[578] The most ancient Reptile hitherto discovered is the Telerpeton Elginense, from the Old Red of Scotland, which will be described in the sequel.
The animals comprehended in the Class of Reptilia constitute, according to Prof. Owen’s arrangement, eight principal groups, or Orders, as follow:—
| The Batrachia; Frogs: the body naked, with only rudimentary ribs; and with two or four feet. Most of these reptiles breathe by branchiæ or gills in their young state, and by lungs in the adult (as for example the Frog); in some (the perenni-branchiata), the branchiæ are persistent through life. |  | Having a tripartite heart (i. e. with two auricles and one ventricle), and simple transverse processes to the cervical and anterior dorsal vertebræ. |
| The Ophidia; Serpents: the body destitute of feet. | ||
| The Lacertia or Sauria; Lizards: the body supported by four or two feet, and covered with scales. | ||
| The Chelonia; Tortoises: the body supported by four feet or paddles, and enveloped in two osseous bucklers, composed of the expanded bones of the sternum and thorax. | ||
| The Enaliosauria; Sea-saurians (extinct): body furnished with four paddles, and destitute of scaly covering. | | Having a quadripartite heart (i. e. with two auricles and two ventricles), and double transverse processes to the cervical and anterior dorsal vertebræ. |
| The Pterosauria; Wing-saurians (extinct): body supported on four feet, the outer finger of each fore-foot greatly lengthened, and forming a support for the wing. | ||
| The Crocodilia; Crocodiles: body supported on four partially webbed feet, and encased with an armour of bony plates or scutes. | ||
| The Deinosauria; Great-saurians (extinct): body supported on four feet. |
Teeth of Reptiles.—The teeth of the animals of this class exhibit considerable diversity of form, but the characteristic type is that of a conical, pointed tooth, with a simple root or fang; for, in no reptile does the base of the tooth terminate in more than one fang, and this is never branched. "Any fossil, therefore, which exhibits a tooth implanted by two fangs in a double socket, must be mammiferous, since the socketed teeth of reptiles have but a single fang; and the only fishes’ teeth which approach such a tooth in form, are those of a bifurcate base, belonging to certain sharks." (Owen.)
These dental organs are only fitted for seizing and retaining the prey or food; for no living reptiles have the power of performing mastication. In the Crocodiles the tooth has a cylindrical shank, with a conical, longitudinally striated, enamelled crown, having a ridge on each side ([Pl. VI. fig. 5]). In the Labyrinthodon (a fossil reptile), the cone is more curved and pointed ([Pl. VI. fig. 3]); in the Hylæosaurus, the shank is cylindrical, and the crown expanded and lanceolate, with blunt margins ([Pl. VI. fig. 6]); in the Megalosaurus, the tooth is laterally compressed, trenchant, and slightly inclined backwards like a sabre, with serrated edges ([Pl. VI. fig. 7]); in the Iguanodon, the shank is cylindrical, and the crown of a prismatic form, greatly expanded, with broad denticulated edges, and longitudinal ridges on one side ([Pl. VI. fig. 4], and Ligns. [221], [223]). In the Serpents, the teeth are very long and pointed; in the Crocodiles and Lizards, may be seen every modification of the conical form, down to a mere hemispherical tubercle or plate. In the fossil Dicynodon, to be hereafter described, the dental system consists of but two tusks or canine teeth, like those of the Walrus, implanted in the upper jaw. The Turtles and some fossil Lacertians are edentulous, i. e. destitute of teeth; their dental organs consisting of the horny trenchant sheaths with which the jaws are covered.
The teeth are very numerous in reptiles; the individuals of some species have more than two hundred. In some genera, they are implanted on the jaws alone; in many, they occupy the palatine, vomerine, and other bones composing the vault of the mouth, as in certain fishes. The teeth are generally anchylosed to the bone; but in some genera they are implanted in distinct sockets, as in the Crocodile and Plesiosaurus; in others, as in the Ichthyosaurus, they are arranged in a deep furrow, and retained only by the integuments; in some, they are supported upon an elevated osseous base. In the Labyrinthodonts, and in the greater part of the Serpent tribes, the tooth is implanted by the base in a shallow socket, with which it is confluent.
In most of the Lacertians, or true Lizards, the attachment of the teeth presents a peculiar modification, of which the lower jaw of the Iguana, [Lign. 205], p. 649, affords a good illustration. The teeth are not placed in sockets, but are attached by the shank to an alveolar plate, or parapet, that extends along the margin of the jaw, as shown in figs. 1 and 3; the crowns of the teeth project above this plate, as seen in figs. 2 and 4. From the anchylosis of the teeth to the side of the jaw, the Lizards possessing this dental structure are termed Pleurodonts.[579]
[579] The Pleurodonts are those lizards in which the teeth are anchylosed to the side of the dentary bone; Acrodonts, those with the teeth fixed to the upper margin or ridge of the jaw-bone; Thecodonts, those having the teeth implanted, either loosely, or anchylosed to the walls of their sockets. Lacertians are also said to be Pleodont (having solid teeth), or Cœlodont (hollow-toothed).
In reptiles, we have, therefore, five essential modifications in the attachment of the teeth; namely, in distinct sockets; in a continuous groove or furrow; attached laterally by the shank to an alveolar parapet; anchylosed by the base to a shallow socket; and attached to an osseous support, without sockets or an alveolar plate.[580]