With respect to situation, the teeth may be present on the jaws only, i.e. the maxillary, pre maxillary, and mandibular bones, as in the croco diles and many lizards : or upon the jaws and roof of the mouth ; and here either upon the pterygoid bones, as in the Iguana and Mosa saur, or upon both palatine and pterygoid bones, as in most serpents, or upon the vomer, as in most Batrachians, or upon both vomerine and pterygoid bones, as in the Axo lote ; or upon the vomerine and sphenoid bones, as in the Salamandra glutinosa, Maclure. With respect to the marginal or jaw teeth, these may be absent in the intermaxillary bones, as in many serpents ; or they may be present in the upper and not in the lower jaw, as in most frogs ; or in both upper and lower jaws, as in the tailed Batrachians ; and among these they may be supported, upon the lower jaw, by the premandibular or den tary piece, as in the Salamanders, Menopome, Amphiume, Proteus ; or upon the splenial piece, as in the Siren ; or upon both splenial and premandibular bones, as in the Axolotl. The palatine and pterygoid teeth may, in the Batrachians, be arranged in several rows, like the " dents en cardes " of fishes. The sphe noid and splenial teeth are always so ar ranged in the few species that possess them. The intermaxillary, maxillary, and preman dibular teeth are uniserial, or in one row, with the exception of the Cwcilia and the extinct Labyrinthodonts, which have a double row of teeth at the anterior part of the lower jaw.
The teeth of reptiles, with few exceptions, present a simple conical firm, with the crown more or less curved, and the apex more or less acute. The cone varies in length and thickness ; its transverse section is sometimes circular, but more commonly elliptical or oval, and this modification of the cone may be traced through every gradation, from the thick, round, canine-like tooth of the croco dile, to the sabre-shaped fang of the Varanus, the Megalosaur, and the Cladeiodon.* Some times, as in the fully formed teeth of the Megalosaur, one of the margins of the com pressed crown of the tooth is trenchant, sometimes both are so ; and these may be simply sharp-edged, .as in the Varanus of Timor, or finely serrated, as in the great Va ranus, the Cladeiodon, and the Megalosaur.t The outer surface of the crown of the tooth is usually smooth ; it may be polished, as in the Leiodon, or impressed with fine lines, as in the Labyrinthodon (fig. 551.), or raised into many narrow ridges, as in the Pleiosaur and Polyptychodon, or broken by a few broad ridges, as in the Iguanodon (fig. 571.), or grooved by a single longitudinal furrow, as in some serpents (fig. 569, a).t The cone is longest and its summit sharpest in the serpents : from these may be traced, chiefly in the lizard tribe, a progressive short ening, expansion of the base, and blunting of the apex of the tooth, until the cone is reduced to a hemispherical tubercle, or plate, as in the Thorictes and Cyclodns (fig. 570.).
In the Pleiosaur the dental cone is three sided, with one of the angles rounded off. The posterior subcompressed teeth, of the alligator ( fig. 573.) present a new modification of form ; here they terminate in a mammillate summit, supported by a slightly constricted neck. In the tooth of the Hylceosaur the
expanded summit is flattened, bent, and spear shaped, with the edges blunted. But the expansion of the crown is greatest in the sub compressed teeth of the extinct Cardiodon and the existing Iguanas, the teeth of which are farther complicated by having the margins notched. The great Iguanodon had the crown of the tooth expanded both in length and breadth, and combining marginal dentations with longitudinal ridges : this tooth (fig. 571.) presents the most complicated external form as yet discovered in the class of reptiles.
In no reptiles does the base of the tooth ever branch into fangs.
a general rule, the teeth of reptiles are anchylosed to the bone which supports them. When they continue distinct, they may be lodged either in a continuous groove, as in the Ichthyosaur §, or in separate sockets, as in the Plesiosaur and Crocodilians (fig. 573.). The base of the tooth is anchy losed to the walls of a moderately deep socket in the extinct Megalosaur and Theocodon. In the Labyrinthodonts and Crecilite, among the Bratrachians ; in most Ophidians ; and in the Geckos, Agamians, and Varanians, among the Saurians, the base of the tooth is imbedded in a shallow socket, and is confluent therewith.
In the Scincoid inns, the Safeguards (Tejus), in most Iguanians, in the Chameleons and most other Lacertian reptiles, the tooth is anchylosed by an oblique surface extending from the base more or less upon the outer side of the crown to an external alveolar plate of bone*, the inner alveolar plate not being developed. In the frogs the teeth are simi larly but less firmly attached to an external parapet of bone. The lizards which have their teeth thus attached to the side of the jaw are termed Pleurodonts. In a few Igua nians, as the Istiures, the teeth appear to be soldered to the margins of the jaws, these have been termed " Acrodonts." In some large extinct Lacertians, e. g. the Mosasaur and Leiodon, the tooth is fixed upon a raised conical process of bone, as shown in my " Odontography," Plate 68. fig. 1., and Plate 72. fig. 2.
These modifications of the attachment of the teeth of reptiles are closely adapted to the destined application of those instruments, and relate to the habits and food of the species ; we may likewise perceive that they offer a close analogy to some of the transitory con ditions of the human teeth. There is a period, for example 1-, when the primitive dental papillae are not defended by either an outer or an inner alveolar process, any more than their calcified homologues which are confluent with the margin of the jaw in the I?hynchocepbalus.t There is another stage§, in which the groove containing the den tal germs is defended by a single external cartilaginous alveolar ridge ; this condition is permanently typified in the Cyelodus (fig. 570.) and most existing lizards. Next there is developed in the human embryo an internal alveolar plate, and the sacs and pulps of the teeth sink into a deep but continuous groove, in which traces of transverse partitions soon make their appearance ; in the ancient ichthy osaur the relation of the jaws to the teeth never advanced beyond this stage.