Crocodiles, the highest living Reptilia
are Lacertilian in form, with long tails and four well-developed
limbs, the anterior pair being the shorter, and possessing
five complete digits, while the hind-feet are four-toed.
With a single exception, the living species have nails on the
three preaxial (radial and tibial) digits, so that two digits are
without nails on the fore-foot, and one on the hind-foot. The
feet are webbed, but the degree to which the web is developed
varies greatly. The nostrils are situated at the end of the
long snout, and can be closed. The tympanic membranes are
exposed, but a cutaneous valve, or earlid, lies above each, and
can be shut down over it. All are partially aquatic in habit,
and some (the Gavials) are completely so. None of the existing
genera are marine, though many ancient Crocodilia inhabited
The dermal armor is composed of scutes covered by epidermic
scales of corresponding form. When the armor is
complete-as in Caiman
alone among existing Crocodilia
, in Teleosaurus
forms-it consists of transverse rows of quadrate bony plates,
disposed so as to form a distinct dorsal and ventral shield,
separated by soft integument, in the trunk, but united into
continuous rings on the tail. The scutes of the same row are
united suturally; those of each row overlap their successors,
which present smooth facets to receive their under-surfaces.
In existing Crocodilia
, in the extinct Crocodilus Hastingsiae
and in Stagonolepis
, each ventral scute consists of two pieces,
a small anterior and a large posterior, united by a suture,
The scutes always exhibit a pitted sculpture, and those of the
dorsal region are ridged longitudinally, while the ventral
scales are always flat. More or fewer dorsal scutes exist in
all crocodiles, and those upon the neck sometimes form distinct
"nuchal" and "cervical" groups, distinct from the dorsal
shield. The dorsal scutes do not always overlap, and the
ventral scutes are absent, or incompletely ossified, in most existing Crocodilia
In these reptiles the vertebral column is always thoroughly
ossified, and marked out into distinct cervical, dorsal, lumbar,
sacral, and caudal regions. The number of the presacral vertebrae
is twenty-four; that of the sacral, two, in all the recent
forms, and probably in the extinct genera also. The number
of the caudal vertebra varies, but is not less than thirty-five.
The number of the cervical, dorsal, and lumbar vertebrae varies;
but there are usually nine of the first, eleven or twelve of the
second, and four, or three, of the third description.
In existing Crocodilia
all the vertebra, except the atlas
and axis, the two sacrals, and the first caudal, are procoelous.
The majority of the pre-cretaceous Crocodilia
have the corresponding
vertebrae amphicoelous, the concavities of the centra
being very shallow. One genus, Streptospondylus
, which is
perhaps Crocodilian, has the anterior vertebrae opisthocoelous.
It is characteristic of the Crocodilia
, that the centra of the
vertebrae are united by fibro-cartilages, and that the neurocentral
sutures persist for a long time, or throughout life.
The atlas is composed of four pieces, an upper median
piece-which is sometimes divided into two, and is developed
in membrane apart from the rest-being added to the three
pieces found in Lacertilia
. A large odontoid
bone is closely united to, but not anchylosed with, the anterior
flat face of the second vertebra. A pair of elongated, single headed
ribs are attached to the inferior piece of the atlas, and
another similar pair to the os odontoidum and to the second
vertebra, by distinct capitular and tubercular processes. The
other cervical vertebrae all possess ribs with distinct and long
capitula and tubercula-the latter attached above the neurocentral
suture to the neural arch, the former to the centrum
below the neurocentral suture. The body of each cervical
rib, after the second, and as far as the seventh or eighth, is
short, and prolonged in front of, as well as behind, the junction
of the capitulum with the tuberculum; and the several ribs
lie nearly parallel with the vertebral column, and overlap one
another. The ribs of the eighth and ninth cervical vertebrae
are longer, and take on more the character of the dorsal ribs,
the ninth having a terminal cartilage.
The points to which the capitula and tubercula of the ribs
are attached are raised into tubercles; and, by degrees, these
become elongated into distinct capitular and tubercular processes,
between which, in the third to the ninth vertebrae, the
neurocentral suture passes. But in the tenth and in the eleventh
vertebrae, the capitular process, which lies nearer the
neurocentral suture in the posterior than in the anterior cervical
vertebrae, rises upon the body of the vertebra to the level
of the neurocentral suture, by which it is traversed, and the
tubercular process becomes longer than it. (See Fig. 5, p. 19.)
