles. Those which reach the sternum are
nearly always divided into vertebral, sternal, and intermediate
portions, and as a rule only the vertebral portion is completely
ossified. In Crocodiles a number of sternal ribs are connected with a
cartilaginous arch, which is attached to the hind end of the sternum,
and represents the xiphisternum. The sacral ribs connecting the
vertebral column with the ilia are very distinct in Crocodiles; in
these animals and _Sphenodon_ the vertebral ribs have backwardly
projecting uncinate processes as in birds.
In the curious arboreal lizard, _Draco volans_, the posterior ribs are
long and straight, and support a parachute-like expansion of the
integument used in its long flight-like leaps. In Chelonia the ribs
are generally combined with the carapace.
In Ichthyosauria, Sauropterygia, Crocodilia and _Sphenodon_, abdominal
splint ribs occur; and probably all except the first of the paired
ossifications forming the plastron of Chelonia are of similar
character. Abdominal ribs have quite a different origin from true
ribs, for while true ribs are cartilage bones, abdominal ribs have no
cartilaginous precursors, but are simply the ossified tendons of the
rectus abdominalis muscle.
THE STERNUM.
A sternum occurs in the following groups of reptiles: Rhynchocephalia,
nearly all Lacertilia, Pythonomorpha, Crocodilia, and Pterosauria, and
is generally more or less rhomboidal or shield-shaped. In Pterosauria
it is keeled and bears some resemblance to that of birds. It may have
been replaced by membrane bone.
[Illustration FIG. 54. VENTRAL VIEW OF THE SHOULDER-GIRDLE OF STERNUM
OF A LIZARD (_Loemanctus longipes_) × 2. (After PARKER.)
1. interclavicle.
2. clavicle.
3. scapula.
4. coracoid.
5. precoracoidal process.
6. glenoid cavity.
7. sternum.
8. xiphisternum.
9. sternal rib.]
The sternum is absent in Sauropterygia, Ichthyosauria, Chelonia,
Ophidia, and most of the snake-like Amphisbaenidae among Lacertilia;
while it is not well known in Theromorpha and Dinosauria. In the
Sauropod _Brontosaurus_, however, two rounded bones occur near the
base of the coracoids, and these probably represent ossified patches
in a sternum, which was mainly cartilaginous; similar structures occur
in _Iguanodon_.
The sternum frequently remains wholly cartilaginous, especially in
Lacertilia; sometimes it becomes calcified, but true ossification does
not as a rule take place.
APPENDICULAR SKELETON.
THE PECTORAL GIRDLE.
The pectoral girdle is well developed in all groups of reptiles except
the Ophidia, occurring even in the limbless Amphisbaenidae. It is very
solid in the Theromorpha. As a rule all three cartilage bones,
scapula, coracoid, and precoracoid are represented, and frequently
also the membrane bones,--clavicles, and interclavicle.
The coracoids are generally flat expanded bones, which sometimes, as
in Sauropterygia and Ichthyosauria, meet in a ventral symphysis;
sometimes, as in Lacertilia, are united with the sides of the sternum.
In Chelonia neither the coracoids nor precoracoids meet one another,
but their free ends are connected by fibrocartilaginous bands. In
Lacertilia the coracoids are pierced by fenestrae.
The precoracoid is generally represented, but the Theromorpha are the
only reptiles in which it is separately ossified; it forms a
well-marked process on the coracoid in Lacertilia (fig. 54, 5). It is
absent in Ichthyosauria, and Dinosauria, and probably in
Sauropterygia. In some Lacertilia and Chelonia the sternal ends of the
coracoids are unossified and form epicoracoids; in some Chelonia there
are also epiprecoracoids; but neither these nor the epicoracoids
overlap their fellows of the opposite side as they do in arciferous
Anura (see p. 185). In some Lacertilia with degenerate limbs the
pectoral girdle is also much reduced, in _Ophisaurus apus_ the ventral
borders of the coracoids are widely separated.
A scapula is always present, and is generally expanded distally, but
in the Chelonia the distal end is cylindrical. In the Theromorpha it
has an acromial process with which the precoracoid articulates, and it
is very large in Dinosauria. In the Chelonia the scapula and
precoracoid are ossified continuously. Among the Pterosauria,
_Pteranodon_ has an unique pectoral girdle; the scapula and coracoid
are ankylosed and the scapula articulates with the neural spines of
several ankylosed vertebrae.
