SAUROPSIDA.
This great group includes the Reptiles and Birds and forms the second
of the three into which the Gnathostomata may be divided. There is
nearly always a strongly-developed epiblastic exoskeleton which has
the form of scales or feathers, and in some cases a dermal exoskeleton
is also well developed. In living forms the notochord never persists,
being replaced by vertebrae, but in some extinct forms the centra are
notochordal. The vertebral centra are ossified, and only in
exceptionally rare cases have terminal epiphyses. The skull is well
ossified and has membrane bones incorporated in its walls.
The occipital segment is completely ossified, and an interorbital
septum or bony partition separating the two orbits is usually
developed to a greater or less extent. The skull generally articulates
with the vertebral column by a single occipital condyle into the
composition of which the exoccipitals and basi-occipital enter in
varying proportions. The pro-otic ossifies, and either remains
distinct from the epi-otic[65] and opisthotic throughout life, or
unites with them only after they have fused with the adjacent bones.
The hyoid and branchial arches are much reduced; and the
representative of the hyomandibular is connected with the auditory
apparatus, forming the auditory ossicles[66]. Each ramus of the
mandible always consists of a cartilage bone, the articular, and
several membrane bones. The mandible articulates with the cranium by
means of a quadrate. The ribs in Birds and some Reptiles bear
_uncinate processes_, i.e. small, flat, bony or cartilaginous plates
projecting backwards from their posterior borders. The sternum is not
transversely segmented as in mammals, and there are commonly distinct
cervical ribs. The ankle joint is intertarsal, or situated between the
proximal and distal row of tarsal bones, not cruro-tarsal as in
Mammalia.
CLASS I. REPTILIA[67].
The axial skeleton is generally long, and that of the limbs frequently
comparatively short, or sometimes absent.
The exoskeleton generally has the form of epidermal scales, which are
often combined with underlying bony dermal plates or scutes and may
sometimes form a continuous armour. Neither feathers nor true hairs
are ever present. The vertebral column is generally divisible into the
five usual regions. The centra of the vertebrae vary enormously, and
may be amphicoelous, procoelous, opisthocoelous or flat, but they
never have saddle-shaped articulating surfaces. The quadrate is always
large, and is sometimes fixed, sometimes movable. A transpalatine bone
uniting the pterygoid and maxillae is generally present.
Free ribs are often borne along almost the whole length of the trunk
and tail, and often occur attached to the cervical vertebrae. The
sacrum is generally composed of two vertebrae which are united with
the ilia by means of expanded ribs. The sternum is rhomboidal, and may
either be cartilaginous or formed of cartilage bone, but never of
membrane bone; it differs from that of birds also in the fact that it
does not ossify from two or more centres. An interclavicle is
generally present. There are always more than three digits in the
manus, and never less than three in the pes. In all living reptiles
the ilia are prolonged further behind the acetabula than in front of
them, and the bones of the pelvis remain as a rule, distinct from one
another throughout life.
The pubes (pre-pubes) and ischia both commonly meet in ventral
symphysis, and the acetabula are wholly or almost wholly ossified. The
metatarsals are not ankylosed together.
_Order 1._ THEROMORPHA[68].
This order includes a number of mainly terrestrial, extinct reptiles,
which differ much from one another, and show remarkable points of
affinity on the one hand with the Labyrinthodont Amphibia, and on the
other with the Mammalia. The vertebrae are nearly always amphicoelous
and sometimes have notochordal centra. The skull is short and has the
quadrate immovably fixed. There is an interparietal foramen, and
generally large supratemporal fossae bounded by supratemporal[2]
arcades, but with no infratemporal[69] arcades; _Elginia_ however has
the whole of the temporal region completely roofed over.
The teeth are placed in distinct sockets and are very variable in
form, the dentition sometimes resembling the heterodont dentition of
mammals. The humerus has distinct condyles and an ent-epicondylar
foramen[70] as in many mammals.
The pubis is fused with the ischium, and both pectoral and pelvic
girdles are remarkably solid. The obturator foramen is remarkably
small or even absent. The anterior ribs have two articulating
surfaces, and each articulates by its tuberculum with the transverse
process, and by its capitulum with the centrum as in mammals.
These reptiles occur chiefly in deposits of Triassic and Permian age.
Some of the best known genera are _Dicynodon_, _Udenodon_, _Placodus_,
_Pariasaurus_ and _Galesaurus_. They will be noticed in the general
account of the skeleton in reptiles.
_Order 2._ SAUROPTERYGIA.