The terminal cartilage is united with the sternum by a sternal
rib, which may become more or less completely converted into
a cartilage-bone, and is articulated with the vertebral rib.
In the twelfth vertebra a sudden change in the character
of the transverse processes takes place. There is no longer a
capitular, distinct from a tubercular, process, but one long
"transverse process" takes the place of both. A sort of step
in the base of this process bears the capitulum of the rib, and
answers to the capitular process of the cervical vertebra, while
the outer end of the process articulates with the tuberculum
of the rib, and represents the tubercular process. The neurocentral
suture, in this and the succeeding dorsal vertebrae, lies
below the root of the transverse process, which, therefore, is
wholly a product of the neural arch. Neither the capitular
processes, nor that part of the dorsal transverse process which
represents them, have distinct centres of ossification. (Thus, if it be a part
of the definition of a "parapophysis," that it is antogenous, there are no
parapophyses in the vertebrae of the Crocodilia
; and if it be
part of the definitation of a "parapophysis" that its arises from the centrum,
the dorsal vertebrae of the Crocodilia
have no parapophyses.)
In the succeeding dorsal vertebrae the "step" of the transverse
process gradually moves outward, until at length it becomes
confounded with the tubercular facet, and a corresponding
change takes place in the proximal ends of the ribs, in the
hindermost of which the distinction between capitulum and
tuberculum is lost.
The lumbar vertebrae have long transverse processes which
arise from the neural arches, i. e., above the neurocentral suture.
The centra of the two sacral vertebrae have their applied
and firmly-united faces flat, their free faces concave; consequently,
the first has the anterior face concave and the posterior
flat, while the second has the anterior surface flat and the
posterior concave. Each sacral vertebra has a strong rib expanded
at its distal end; and wedged in at its proximal end, between rough sutural surfaces furnished by the neural arch
above and the centrum below.
The first caudal vertebra is biconvex, but all the others are
procoelous; those of the anterior moiety of the tail have long
ribs fixed in between the neural arches and centra, as in the
sacrum, and becoming anchylosed in that position. Chevronbones
are attached to the posterior edges of the centra of the
vertebrae, except that of the first, and those of the posterior
part of the tail.
From seven to nine of the anterior dorsal ribs are united
with the sternum by sternal ribs, the form of which varies a
good deal in different Crocodilia
, being sometimes narrow,
sometimes broad and flattened. An elongated plate of cartilage,
which may be partially converted into cartilage-bone, is
attached to the hinder margin of several of the most anterior
ribs, above the junction between the ossified and the cartilaginous
part of the vertebral rib. (Fig. 5, P.u.
) These are the
so-called "uncinate processes," which also exist in Hatteria
and reappear in Birds.
The sternum consists of a rhomboidal plate of cartilagebone,
with the posterolateral edges of which two pairs of
sternal ribs articulate. The posterior angle of the plate is continued
into a median prolongation, which, at length, divides
into two curved divergent cornua. From five to seven pairs
of sternal ribs are united with the prolongation and its cornua.
A long and slender interclavicle lies in a groove of the middle
of the ventral face of the rhomboidal part of the sternum.
In the ventral wall of the abdomen, superficial to the recti
muscles, lie seven transverse series of membrane-bones, which
are termed "abdominal ribs;" though it must be recollected
that they are quite distinct from true ribs, and rather correspond
with the dermal ossicles of the Labyrinthodonta
series is composed of four elongated and more or less curved
ossicles, pointed at each end, and so disposed that inner ends
of the inner pair meet at an angle, open backward in the
middle line, while their outer ends overlap the inner ends of
the outer pair. The most posterior of these ossicles are
stronger than the others, and are closely connected with the
|Fig. 77. - Longitudinal and vertical section of the hinder part of the skull of a Crocodile; Eu, Eustachian tube: P N, posterior nares; P, pituitary fossa.
