388 ANATOMY OF VERTEBRATES. 



marginal dentations with longitudinal ridges: this tooth, fig. 273, 

 presents the most complicated external form as yet discovered in 

 the class of Reptiles. 



In no Reptile does the base of the tooth ever branch into fangs. 



Attachment. - - As a general rule, the teeth of Reptiles are 

 anchylosed to the bone which supports them. When they con- 

 tinue distinct, they may be lodged either in a continuous groove, 

 as in the Ichthyosaur, 1 or in separate sockets, as in the Plesiosaur 

 and Crocodilians, fig. 275. The base of the tooth is anchylosed 

 to the wall of a moderately deep socket in the extinct Megalosaur 

 and Thecodon. In the Labyrinthodonts and Csecilise, among the 

 Batrachians, in most Ophidians, and in the Geckos, Agamians, 

 and Yaranians, among the Saurians, the base of the tooth is 

 imbedded in a shallow socket, and is confluent therewith. 



In the Sciiicoids, the Safeguards ( Tejus\ in most Iguaniaiis, in 

 the Chameleons and many Lacertian reptiles, the tooth is anchy- 

 losed 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, 2 the inner alveolar plate not being developed. In the Frogs 

 the teeth are similarly 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 Iguanians, as the Istiures and Rhynchocephalus, the teeth are 

 soldered to the margins of the jaws ; and in some large extinct 

 Lacertians, e. g. the Mosasaur and Leiodon, each tooth is fixed 

 upon a conical process of the alveolar border : these Sauria are 

 termed f Acrodonts.' 



Such modifications of the attachment of the teeth of Reptiles 

 are adapted to the habits and food of the species ; and they likewise 

 offer an analogy to some of the transitory conditions of the human 

 teeth. There is a period, for example, when the primitive dental 

 papilla? are not defended by either an outer or an inner alveolar 

 process, any more than their calcified homologues, which are con- 

 fluent with the margin of the jaw in the Rhynchocephalus. 3 

 There is another stage in which the groove containing the dental 

 germs is defended by a single external cartilaginous alveolar 

 ridge ; this condition is permanently typified in the Cyclodus., 

 fig. 272, 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 appear- 



1 v. pi. 13, fig. 9. 2 v. pi. 67. 



3 CLVIII. pt. 2, pi. 6, figs. 5 & 6, p. 83. 



