DENTAL SYSTEM OF REPTILES. 253 



next to tliis; a layer of softer vascular dentine forming 

 the inner half of the crown ; and a portion of firm osteo- 

 dentine in the middle of the grinding surface, formed by 

 the ossified remnant of the tooth-pulp. The series of 

 complex teeth, so constructed, seems to have been admi- 

 rably adapted to the cropping and comminution of such 

 tough vegetable food as the clathrarke and similar now 

 extinct plants, the fossil remains of which are found 

 buried with those of the iguanodon. No existing reptile 

 now presents so complicated a structure of the tooth in 

 relation to vegetable food. The still more complex, and 

 indeed marvellous structure of the teeth of the extinct 

 gigantic lizard-like toad, called Lahyrinthodon^ has been 

 already noticed (Fig. 56, p. 236). But, perhaps, the most 

 singular dental structure yet found in the ancient mem- 

 bers of the class Eeptilia, is that preserited by certain 

 species of fossil found in South Africa, and probably 

 from a geological formation nearly as old as our coal 

 strata. I have called them "Dicynodonts," from their 

 dentition being reduced to one long and large canine 

 tooth on each side of the upper jaw. As these teeth give, 

 at first sight, a character to the jaws like that which the 

 long poison-fangs give, when erected, to the jaws of the 

 rattlesnake, I shall briefly notice their characters before 

 entering upon the description of the more normal saurian 

 dentition. 



Fig. 64 gives a reduced side view of the skull and 

 teeth of the Dicynodon lacerticeps. 



The maxillary bone, 21, is excavated by a wide and 

 deep alveolus, with a circular area of half an inch, and 

 lodges a long and strong, slightly curved, and sharp- 

 pointed canine tooth or tusk, which projects about two- 

 thirds of its length from the o])en extremity of the socket. 



99 