This order includes a number of extinct marine reptiles, devoid of an
exoskeleton. The tail is short, the trunk long, and the neck in the
most typical forms extremely long. The vertebrae have slightly
biconcave, or nearly flat centra. The skull is relatively small and
has large supratemporal fossae. The teeth are placed in distinct
sockets, and are generally confined to the margins of the jaws; they
are sharp and curved and are coated with grooved enamel. The
premaxillae are large, and there is an interparietal foramen. The
quadrate is firmly united to the cranium. The anterior nares are
separate and are placed somewhat close to the orbits. There is no
ossified sclerotic ring. The palatines and pterygoids meet the vomers,
and more or less completely close the palate, and in some forms, e.g.
_Plesiosaurus_, there is a distinct parasphenoid. Thoracic ribs are
strongly developed and each articulates with its vertebra by a single
head. The cervical vertebrae have well-marked ribs, which articulate
only with the centra, in this respect differing from those of
Crocodiles. The caudal vertebrae bear both ribs and chevron bones, and
abdominal splint-ribs are largely developed.
In the shoulder-girdle the coracoids are large and meet in a ventral
symphysis; precoracoids and a sternum are apparently absent, but parts
generally regarded[71] as the clavicles and interclavicle are well
developed. In the pelvis, the pubes and ischia meet in a long
symphysis. The limbs are pentedactylate, and in the best known forms,
the Plesiosauridae, form swimming paddles.
The Sauropterygia occur in beds of Secondary age, and some of the best
known genera are _Plesiosaurus_, _Pliosaurus_ and _Nothosaurus_.
_Order 3._ CHELONIA.
In the Tortoises and Turtles the body is enclosed in a bony box,
formed of the dorsal carapace, and a flat ventral buckler, the
plastron. Except in _Dermochelys_ the carapace is partly formed from
the vertebral column and ribs, partly from dermal bones. Both carapace
and plastron are, except in _Dermochelys_, _Trionyx_ and their allies,
covered with an epidermal exoskeleton of horny plates, which are
regularly arranged, though their outlines do not coincide with those
of the underlying bones. The thoracic vertebrae have no transverse
processes, and are quite immovably fixed, but the cervical and caudal
vertebrae are very freely movable. There are no lumbar vertebrae. The
skull is extremely solid, and frequently has a very complete false
roof. Teeth have been detected in embryos of _Trionyx_ but with this
exception the jaws are toothless, and are encased in horny beaks. The
quadrate is firmly fixed. The facial part of the skull is very short,
and the alisphenoidal and orbitosphenoidal regions are unossified. In
living forms there are no separate nasal bones, while large
prefrontals and postfrontals are developed. There is a comparatively
complete bony palate chiefly formed of the palatines and pterygoids.
The anterior nares are united and placed at the anterior end of the
skull, and the premaxillae are very small. There is no transpalatine
bone and the vomer is unpaired. The dentaries are generally fused
together.
There are ten pairs of ribs, and each rib has only a single head and
is partially attached to two vertebrae; there are no cervical or
sternal ribs. There is no true sternum.
The three anterior elements of the plastron are respectively
homologous with the interclavicle and two clavicles of other reptiles,
while the remaining elements of the plastron are probably homologous
with the abdominal ribs of Crocodiles. The pectoral girdle lies within
the ribs, and the precoracoids and coracoids do not meet in ventral
symphyses. The scapula and precoracoid are ossified continuously. The
pubis probably corresponds with the pre-pubis of Dinosaurs. There are
four limbs each with five digits.
The order includes three suborders:--
_Suborder (1)._ TRIONYCHIA.
The carapace and plastron have a rough granular surface covered with
skin and without any horny shields.
The plastron is imperfectly ossified, and marginal bones may be
absent, or if present are confined to the posterior portion of the
carapace. The pelvis is not united to the plastron. The cranium has
not a complete false roof and the head can be drawn back under the
carapace.
The first three digits of both manus and pes bear claws, and the
fourth digit in each case has more than three phalanges. The most
important genus is _Trionyx_.
_Suborder (2)._ CRYPTODIRA.
The carapace and plastron vary in the extent to which they are
ossified, and except in _Dermochelys_[72] and its allies are covered
by horny plates. Marginal bones are always present. The head can
generally be drawn back under the carapace. The pelvis is not firmly
united to the plastron. The cranium often has a complete false roof,
and in the mandibular articulation the cup is borne by the cranium,
and the knob by the mandible. Among the more important genera are
_Dermochelys_, _Chelone_, and _Testudo_.
_Suborder (3)._ PLEURODIRA.
The carapace and plastron are strongly ossified, and firmly united to
the pelvis. The head and neck can be folded laterally under the
carapace, but cannot be drawn back under it. The cranium has a more or
less complete false roof, and in the mandibular articulation the knob
is borne by the cranium, and the cup by the mandible. _Chelys_ is a
well-known genus.
_Order 4._ ICHTHYOSAURIA[73].