Clavicles occur in some Theromorpha such as _Pariasaurus_, and also in
the Ichthyosauria, Sauropterygia, Rhynchocephalia, and most
Lacertilia. They are absent in the Pterosauria, the Chamaeleons among
Lacertilia, the Ophidia and the Crocodilia. They are wanting too in
the Chelonia, unless the first pair of ossifications in the plastron
are to be regarded as clavicles. In the Sauropterygia bones regarded
as the clavicles and interclavicle are generally well developed. The
unpaired ossification in the plastron of Chelonia is an interclavicle,
and a representative of the same bone occurs arising from the sternum
in Pterosauria. A well developed =T=-shaped interclavicle is found in
Ichthyosauria, Rhynchocephalia, Lacertilia, and some Theromorpha, such
as _Pariasaurus_.
THE LIMBS.
In most reptiles there are two pairs of pentedactylate limbs provided
with claws, but in nearly all Ophidia and some Lacertilia
(_Amphisbaena_, _Lialis_, _Anguis_) the limbs have entirely
disappeared. In a few Ophidia such as _Python_ traces of the posterior
limbs occur, and in _Chirotes_ among the Amphisbaenidae there are
minute anterior limbs. The Lacertilians, _Chalcides_ (_Seps_) and
_Ophisaurus_ (_Bipes_, _Pseudopus_) have very small posterior limbs.
The limbs are as a rule adapted for walking, but in Ichthyosauria,
Sauropterygia, Pythonomorpha and some Chelonia, they have the form of
swimming paddles, the relative size of the manus and pes being
increased, while that of the proximal and middle portions of the limbs
is reduced. This reduction is carried to its furthest extent in the
Ichthyosauria in which radius and ulna, tibia and fibula, have the
form of short polygonal bones similar to those constituting the manus
and pes. In the Pythonomorpha the reduction of the limb bones is not
quite so marked, in the Sauropterygia it is less, and still less in
the Chelonia. In the earlier Ichthyosauria too, the limb bones are not
so short as they are in the later forms. The Ichthyosaurian limb is
also remarkable, firstly for the fact that both humerus and femur are
terminated by concave articulating surfaces instead of by convex
condyles, and secondly for the great multiplication of the phalangeal
bones, each digit being sometimes composed of a series of over twenty.
Sometimes too the number of series is increased, either by the
bifurcation of some of the digits or by the development of marginal
bones. In the Sauropterygia the phalanges are likewise increased above
the normal but not so much as in Ichthyosauria. The humerus and femur
of Sauropterygia are noticeable for the enormous size of the terminal
epiphyses which form in each case by far the greater part of the bone.
THE ANTERIOR LIMB.
The anterior limb is usually approximately equal in length to the
posterior, but in many Dinosauria it is considerably the shorter of
the two. The humerus is generally without distinct condyles, but they
are well developed in the Theromorpha, the Lacertilia and _Sphenodon_.
In the Theromorpha, some Rhynchocephalia, and some Sauropterygia, such
as _Mesosaurus_, the humerus has an ent-epicondylar foramen; in
Lacertilia, Chelonia and some Dinosauria there is an ect-epicondylar
foramen or groove; _Sphenodon_ possesses both ent- and ect-epicondylar
foramina. The radius and ulna are always separate. In some Chelonia,
such as _Chelydra_, the carpus has a very simple arrangement, namely,
a proximal row of three bones, the radiale, intermedium and ulnare,
and a distal row of five carpalia, with one bone, the centrale,
between the two rows. Many reptiles have a carpus only slightly
different from this. Thus the carpus in _Sphenodon_ differs mainly in
having two centralia, that of most Lacertilia, in having the centrale
and intermedium fused.
Crocodiles have a much reduced carpus with the radiale and ulnare
considerably elongated. The manus in Chamaeleons is curiously
modified, having the first three digits arranged in one group and
turned inwards, and the fourth and fifth in another group turned
outwards; carpalia 3 and 4 are united.
In the Pterosauria the anterior limbs form wings, the phalanges of the
fifth digit being very greatly elongated to support the wing membrane.
The first digit is vestigial and the second, third, and fourth are
clawed.
THE PELVIC GIRDLE.
The pelvic girdle is well developed in all reptiles which have
posterior limbs, but is absent or quite vestigial in Ophidia and those
Lacertilia which have no posterior limbs. The ilium and ischium agree
in their general characters throughout all the various groups of
reptiles, but that is not the case with the pubis.
