The Classification and the Osteology of the Reptilia

The province Sauropsida is divisible into the two classes, Reptilia and Aves.

All Reptilia, so far as their organization is known to us, are distinguished from Aves by the following characters:
1. The exoskeleton is composed of horny plates (scales), or bony plates (scutes), never of feathers.
2. The centra of the vertebrae may be amphicoelous, procoelous, opisthocoelous, or may have nearly flat articular faces; but these faces are spheroidal or oval, and are never cylin droidal, even in the cervical region. (The articular faces of the vertetrae of some Pterosauria are very much elongated transversely.)
3. When reptiles possess a sacrum, the sacral vertebrae have large expanded ribs, with the ends of which the ilia articulate.
4. The sternum is rhomboidal; and, when many ribs are connected with it, the hindermost of these are attached to a single, or double, median backward prolongation (except, perhaps, in the Pterosauria). The sternum may be converted into cartilage-bone, but (with the possible exception of the Pterosauria) is never replaced by membrane-bone, and does not ossify from two, or more, definite centres.
5. When an interclavicle exists, it remains distinct from the clavicles.
6. The manus contains more than three digits (? Dinosauria), and the three radial digits, at fewest, have claws.
7. In all existing reptiles, the ilia are prolonged farther behind the acetabulum, than in front of it; and the inner wall of the acetabulum is wholly, or almost completely, ossified. The pubes are directed dovraward and forward, and, like the ischia, meet in a ventral symphysis. In the extinct Dino sauria, the pelvis exhibits forms transitional between the reptilian and the ornithic arrangement.
8. The digits of the pes are not fewer than three; and the metatarsal bones are not anchylosed together, or with the distal tarsal bones.
9. In existing reptiles not fewer than two aortic arches (a right and a left) persist. Two arterial trunks are given off from the right ventricle, or the part of the single ventricle which answers to it. The venous and arterial currents of the blood are connected, either in the heart itself, or at the origins of the aortic arches.
10. The blood is cold. There are usually two semilunar valves at the origins of the aortic and pulmonary trunks.
11. The corpora bigemina lie upon the upper surface of the brain.

In Aves, on the contrary:
1. The exoskeleton consists of feathers. Ossifications of the dermis are rare, and never take the form of scutes.
2. In all recent birds, the centra of the cervical vertebrae, at least, have subcylindrical articular faces. If, as in some birds, the faces of the centra of the other vertebrae are spheroidal, they are opisthocoelous, which is the rarest arrangement among reptiles.
3. The proper sacral vertebrae of birds-that is to say, those between, or through, the arches of which the roots of the sacral plexus pass-have no expanded ribs abutting on the ilia.
4. The sternum has no costiferous median backward prolongation, all the ribs being attached to its sides. The cartilaginous sternum is replaced, in the adult, by membrane-bone, and ossifies from two, to five or more, centres,
5. When an interclavicle exists it is confluent with the clavicles.
6. The manus does not contain more than three digits, and not more than the two radial digits have claws.
7. The ilia are greatly prolonged in front of the acetabulum, the inner wall of which is membranous. The pubes and ischia are directed backward, more or less parallel with one another, and the ischia never meet in a ventral symphysis.
8. The astragalus sends up a process on to the front face of the tibia, and early anchyloses with the latter bone. In this character, Birds differ from all existing Reptiles. The foot contains not more than lour digits. The first metatarsal is, almost always, free, shorter than the rest, and incomplete above. The other three are anchylosed together, and with the distal tarsal bone, to form a tarso-metatarsus.

Some of the extinct Dinosauria closely resembled birds in the form of the tibia and astragalus, the immovable union of the two bones, and the reduction of the number of the digits.

9. Only one aortic arch, the right, is present. Only one arterial trunk, the pulmonic, is given off from the right ventricle. The arterial and venous currents communicate only by the capillaries.
10. The blood is hot. There are three semilunar valves at the origins of the aortic and pulmonary trunks. In all existing birds the extremities of the chief pulmonary passages terminate in air-sacs. There is a rudiment of this structure in the Chamaeleons, and the extinct Pterodactyles very probably possessed such sacs.
11. The corpora bigemina are thrown down to the sides and base of the brain.

The Reptilia. - This class is divisible, by well-defined characters, into the following groups :

1. The dorsal vertebrse (which, like all the other vertebra, are devoid of transverse processes) are not movable upon one another, nor are the ribs movable upon the vertebrae (Pleurospondylia). Most of the dorsal vertebrae and ribs are restrained from motion by the union of superficial bony plates, into which they pass, to form a carapace.
   Dermal bones, usually nine in number, one of which is median and symmetrical, and the others lateral and paired, are developed in the ventral walls of the thorax and abdomen, forming a plastron.
I. - Chelonia.
2. The dorsal vertebrae (which have either complete, or rudimentary, transverse processes) are movable upon one another, and the ribs upon them. There is no plastron.
1. The dorsal vertebrae have transverse processes, which are either entire, or only very imperfectly divided into terminal facets (Erpetospondtylia).
1. The transverse processes are long; the limbs well developed, with the digits united by the integument into a paddle; the sternum and sternal ribs are absent or rudimentary.
II. - Plesiosauria.
2. The transverse processes are short, and sometimes rudimentary; the limbs present or absent; when they are fully developed, the digits are free, and there is a well-developed sternum with sternal ribs.
1. A pectoral arch and urinary bladder.
III. - Lacertilia.
2. No pectoral arch, and no urinary bladder.
IV. - Ophidia.
2. The dorsal vertebrre bave double tubercles in the place of transverse processes (Ferospondylia). The limbs are paddle-like.
V. - Ichthyosauria.
3. The anterior dorsal vertebrae have elongated and divided transverse processes, the tubercular being longer than the cabitular division (Suchospondylia).
1. With only two vertebrae in the sacrum.
VI. - Crocodilia.
2. With more than two vertebrae in the sacrum.
1. The manus without a prolonged ulnar digit
1. The hind-limb saurian.
VII. - Dicynodontia.
2. The hind-limb ornithic.
VIII. - Ornithoscelida.
2. The manus having an extremely prolonged ulnar digit.
IX. - Plerosauria.

I shall describe the exoskeletal, endoskeletal, and dental systems of the chief groups of the Reptilia, under the several heads here enumerated, and I shall then give an account of these systems in Aves. But modifications in the myology, neurology, splanchology, and development of the two classes may be conveniently considered together, under those several heads, in another chapter.