The order includes a number of large extinct marine reptiles whose
general shape is similar to that of the Cetacea. The skull is
enormously large, and the neck short. The tail is very long, and is
terminated by a large vertically-placed bilobed fin, the vertebral
column running along the lower lobe. The very numerous vertebrae are
short and deeply biconcave. The vertebral column can be divided into
caudal and precaudal regions only, as the ribs which begin at the
anterior part of the neck are continued to the posterior end of the
trunk without being connected with any sternum or sacrum. The precaudal
vertebrae bear two surfaces for the articulation of the ribs, while
in the caudal vertebrae the two surfaces have coalesced. The caudal
region is also distinguished by its chevron bones. The vertebrae have
no transverse processes, and the neural arches are not firmly united
to the centra, and have only traces of zygapophyses. The atlas and
axis are similar to the other vertebrae, but there is a wedge-shaped
intercentrum between the atlas and the skull, and another between
the atlas and the axis. The skull is greatly elongated (fig. 32) and
pointed, mainly owing to the length of the premaxillae. The orbits
are enormous, and there is a ring of bones in the sclerotic (fig. 32,
15). The anterior nares are very small; and are placed far back just
in front of the orbits. There is an interparietal foramen, and the
supratemporal fossae (fig. 32, 9) are very large, while there are no
infratemporal fossae. An epipterygoid occurs. The quadrate is firmly
fixed to the cranium, and there is a large parasphenoid. There are
large prefrontals, but the frontals are very small. The very numerous
teeth are large and conical, and are placed in continuous grooves
without being ankylosed to the bone. They are confined to the jaw-bones.
[Illustration FIG. 32. LATERAL (BELOW) AND DORSAL (ABOVE) VIEWS OF THE
SKULL OF AN _Ichthyosaurus_. (Modified from Deslongchamps.)
1. premaxillae.
2. maxillae.
3. nasal.
4. prefrontal[1].
5. frontal.
6. postfrontal[74].
7. anterior nares.
8. orbit.
9. supratemporal fossa.
10. interparietal foramen.
11. parietal.
12. squamosal.
13. supratemporal.
14. quadratojugal.
15. sclerotic ring.
16. postorbital.
17. jugal.
18. lachrymal.
19. dentary.
20. articular.
21. angular.]
The ribs are long, and the anterior ones have capitula and tubercula.
There is no sternum, but the ventral body wall is strengthened by a
complex system of abdominal splint ribs.
The pectoral girdle is strongly developed, the scapulae are narrow,
the coracoids broad, and meet ventrally without overlapping. There are
probably no precoracoids, but clavicles and a T-shaped interclavicle
are well developed.
The limbs are very short, and completely modified into swimming
paddles. The humerus and femur are both short, while the radius and
ulna, tibia and fibula are generally still further reduced to the form
of short polygonal bones.
The digits are formed of longitudinal series of very numerous small
bones. The number of digits is five, but there sometimes appear to be
more owing to the bifurcation of certain of them, or to the addition
of marginal bones, either to the radial or ulnar side of the limb. The
humerus has no foramen, and both humerus and femur are unique in that
they are distally terminated by concave surfaces instead of by convex
condyles. The pelvic limb is much smaller than the pectoral. The
pelvis has no bony connection with the vertebral column, and all the
component bones are small and rod-like.
The Ichthyosauria are confined to beds of Secondary age and by far the
best known genus is _Ichthyosaurus_.
_Order 5._ RHYNCHOCEPHALIA.
This order includes the living _Sphenodon_ (_Hatteria_) and various
extinct forms. The general shape of these animals is lizard-like and
the tail is long.
The vertebrae are amphicoelous or sometimes nearly flat, and the
notochord sometimes persists to some extent. _Proterosaurus_ differs
from the other members of the order in having opisthocoelous cervical
vertebrae.
The sacrum is composed of two vertebrae. Ossified inter centra
(interdorsalia) generally occur in the cervical and caudal regions,
and sometimes throughout the whole vertebral column. In the skull the
quadrate is immovably fixed and united to the pterygoid. The palate is
well ossified, while the premaxillae which are often beak-like are
never ankylosed together. The jaws may be toothless or may be provided
with teeth which are usually acrodont (see p. 199). The palatines
frequently bear teeth, and in _Proterosaurus_ teeth occur also on the
pterygoids and vomers. The rami of the mandible are united by ligament
at the symphysis except in the Rhynchosauridae, in which the union is
bony. Superior and inferior temporal arcades occur.
The ribs have capitula and tubercula, and often uncinate processes
(see p. 190) as in birds. A pectoral girdle and sternum, with
clavicles and a T-shaped interclavicle are developed, and abdominal
ribs are always found. The precoracoid is however absent. The limbs
are pentedactylate.
_Sphenodon_[75] (Hatteria) now living in some of the islands of the
New Zealand group, is certainly the most generalised of all living
reptiles. Though lizard-like in form it differs from all living
lizards in the possession of two temporal arcades, abdominal ribs and
a fixed quadrate; and is often considered to be nearly allied in many
respects to the type of reptile from which all the others took their
origin.