In the Crocodilian skull the following are the chief peculiarities
which are worthy of especial notice:
1. There is an interorbital septum, and the presphenoidal
and orbitosphenoidal regions remain cartilaginous, or very
2. All the bones of the skull (except the mandible, stapes,
and hyoid) are firmly united by sutures, which-persist throughout
3. There are large parotic processes. Both the upper and
the lower temporal arcades are completely ossified, and formed
by post-frontal, squamosal, jugal, and quadrato-jugal bones;
supra-temporal, lateral-temporal, and post-temporal fossae are
formed, as in the Lacertilia
, though their relative sizes are
4. The maxillary and the palatine bones develop palatine
plates, which unite suturally in the middle line, and separate
the nasal passages from the cavity of the mouth, as in Mammalia
and in all existing Crocodiles, but not in Teleosaurus
, the pterygoids are also modified in the same way
(as in Mymercophaga
among Mammals), so that the posterior
nares are situated very far back beneath the base of the skull.
5. In consequence of the development of these palatine
plates of the maxillary and palatine bones, the two vomers
are, in most Crocodiles, invisible upon the under-surface of
the bony roof of the mouth.
6. There are larger alisphenoids, but the orbitosphenoids
are absent or radimentary.
7. There is no parietal foramen.
8. The quadrate bone is very large, and fixed immovably
to the walls of the skull, as in the Chelonia
; and, as in the
latter, the pterygoid bone is firmly connected with the base
of the skull, and united only with the upper and inner surface
of the quadrate bone.
9. The pterygoid sends down a large free process, against
the broad outer edge of which the inner surface of the mandible
10. The tympanic cavity is completely bounded by bone.
The prootic and opisthotic (which is united with the ex-occipital)
form its inner walls, the quadrate its outer wall, the
squamosal and post-frontal its roof, and the quadrate, the basi-occipital,
and basisphenoid its floor. The two tympana are
placed in communication with the cavity of the mouth by three
canals-one large, opening in the middle line; and two smaller
ones at the sides, on the base of the skull, behind the posterior
nares. The large canal passes up between the basisphenoid
and basi-occipital, and divides between those bones into a
right and left lateral canal. Each lateral canal subdivides
into an anterior branch, which traverses the basisphenoid, and
a posterior, which passes up in the basi-occipital. The
posterior branch receives the narrow lateral canal of its side
(which runs vertically up to it), and then opens into the
posterior part of the floor of the tympanum. The anterior
branch opens into its anterior wall.
The tympanic cavities of embryonic Crocodiles communicate
with the mouth by wide and simple apertures, and the
complicated arrangement of canals just described results from
the great downward development of the basisphenoid and basi-occipital,
and their encroachment upon these apertures on the
inner side, while the quadrate bone narrows them on the outer.
In adult Crocodilia
, air-passages extend from each tympanum
to that of the opposite side, through the bones which
form the roof of the posterior region of the skull. On the
other hand, they excavate the quadrate bone, whence the
air passes through a membranous tube into the hollow articular
piece of the mandible. The hyoidean apparatus is
greatly simplified, consisting only of a broad plate of cartilage,
which may become partially ossified, and two ossified
cornua which are not directly connected a with the skull. A
minute styliform cartilage, which lies in close proximity with
the portio dura
, on the upper part of the posterior face of the
quadrate bone, represents the stylohyal
, or proximal end of
the hyoidean arch.
The pectoral arch has no clavicle, and the coracoid has no
distinct epicoraooidal element, nor any fontanelle. The carpus
consists proximally of two elongated and somewhat hour-glassshaped
bones, articulated respectively with the radius and the
ulna. The radial is the larger, and is partially articulated
with the ulna. Behind these, and directed transversely, lies
another curved ossification, the upper concave face of which
articulates with the ulna. It is united with the latter bone
on the one hand, and with the fifth metacarpal, on the other,
by strong ligaments, and represents a pisiform bone. Distally,
there lies on the ulnar side the so-called lenticular
oval ossicle interposed between the ulnar proximal carpal and
the second, third, fourth, and fifth metacarpals, the last three
of which it supports altogether. On the radial side, a disk
of cartilage, which never becomes completely ossified, is connected
by ligament with the lenticulare
, and is interposed
between the radial proximal bone and the head of the metacarpal
of the pollex. From the ulnar side of the head of this
bone a cartilaginous ligamentous band proceeds, over the head
of the second metacarpal, to the radial side of the lenticulare
The three radial digits are much stronger than the two
ulnar, and the numbers of the phalanges are 2, 3, 4, 4, 3,
counting from the radial to the ulnar side.
