244 A MANUAL OF DENTAL ANATOMY. 



known whether these were sheathed in horny cases like 

 those of the turtles, or whether the bones themselves came 

 into use, as in Hatteria. But the most striking peculiarity 

 of Dicynodon was the co-existence with such jaws of a pair 

 of very large -caniniform tusks, extending downwards and 

 forwards from the upper jaw, and growing from persistent 

 pulps, a thing altogether exceptional in the reptilian class. 



The dentition of Ophidian reptiles (snakes) is very uni- 

 form; they maybe conveniently divided into two groups, the 

 poisonous and the non-venomous snakes. 



Non-venomous snakes have one row of teeth in the lower 

 jaw, and two rows in the upper jaw; in the latter the 

 maxillary bones carry one row, while a parallel internal row 

 is supported upon the palatine and pterygoid bones. 



The teeth are in both groups strongly recurved, and are 

 firmly anchylosed to the bone ; they consist of a central 

 body of unvascular dentine, coated by a very thin layer of 

 enamel (there is not, as is generally supposed, any layer of 

 cementum, the enamel having been erroneously supposed to 

 be such). 



The two halves of the lower jaw are connected at the 

 symphysis by an exceedingly elastic ligament ; their articu- 

 lation with the base of the skull through the medium of an 

 elongated movable quadrate bone, is also such as to allow 

 of their being widely separated from the skull and from one 

 another, which allows of the dilatation rendered necessary 

 by the large size of the creatures which a snake swallows 

 whole. 



The teeth of the snake are simply available for seizing- 

 prey and retaining it, as the snakes invariably swallow their 

 prey whole, and in no sense masticate it. 



As the object to be swallowed is often so disproportionately 

 large as to make the process of deglutition appear an im- 

 possibility, the mouth and pharynx have to undergo great 

 dilation. The arrangements which combine to give to the 



