260 MAMMALIAN DENTITION 



comys. In both of these types the roots are fully completed 

 in the adult teeth and do not grow persistently as in the case 

 of the Horse which represents a much more advanced stage. 

 Again attrition is compensated for in a totally different man- 

 ner by forcing up of the stump of the worn tooth so that the 

 last possible wear can be obtained even after the crown is 

 practically nonexistent. This condition also is well illustrated 

 in the Horse. All three methods are manifested in conjunction 

 though in differing degrees by many Mammals. 



While progression toward complete hypsodonty is usually 

 associated with increase in complexity of crown pattern this 

 is not invariably the case. Among the extinct Ungulates pe- 

 culiar to South America there lived during the Pleistocene a 

 stoutly built semi-aquatic lumbering animal named Toxodon. 



Fig. 94. — Upper teeth of Jerboa Pouched Mouse (Antechinomys laniger, after 

 Gregory). As in all primitive Mammals the canines, premolars and molars possess 

 two large external roots. 



Further back and separated from Toxodon by a vast geological 

 period there existed in the Miocene a smaller and lighter beast 

 Nesodon which we have already mentioned because of a pecu- 

 liarity in its dentition (page 253). It is probable that Nesodon 

 is actually the ancestor or at any rate very closely related to 

 the ancestor of Toxodon. The molars of both animals were 

 hypsodont but the curious fact is that the crowns in Nesodon 

 were decidedly more complex than those of its descendant 

 Toxodon. We must then be always upon the watch for excep- 

 tions to general rules which we find it necessary to formulate. 

 In a typical specialized hypsodont Ungulate the cusps are ob- 

 served to be long but a more striking feature is the loss of 

 distinction between crown and root. The neck of the tooth 

 does not exist. Further the root, if one may term it such, is 