Among the better known extinct forms are _Proterosaurus_ of Permian
and _Hyperodapedon_ of Triassic age.
_Order 6._ SQUAMATA.
This order includes the extinct Mosasaurians, and the lizards and
snakes which form the vast majority of living reptiles. The trunk may
be moderately elongated and provided with four short limbs as in
lizards, or it may be limbless, extremely elongated, and passing
imperceptibly into the tail. The surface is generally completely
covered with overlapping horny epidermal scales, below which bony
dermal scutes may be developed.
The vertebrae are procoelous, rarely amphicoelous. There are no inter
centra, and the neural arches are firmly united to the centra.
Additional articulating surfaces, the zygosphenes and zygantra, are
often developed[76]. The sacrum is formed of two or rarely three
vertebrae, or may be wanting as in Ophidia. In the skull an
infratemporal arcade forming the lower boundary of the infratemporal
fossa is absent, and the quadrate, except in the Chamaeleons, is
movably articulated to the squamosal. The palatal vacuities are large
and the nares are separate. There is often a distinct parasphenoid.
The teeth are either _acrodont_ (i.e. ankylosed to the summit of the
jaw), or _pleurodont_, i.e. ankylosed to the inner side of the jaw.
The thoracic ribs each have a single head which articulates with the
centrum of the vertebra; while uncinate processes and abdominal ribs
never occur.
A pectoral girdle and sternum may be present, or may be completely
absent as in snakes. Except in snakes there are generally four
pentedactylate limbs which may either form paddles or be adapted for
walking.
_Suborder (1)._ LACERTILIA[77].
The body is elongated, and as a rule four short pentedactylate limbs
are present, but sometimes limbs are vestigial or absent. The
exoskeleton generally has the form of horny plates, spines, or scales;
while sometimes as in the Chamaeleons and Amphisbaenians it is absent.
In other forms such as _Tiliqua_ and _Scincus_, the body has a
complete armour of bony scutes, whose shape corresponds with that of
the overlying horny scales.
The vertebrae are procoelous, rarely as in the Geckos amphicoelous;
they are usually without zygosphenes and zygantra, but these
structures occur in the Iguanidae. The sacral vertebrae of living
forms are not ankylosed together, and the caudal vertebrae usually
have well-developed chevron bones.
In the skull[78] the orbits are separated from one another, only by an
imperfectly developed interorbital septum, the cranial cavity not
extending forwards between them, while the alisphenoidal region is
unossified. The premaxillae may be paired or united (Amphisbaenidae),
and there is usually an interparietal foramen. There may be a complete
supratemporal[79] arcade bounding the lower margin of the
supratemporal fossa, or the supratemporal fossa may be open below. The
quadratojugal is not ossified, and the quadrate articulates with the
exoccipital. There is no infratemporal arcade. There is commonly a
rod-like epipterygoid[80] (fig. 33, 14) connecting the pterygoid and
parietal.
Teeth are always present, and may be confined to the jaws or may be
developed also on the pterygoids and rarely on the palatines; they are
either acrodont or pleurodont. The rami of the mandible are suturally
united.
A pectoral girdle is always present, and generally also a sternum.
Clavicles and a T-shaped interclavicle are commonly present, but are
absent in the Chamaeleons.
[Illustration FIG. 33. A, LATERAL VIEW, AND B, LONGITUDINAL SECTION OF
THE SKULL OF A LIZARD (_Varanus varius_). × 3/5. (Brit. Mus.)
1. premaxillae.
2. maxillae.
3. nasal.
4. lateral ethmoid.
5. supra-orbital.
6. lachrymal.
7. frontal.
8. postfrontal.
9. prefrontal.
10. basisphenoid.
11. pro-otic.
12. epi-otic.
13. pterygoid.
14. epipterygoid (columella cranii).
15. jugal.
16. transpalatine.
17. parasphenoid.
18. quadrate.
19. parietal.
20. squamosal.
21. supratemporal.
22. exoccipital.
23. dentary.
24. splenial.
25. supra-angular.
26. angular.
27. coronoid.
28. articular.
29. vomer.
30. basi-occipital.
31. orbitosphenoid.]
There is no separate precoracoid but a precoracoidal process (fig. 34,
7) of the coracoid is generally prominent.
[Illustration FIG. 34. LATERAL VIEW OF THE SHOULDER-GIRDLE OF
_Varanus._ × 3/5.
(Brit. Mus.).
1. suprascapula.
2. scapula.
3. glenoid cavity.
4. coracoid.
5. clavicle.
6. interclavicle.
7. precoracoidal process.]
Sternal ribs are present in chamaeleons and scinks. The limbs are in
the great majority of cases pentedactylate and the digits are clawed.