The pelvis (Fig. 78, C) possesses large ilia, which are firmly
united with the expanded ends of the strong ribs of the sacrum.
The ischium unites with its fellow in a median ventral symphysis,
and, with the ilium, forms almost the whole of the acetabulum.
The pubes take hardly any share in the formation of the
latter cavity in the adult. Their axes are directed forward
and inward, and they coalesce in the middle line; but as the
inner, or median, moiety of each pubis remains cartilaginous,
or imperfectly ossified, the bones, in imperfectly prepared
skeletons, appear as if they formed no symphysis.
The tarsus presents, proximally, an astragalo-navicular
bone and a calcaneum, which are less closely united than in
the Lizards. The latter bone has a large calcaneal process on
Its posterior face, the Crocodile being the only Sauropsid vertebrate
in which such a process is developed (Fig. 78, C. Ca
Two rounded distal tarsal bones, of which the fibular is
much the larger, lie between the calcaneum and the third,
fourth, and rudimentary fifth, metatarsals. A thin plate of
cartilage is interposed between the distal end of the astragalonavicular
and the second metatarsal, and unites with the head
of the first metatarsal.
As in the manus, the three, pre-axial, clawed, digits are
stronger than the others. The fifth is represented only by an
imperfect metatarsal. The numbers of the phalanges are
2, 3, 4, 4, counting from the tibial to the fibular side.
In the Crocodilia
the teeth are confined to the premaxillae,
maxillae, and dentary part of the mandible. They are simple
in structure, have large pulp-cavities, are lodged in distinct
alvcoli, and are replaced by others developed upon their inner
sides. The development of the new tooth causes absorption
of the inner wall of the base of the old one, and the replacing
tooth thus comes to lie within the pulp-cavity of its predecessor.
The teeth vary much in shape, having either long, curved,
and acute, or short and obtuse, or almost globular and straight,
crowns. Very often they possess sharp anterior and posterior
edges, which may be finely serrated.
are to be found in the rivers of all continents
and the larger islands in the hotter parts of the world.
None of the existing species are truly marine, though many
of the extinct species were. They are first known to occur in
strata of Triassic age, and abound, under forms which differ
but little from some of those which now exist, in the Mesozoic
and Cainozoic formations.
They may be divided into the following groups:
- With procoelous presacral vertebrae, and posterior nares bounded
below by the pterygoids. (All existing Crocodilia, and the fossil
forms of cretaceous and later formations, are included in this division.)
- The nasals enter into the formation of the nasal aperture.
- The head short and broad. The teeth very unequal; the first
and fourth of the mandibles biting into pits of the upper
jaw. The premaxillo-maxillary suture straight or convex
forward. The mandibular symphysis not extending beyond
the fifth tooth, and the splenial element not entering into it.
The cervical scutes distinct from the tergal.
Alligator. Caiman, Jacare.
- The head longer. The teeth unequal. The first mandibular
tooth biting into a fossa; the fourth, into a groove, at the
side of the upper jaw. The premaxillo-maxillary suture
straight or convex backward. The mandibular symphysis
not extending beyond the eighth tooth, and not involving
the splenial elements. The cervical scutes sometimes distinct
from the tergal, sometimes united with them.
- The nasals are excluded from the external nasal aperture. The head very long; the teeth subequal. Both the first and the fourth mandibular teeth bite into grooves in the margin of the upper jaw. The premaxillo-maxillary suture acutely angulated backward. The mandibular symphysis extends to at least the fourteenth tooth, and the splenials enter into it. The cervical and tergal seutes form a continuous series.
- With the presacral vertebrae amphieoelons (the anterior vertebrae
sometimes opisthocoelous (?) ); and the posterior nares bounded by
the palatines, the pterygoids not being united below. (All these
Crocodiles are extinct and pre-cretaceous.)
- With the external nares terminal.
Streptospondylus. Stagonolepis. Galesaurus (?).
- With the external nares on the upper part of the base of the snout
near the orbits.
There is a large number of extinct Reptilia
in the characters of their pre-sacral vertebrae,
but differ from them, and resemble Lacertilia Chelonia
Birds, in other respects.
These are the Dicynodontia
, the Ornithoscelida
, and the Pterosauria.