TEETH OF REPTILES. 389 



ance ; in the ancient Ichthyosaur the relation of the jaws to 

 the teeth never advanced beyond this stage. Finally, the dental 

 groove is divided by complete partitions, and a separate socket is 

 formed for each tooth ; and this stage of developement is attained 

 in the highest-organised Reptiles, e. g. the Crocodiles, figs. 95, 

 275. 



Substance. — This may be four-fold, and a single tooth may be 

 composed of dentine, cement, enamel, and bone : but the dentine 

 and cement are present in the teeth of all Reptiles. 



In the Batrachians and Ophidians a thin layer of cement 

 invests the central body of dentine, and, as usual, follows any 

 inflections or sinuosities that may characterise its surface. Be- 

 sides the outer coat of cement, which is thickest at the base, a 

 generally thin coat of enamel defends the crown of the tooth in 

 most Saurians, and the last remains of the pulp are not unfre- 

 quently converted into a coarse bone, both in the teeth which 

 are anchylosed to the jaw, and in some teeth, as those of the 

 Ichthyosaur, which remain free. The modification called ' vaso- 

 dentine ' is peculiar in the present class to the teeth of the 

 Iguanodon. 1 



Structure. — The varieties of dental structure are few in the 

 Reptiles as compared with Fishes or Mammals, and its most com- 

 plicated condition arises from interblending of the dentinal and 

 other substances rather than from modifications of the tissues 

 themselves. In the teeth of most Reptiles the intimate structure 

 of the dentine is of the hard or unvascular kind, presenting only 

 plasmatic or dentinal tubes, diverging from the pulp cavity, at 

 right angles to the external surface of the tooth. Such dentine 

 may be folded inwardly upon itself, so as to produce a deep longi- 

 tudinal indentation on one side of the tooth ; it is the expansion of 

 the bottom of such a longitudinal deep fold that forms the central 

 canal of the venom-fang of the Serpent: but its structure remains 

 unaltered ; and although the pulp-cavity, fig. 270, p, is reduced 

 to the form of a crescentic fissure, the dentinal tubes continue to 

 radiate from it according to the usual law. By a similar inflection 

 of many vertical longitudinal folds of the cement and external 

 surface of the tooth at regular intervals around the entire cir- 

 cumference of the tooth, and by a corresponding extension of 

 radiated processes of the pulp-cavity and dentine into the inter- 

 spaces of such inflected and converging folds, a modification of 

 dental structure is established in certain extinct Reptiles, which, 



1 v. pi. 71, Iguanodon. 