The phalanges articulate by means of condyles. Sometimes one or both
pairs of limbs are absent. When the posterior limbs are absent the
pelvis is also wanting, though the loss of the anterior limbs does not
lead to a corresponding loss of the pectoral girdle.
The pubis corresponds to the pre-pubis of Dinosaurs, and both pubes
and ischia meet in ventral symphyses.
The suborder includes the Lizards, Chamaeleons and Amphisbaenians.
_Suborder (2)._ OPHIDIA[81].
The Ophidia or snakes are characterised by their greatly elongated
body and want of limbs. The body is covered with overlapping horny
scales and bony dermal scutes are never present. The vertebrae are
procoelous, and are distinguishable into two groups only, precaudal or
rib-bearing, and caudal or ribless. The atlas vertebra is also
ribless. The neural arches are always provided with zygosphenes and
zygantra. Many of the vertebrae have strong hypapophyses, and the
caudal vertebrae are without chevron bones.
In the skull the cranial cavity extends forwards between the orbits,
and is closed in front by downgrowths from the frontals and parietals
which meet the well-ossified alisphenoids and orbitosphenoids[82]. The
cranium is strongly ossified, and there are no parotic processes or
interparietal foramen. There are no temporal arcades and no
epipterygoid. The premaxillae if present are very small (fig. 51, 1)
and usually toothless. The quadrates articulate with the squamosals,
and do not as in Lacertilia meet the exoccipitals. The palatines do
not unite directly with the vomers or with the base of the cranium,
and the whole palato-maxillary apparatus is more loosely connected
with the cranium than it is in Lacertilia. The pterygoids, and in most
cases also the palatines, bear teeth. The dentition is acrodont, and
the rami of the mandible are united only by an elastic ligament--an
important point serving to distinguish the Ophidia from the
Lacertilia. There is an imperfectly developed interorbital septum, the
ventral part of which is formed by the parasphenoid. The postfrontal
is generally well developed, while the jugals and quadratojugals are
absent. There are never any traces of the anterior limbs or pectoral
girdle, but occasionally there are vestiges of a pelvis and posterior
limbs.
_Suborder (3)._ PYTHONOMORPHA[83].
This suborder includes _Mosasaurus_ and its allies, a group of
enormous extinct marine reptiles found in beds of Cretaceous age.
The skin is in most forms at any rate unprovided with dermal scutes.
The vertebrae may be with or without zygosphenes and zygantra. The
skull resembles that of lizards, having an interparietal foramen, and
a cranial cavity open in front. The squamosal takes part in the
formation of the cranial wall, and the quadrate articulates with the
squamosal, not as in Lacertilia with the exoccipital. There are large
supratemporal fossae, bounded below by supratemporal arcades. The
teeth are large and acrodont, and occur on the pterygoids as well as
on the jaws. The two rami of the mandible are united by ligament only.
Pectoral and pelvic girdles are present, but clavicles are wanting,
and the pelvis is not as a rule united to any sacrum.
The limbs are pentedactylate, and are adapted for swimming, while all
the limb bones except the phalanges are relatively very short. The
number of phalanges is not increased beyond the normal, and they
articulate with one another by flat surfaces. The terminal phalanges
are without claws.
_Order 7._ DINOSAURIA[84].
The extinct reptiles comprising this order were all terrestrial, and
include the largest terrestrial animals known. They vary greatly in
size and in the structure of the limbs, some approach close to the
type of structure met with in birds, others are allied to crocodiles.
Passing to the more detailed characters:--there is sometimes a
well-developed exoskeleton having the form of bony plates or spines.
The vertebrae may be solid or their centra may be hollowed internally;
their surfaces may be flat, biconcave or opisthocoelous. The sacrum is
composed of from two to six vertebrae.
As regards the skull, the quadrate is large and fixed, and
supratemporal and infratemporal fossae bounded by bone occur. The
teeth are more or less laterally compressed, and often have serrated
edges; they may be placed in distinct sockets or in a continuous
groove. The ribs have capitula and tubercula, and sternal ribs often
occur. The scapula is very large, the coracoid small, and there is no
precoracoid, or T-shaped interclavicle. Clavicles are only known in a
few cases. In the pelvis the ilium is elongated both in front of, and
behind, the acetabulum, sometimes the pre-pubis, sometimes the
post-pubis is the better developed. The anterior limbs are shorter
than the posterior, and the long bones are sometimes solid, sometimes
hollow.
There are three well-marked suborders of the Dinosauria.
_Suborder (1)._ SAUROPODA[85].
The reptiles belonging to this group were probably quadrupedal and
herbivorous.
They have the cervical and anterior trunk vertebrae opisthocoelous,
while the posterior vertebrae are biconcave; all the presacral, and
sometimes the sacral vertebrae are hollowed internally. The teeth are
spatulate and without serrated edges, they are always planted in
distinct sockets, and some of them are borne by the premaxillae.
