TEETH 



317 



fossil forms) a heterodont dentition appears to be indicated. 1 In 

 spite of the great modifications which have taken place in the 

 palate of Crocodiles, their teeth, which have a conical form, show 

 the least amount of differentiation in the course of their phylo- 

 genetic history. Almost all Keptiles are polyphyodont, but in some 

 cases certain of the teeth are not replaced (e.g. Agama colonorum), 

 and in others some of them undergo reduction (e.g. Typhlopidae). 



In poisonous Snakes a varying number of maxillary teeth are 

 differentiated to form poison-fangs, which, like those of the lower 

 jaw of the poisonous Lizard, Heloderma, are longitudinally grooved 

 anteriorly. In the Viperine forms there are on each side a 

 number of poison-fangs arranged in rows ; the stronger ones project 



A B 



FIG. 232. A, DIAGRAMMATIC TRANSVERSE SECTIONS THROUGH THE JAWS OF 

 REPTILES, SHOWING PLEURODONT (a), ACRODONT (6), AND THECODONT (c) 

 DENTITIONS. B, a, LOWER JAW OF Zootoca vivipai-a, and 6, OF Anyuis 



frayili*. (After Leydig.) 



freely, while the lesser, reserve teeth lie within the gum (Fig. 

 233, A) ; only one of these teeth, however, is firmly fixed to the 

 maxilla at a time. Each fang is perforated by a poison-canal 

 formed by the meeting of the edges of the longitudinal groove, 

 and is incompletely surrounded by the pulp-cavity, the latter 

 having the form of a half-ring in transverse section (Fig. 233, 

 B, c,): the duct of the poison-gland passes into an aperture at 

 the base of the tooth which leads into the poison-canal, and the 

 latter opens by a slit at a short distance from the apex of the 

 tooth on the premaxilla (Fig. 233, A). 2 



1 The teeth of Hatteria are at first simple in form and their attachment is 

 acrodont ; the apparent heterodont condition in the adult is due to the fusion of 

 more than one generation of teeth to the bone, which embraces them basally and 

 grows down beyond the gums. In this animal, the vomerine teeth are in the 

 course of suppression ; they are usually absent, but may occur on one or both 

 vomers. Functionless teeth are present in the embryo which later disappear, 

 and the same is true of Crocodilus porosus and Iguana tuberculata. 



2 A peculiar, broad, lancet-like tooth, originally paired, is present in 

 embryos of Lizards and some Snakes. It projects considerably beyond its 

 neighbours in the median line of the snout, and serves the young as a means of 

 breaking through the parchment-like egg-shell. This must not be confounded 

 with the analogous structure present in Rana opisthodon or with the horny 

 "neb" in Hatteria, Crocodiles, Chelonians, Birds, and Monotremes, which is 

 purely of an epithelial nature. 



