TYPES OP MOLAR TEETH OF MAMMALIA EDUCABILIA. 



79 



separate crescents. 



Fis. 



Anthracotherinm vela lai urn. 



Hynpota 



inus americaniis 



Fig. 



Fig. 10. 



Another intermediate form is seen in the genus Anthracothe- 

 rium, where the tubercles of ^. g 



the mandibular teeth are 

 compressed, while they re- 

 main conic (Fig. 7); seleno- 

 dont forms of Omnivora pre- 

 sent us with near approaches 

 to these Bunodont genera. Thus in Hyopotamus and Am- 

 plotherium, the crowns, when unworn, present four principal 

 tubercles, which are openly V-shaped in section, and which are separated by open 

 valleys. The latter are deeper than those of the truly Bimodont genera, but much 

 shallower than those of the typical Selenodonts. In Oreodon the valleys are some- 

 what deepened and the crescents elevated, while in the deer the same infolding 



is carried still further. In the 

 cavicornia the type reaches its full- 

 est expression in the loss of the 

 shoulder at the base of the crown, 

 the great elongation of the latter, 

 and correspondingly deep infolding 



„ - . . , 11 3Ierychyus major. 



of the terminal valleys. 



Prof. Lartet* states that the most ancient deer have very short-crowned molars, 

 and the depressions on the surface are so shallow that the bottom is always visible, 

 while in the CervidcB of the more recent tertiary periods, and especially the Plisto- 

 cene and living species, these same cavities are so deep that whatever be the state 

 of attrition, the bottom cannot be seen. This, he says, is a perfectly reliable rule 

 for distinguishing the ancient from the more modern forms of deer, and can be 

 applied to other animals as well as the Cervidse, 



The writer nearly contemporaneouslyf recalled the observations of Leidy that 

 the teeth of the Oreodont ''Merychyus are more prismatic, have larger crowns and 

 shorter roots, approaching the sheep, as Oreodon does the deer." Now Oreodon is 

 Miocene, and Merychyus PHocene. It was then observed: "This phenomenon 

 suggests an explanation on the score of adaptation which the other cases do not. 

 The existence during the later period, of a [hardjer material of diet would increase 

 the rapidity of wearing of the crown of the tooth, and require a longer crown and 

 greater rapidity of protrusion. This necessitates a diminution of the basal shoulder 

 and shortening of the roots producing the prismatic form aforesaid." 



* Comptes Eendus, 1868, p. 1119. 



t Proceed. Acad. Nat. Sciences of Philadelphia, 1868, p. 274: Origiu of Genera, 44. 



Procamelus robustus. 