[Illustration FIG. 35. RESTORED SKELETON OF _Ceratosaurus nasicornis_.
× 1/30. (After Marsh.)
1. anterior nares.
2. prominence on the nasal bones which probably carried a horn.
3. pre-orbital vacuity.
4. orbit.
5. scapula.
6. coracoid.
7. ilium.
8. pubis (pre-pubis).
9. ischium.]
The nares have the form of long slits and there are large pre-orbital
vacuities.
The limb bones are solid, and the anterior limbs are not much shorter
than the posterior ones. All the limbs are plantigrade and
pentedactylate, and the digits of the pes are clawed. There is a large
pre-pubis directed downwards and forwards, meeting its fellow in a
ventral symphysis, but there is no post-pubis.
The Sauropoda are found in the secondary rocks of Europe and N.
America and include the largest land animals that are known to have
existed. Many of the best known forms such as _Brontosaurus_ and
_Morosaurus_ are North American.
_Suborder (2)._ THEROPODA.
The members of this suborder were all carnivorous, and from the small
comparative size of the anterior limbs many of them were probably
bipedal.
The vertebrae are opisthocoelous or amphicoelous, their neural arches
are provided with zygosphenes and zygantra, and their centra are
frequently hollowed internally; the limb bones are also hollow, and in
fact the whole skeleton is extremely light. The tail is of great
length. The teeth are pointed and recurved, and have one or both
borders serrated; they are always planted in distinct sockets, and
some of them are borne by the premaxillae. There are large pre-orbital
vacuities. The digits of both manus and pes are terminated by pointed
ungual phalanges which must have borne claws. In the pelvis the
pre-pubes and ischia are slender bones, the former meeting in a
ventral symphysis. The ilia are very deep vertically and there are no
post-pubes. The astragalus is closely applied to the tibia, in front
of which it sends an ascending process, sometimes the two bones appear
to have been ankylosed together, as in birds. The metatarsals are
elongated and the feet digitigrade.
The Theropoda vary greatly in size, one of the best known genera
_Compsognathus_ was about as large as a cat, another, _Megalosaurus_,
perhaps as large as an elephant. _Ceratosaurus_ is the name of a
well-known North American form regarded by many authorities as
identical with _Megalosaurus_.
_Suborder (3)._ ORTHOPODA.
This suborder includes the most specialised of the Dinosaurs, certain
of which resemble the Theropoda in being bipedal. In some of them such
as _Stegosaurus_ the exoskeleton is strongly developed, in others such
as _Iguanodon_ it is absent.
The vertebrae are solid and may be opisthocoelous, biconcave, or flat.
The teeth are compressed and serrated, often irregularly, and are
frequently not set in distinct sockets. The anterior part of the
premaxillae is without teeth, and a toothless predentary or
mento-meckelian bone is present. The pre-orbital vacuities are small
or absent, and the nares are large and placed far forwards.
The most characteristic features of the group are found in the pelvis
which, except in the Ceratopsia, bears a striking resemblance to that
of birds. The ischium and post-pubis are long slender bones directed
backwards parallel to one another, and the pre-pubis is also well
developed. The ischium has an obturator process. The limb bones are
sometimes hollow, sometimes solid. The anterior limbs are much shorter
than the posterior, pointing to a bipedal method of progression. The
pes is digitigrade or plantigrade, and has three, rarely four,
digits.
The suborder Orthopoda may be further subdivided into three
sections:--
A. STEGOSAURIA.
A dermal exoskeleton is strongly developed. The vertebral centra are
flat or biconcave, and neither they nor the limb bones are hollowed
out by internal cavities. The limbs are plantigrade, the anterior ones
short, the posterior ones very large and strong. The post-pubis is
well developed;
e.g. _Stegosaurus_ from the Upper Jurassic of Colorado.
B. CERATOPSIA.
There is sometimes a well-developed dermal exoskeleton formed of small
granules and plates of bone. The bones are solid, and the vertebral
centra flat. The cranium bears a pair of enormous pointed frontal
horns, and the parietal is greatly expanded and elevated behind,
forming with the squamosals a shield which overhangs the anterior
cervical vertebrae. The premaxillae are united, and in front of them is
a pointed beak-like bone which bites upon a toothless predentary
ossification of the mandible. The teeth have two roots. The anterior
limbs are but little shorter than the posterior ones. There is no
post-pubis;
e.g. _Polyonax_ from the uppermost Cretaceous of Montana.
C. ORNITHOPODA[86].
There is no dermal exoskeleton. The cervical vertebrae are
opisthocoelous, and so are sometimes the thoracic. The limb bones are
hollow and the anterior limbs are much shorter than the posterior
ones. The feet are digitigrade and provided with long pointed claws.
The post-pubis is long and slender and directed back parallel to the
ischium;
e.g. _Iguanodon_ from the European Cretaceous.