In many reptiles such as Chelonia, Ichthyosauria and Lacertilia the
ilia are small, more or less cylindrical bones either directed
backwards, or vertically placed as in the Chamaeleons. In the
Crocodilia they are larger and more expanded, while in Dinosauria and
Pterosauria they are greatly elongated both in front of, and behind,
the acetabulum. The ischia are generally strongly developed somewhat
square bones meeting in a ventral symphysis. In Dinosauria the ischium
(fig. 35, 9) is a much elongated and backwardly-directed bone,
bearing a forwardly projecting obturator process. In Pterosauria the
ischium is fused with the ilium, and in both pterosaurs and crocodiles
the ilium and ischium are the only bones taking part in the formation
of the acetabulum. In most Lacertilia there is an unpaired structure,
the _hypo-ischium_ or _os cloacae_ projecting back from the symphysis
ischii, which is usually separated from the symphysis pubis by a large
space, the _foramen cordiforme_. In some Lacertilia and Chelonia there
is a cartilaginous bar dividing the foramen cordiforme into two
obturator foramina; in many Chelonia this bar is ossified. Among
_Ophidia_, _Python_, _Tortrix_, _Typhlops_ and their allies have a
structure representing a vestigial ischio-pubis: but in most Ophidia
there is no trace of the pelvis. In some Theromorpha all the bones of
the pelvis are completely fused, forming an os innominatum as in
mammals; the pubes and ischia are so completely fused that sometimes
as in _Pariasaurus_ even the obturator foramina are closed.
Concerning the reptilian pubis there are considerable difficulties.
Sometimes there is only a single pubic structure present, sometimes
there are two. The reptilian pubis is best understood by comparing the
arrangements met with in the various other groups with that in the
Orthopod Dinosaurs such as _Iguanodon_. In _Iguanodon_ the pubis
consists of two portions, viz. of a moderately broad pre-pubis
directed downwards and forwards, and of a narrow greatly elongated
post-pubis directed backwards parallel to the ischium. The pubis is
united to both ilium and ischium, the acetabulum has a large
unossified space, and neither pre-pubes nor post-pubes meet in ventral
symphyses. The arrangement bears a great resemblance to that of Ratite
birds. In Lacertilia, Chelonia, Rhynchocephalia and Ichthyosauria
together with Theropod and Sauropod Dinosaurs the pubis corresponds to
the pre-pubis of _Iguanodon_ and is a more or less cylindrical bone
expanded at both ends, meeting its fellow in a ventral symphysis. In
Chelonia and Lacertilia the pubis bears a lateral process which is
homologous with the post-pubis of Iguanodon. In Lacertilia and
sometimes in Chelonia there is a cartilaginous epipubis attached to
the anterior border of the pubic symphysis; this is well developed in
the Chamaeleons and Geckos. In Crocodilia there is, forming the
anterior and ventral portion of the acetabulum, a patch of cartilage
(fig. 49, 3) which is probably the true pubis homologous with that of
lizards and with the pre-pubis of _Iguanodon_. The large bone
generally called the pubis in Crocodiles is probably an epipubis.
THE POSTERIOR LIMB.
The posterior limb is entirely absent in some Lacertilia and in most
Ophidia, though traces occur in _Python_, _Tortrix_ and _Typhlops_. In
the Ichthyosauria, Sauropterygia and Pythonomorpha the posterior limbs
form swimming paddles and have been already referred to.
The arrangement of the proximal and middle segments of the limb is
fairly constant in all reptiles with limbs adapted for walking, and
the tibia and fibula are always separate. The pes is however subject
to a considerable amount of variation, especially as regards the
tarsus. In some Chelonia the tarsus like the carpus has an extremely
simple arrangement, consisting of a proximal row of three bones, the
tibiale, intermedium and fibulare, a centrale, and a distal row of
five tarsalia. In most living reptiles, however, the tibiale and
intermedium are as in mammals united, forming the astragalus. In
Crocodiles (fig. 48, B, 15) the centrale is also united with the
tibiale while the distal tarsalia are very slightly developed. The
calcaneum in Crocodiles is drawn out into a long process forming a
heel in a manner almost unique among Sauropsida. In _Sphenodon_ and
Lacertilia the tibia and fibula articulate with a single large bone
representing the whole proximal row of tarsalia.
The pes is generally pentedactylate, but in some Crocodiles the fifth
digit is vestigial (fig. 48, B), and in some Dinosauria (fig. 35)
there are only three digits. The North American Dinosaurs present a
continuous series ranging from a pentedactylate plantigrade form like
_Morosaurus_, to such a form as _Hallopus_ with a highly digitigrade
and specialised pes reduced to three functional digits, and a
vestigial fifth metatarsal. The second, third and fourth metatarsals
in this form are nearly two-thirds as long as the femur, and the
calcaneum is drawn out into a heel much as it is in most mammals.
In Lacertilia, Orthopoda and many Chelonia, the ankle joint comes to
lie between the proximal and distal row of tarsals as in birds.
FOOTNOTES:
[91] See pp. 214 and 215.
[92] These terms are defined on p. 199.
[93] E.T. Newton, _Phil. Trans._ vol. CLXXXIV, B, p. 431 (1893).