_Order 8._ CROCODILIA[87].
This order includes the Crocodiles, Alligators and Garials and various
extinct forms, some of which are closely allied to the early
Dinosaurs.
There is always a more or less complete exoskeleton formed of bony
scutes overlain by epidermal scales; these bony scutes are specially
well developed on the dorsal surface but may occur also on the
ventral. The vertebral column is divisible into the five regions
commonly distinguishable. In all living forms the vertebrae, with the
exception of the atlas and axis, the two sacrals, and first caudal,
are procoelous, but in many extinct forms they are amphicoelous. The
atlas (fig. 71) is remarkable, consisting of four pieces, and the
first caudal is biconvex.
The teeth are, in the adult, planted in separate deep sockets. The
skull is very dense and solid, and all the component bones including
the quadrate are firmly united. The dorsal surface of the skull is
generally characteristically sculptured. There is an interorbital
septum, and the orbitosphenoidal and presphenoidal regions are
imperfectly ossified. Supratemporal, infratemporal, and post-temporal
fossae occur, but no interparietal foramen. In living genera there is
a long secondary palate formed by the meeting in the middle line of
the palatines, pterygoids and maxillae (fig. 43, A).
Cervical ribs (fig. 41, 8 and 9) are well developed, and articulate
with rather prominent surfaces borne on the neural arches and centra
respectively. The thoracic ribs articulate with the long transverse
processes, and sternal ribs and abdominal splint ribs (fig. 46, 4)
occur. The sternum is cartilaginous, and both it and the
shoulder-girdle are very simple. The precoracoid is represented by
merely a small process on the coracoid, while the clavicles are
absent, except in the Parasuchia. In the pelvis (fig. 49) there is a
large ilium, and an ischium meeting its fellow in a ventral symphysis;
these two bones form almost the whole of the acetabulum. In front of
the acetabulum, in the Eusuchia, projects a bone which is generally
called the pubis, but is in reality rather an epipubis (fig. 49, 4),
the true pubis being probably represented by a fourth element which
remains cartilaginous for some time, and later on ossifies and
attaches itself to the ischium. The limbs are small in proportion to
the size of the body, and are adapted for swimming or for shuffling
along the ground; they are plantigrade and the bones are all solid. In
living forms the anterior limbs have five digits and the posterior
four, the fifth being represented only by a short metatarsal. The
first three digits in each case are clawed. The calcaneum has a large
backwardly-projecting process.
The order Crocodilia may be subdivided into two suborders.
_Suborder (1)._ PARASUCHIA.
The vertebral centra are flat or biconcave. The premaxillae are very
large, and the nares are separated, and placed far back. The posterior
narial openings lie comparatively far forward between the anterior
extremities of the palatines.
The palatines and pterygoids do not form a secondary palate. The
supratemporal fossae are small, and open posteriorly, the lateral
temporal fossae are very large. The parietals and frontals are paired.
Clavicles are present. The best known and most important genus of
these extinct crocodiles is _Belodon_.
_Suborder_ (2). EUSUCHIA.
The vertebrae are either biconcave or procoelous. The premaxillae are
small, and the anterior nares are united and placed far forwards. The
posterior nares lie far back, the palatines and in living genera the
pterygoids, meeting in the middle line, and giving rise to a closed
palate. The supratemporal fossae are surrounded by bone on all sides,
and the parietals, and often also the frontals are united. There are
no clavicles. The suborder includes the genera _Crocodilus_,
_Alligator_, _Garialis_ and others living and extinct.
_Order 9._ PTEROSAURIA[88].
These animals, called also the pterodactyles or Ornithosauria, are a
group of extinct reptiles, whose structure has been greatly modified
from the ordinary reptilian type for the purpose of flight.
The skin was naked and they vary greatly in size and in the length of
the tail. The vertebrae and limb bones are pneumatic just as in birds.
The presacral vertebrae are procoelous and have their neural arches
firmly united to the centra. The neck is long, the caudal vertebrae
are amphicoelous, and from three to five vertebrae are fused together
in the sacral region. The skull is large and somewhat bird-like, the
facial portion being much drawn out anteriorly, and the sutures being
obliterated. It resembles that of other reptiles in having large
supratemporal fossae; large pre-orbital vacuities also occur. The jaws
may be toothed or toothless, and the teeth, when present, are imbedded
in separate sockets. The premaxillae are large, and the quadrate is
firmly attached to the skull. The rami of the mandible are united at
the symphysis, and there is an ossified ring in the sclerotic. The
occurrence of a postfrontal and its union with the jugal behind the
orbit, are characteristic reptilian features.
The ribs have capitula and tubercula, and sternal and abdominal ribs
occur. The sternum has a well-developed keel, and the scapula and
coracoid are large and bird-like. There are no clavicles or
interclavicle.
The anterior limbs are modified to form wings by the great elongation
of the fifth digit, to which a membrane was attached. The second,
third and fourth digits are clawed and are not elongated in the way
that they are in bats. The pollex, if present at all, is quite
vestigial.
The pelvis is weak and small, and though the ilia are produced both in
front of and behind the acetabula, in other features the pelvis is not
bird-like. The ischia are short and wide, and the pubes are
represented only by the pre-pubes. The posterior limbs are small and
the fibula is much reduced. The pes is quite reptilian in type, and
has five separate slender metatarsals. The two best known genera are
_Pterodactylus_, in which the tail is short, and _Rhamphorhynchus_, in
which it is long. The Pterosauria are found throughout the Jurassic
and Cretaceous formations in both Europe and North America.
FOOTNOTES:
[65] According to Baur a distinct epi-otic is not recognisable in the
reptilian skull.
[66] H. Gadow, _Phil. Trans._, vol. 179, 1888.
[67] See G. Baur, _J. Morph._, vol. I., 1887. R. Lydekker, _Catalogue
of the Fossil Reptilia and Amphibia in the British Museum_, Parts I. &
II. C.K. Hoffmann, _Reptilien_, in Bronn's _Classen und Ordnungen des
Thierreichs_, Bd. VI., 3 abth. 1879-90.
[68] T.H. Huxley, _Quart. J. Geol. Soc._, vol. XV. p. 649, 1859. R.
Owen, _Catalogue of Fossil Reptiles of S. Africa in the British
Museum_, London, 1876. H.G. Seeley, various papers published in the
_P.R. Soc. London_, and _Phil. Trans._
[69] See pp. 281-283.
[70] An ent-epicondylar foramen is one piercing the humerus on its
inner side just above the condyle.
[71] According to Hulke they should be regarded as the
omosternum,--the clavicles and interclavicle being wanting.
[72] See p. 272.
[73] R. Lydekker, _Nat. Sci._ vol. I. p. 514, 1892. Further references
are there given.
[74] The exact position of the suture between the prefrontal and
postfrontal is not known.
[75] A. Günther, On the Anatomy of Hatteria, _Phil. Trans_, vol. 157,
1867, p. 595.
[76] =Zygosphenes= are extra articulating surfaces borne upon the
anterior face of the neural arch; they fit into corresponding
structures, the =zygantra=, which are borne on the posterior surface
of the neural arch of the preceding vertebra. Ordinary zygapophyses
always accompany them.
[77] See E.D. Cope, _P. Amer. Phil. Soc_. vol. xxx. p. 185.
[78] See W.K. Parker, _Phil. Trans._ vol. 170, 1879, p. 595.
[79] See p. 281.
[80] Often called the columella cranii.
[81] See C.K. Hoffmann, in Bronn's _Klassen und Ordnungen des
Thierreichs_, Bd. VI., 3 abth. 1885-90.
[82] Some anatomists consider that the closing in of the brain case in
front is entirely due to the frontals and parietals.
[83] E.D. Cope, _Rep. U.S. Geol. Surv._, 1875, vol. II., The
vertebrata of the Cretaceous formations of the west. E.D. Cope, _P.
Boston Soc._ 1862, XII. p. 250. O.C. Marsh, _Amer. J. Sci._, 1872,
vol. 3. R. Owen, _Quart. J. Geol. Soc._, 1877, and 1878.
[84] J.W. Hulke, _Presidential address to the Geol. Soc. of London_,
1883 and 1884. O.C. Marsh, many papers in the _Amer. J. Sci._ from
1878 onwards, also in the _Geol. Mag._ R. Owen, _History of British
fossil reptiles: Dinosauria_ (Palaeont. Soc.).
[85] The diagnostic characters of the different groups of Dinosaurs
are in the main those given by von Zittel.
[86] See O.C. Marsh, _Amer. J. Sci._ (3), vol. 48, 1894, p. 85.
[87] See C.B. Brühl, _Das Skelet der Krokodiliden_, Wien, 1862. C.K.
Hoffmann in Bronn's _Klassen und Ordnungen des Thierreichs_, Bd. VI.
Abth. III. 1881-85. T.H. Huxley, _Proc. Linn. Soc._ (Zoology) 1860
vol. IV. p. 1. R. Owen, _History of British fossil Reptiles_.
_Crocodilia_ (Palaeont. Soc.). A. Smith Woodward, _Geol. Mag._ 1885,
3rd dec. II. p. 496. A. Smith Woodward, _Proc. of Geologists' Assoc._
vol. IX. p. 288, 1886.
[88] See H.G. Seeley, On the Organisation of the Ornithosauria,
_Journ. Linn. Soc._ (Zoology) vol. XIII. p. 84. K.A. Zittel, Ueber
Flugsaurier aus dem lithographischen schiefer, _Palaeontograph._ XXIX.
p. 49.