MOLAE TEETH OF THE MAMMALIA EDUCABILIA. 253 



forms, if any there be, which give color to the supposition that 

 the various divisions of Lophodonts have descended from Buno- 

 dont predecessors. Here, then, I mention a fact of prime impor- 

 tance, i. e., that in America, at least, no Selenodonts are known 

 from formations of older age than Miocene ; while the greatest 

 development of Bunodonts is in the beds of the next older epoch, 

 the Eocene. 



The special forms of Lophodonts may be separately considered 

 as follows : 



First, as to the opposite and alternate types, or the Antiodont 

 and Amoebodont. They pass into each other by many intermedi- 

 ate conditions among the Bunodonts, as in Notharchis, Lim- 

 notherium, etc., of the Wyoming Eocene. There is reason, also, 

 to believe that this has been the case with some of the Lophodonts 

 after they had left the bunodont stage behind. Thus Equus is an 

 antiodont as to its upper molars, but has been probably derived 

 from Palmotlieriodont ancestors, which are amoebodont ; this is 

 rendered especially probable by the fact that the mandibular teeth 

 are of the amoebodont division (hippodont). It is also highly 

 probable that the antiodont genus Tapirus, though so near to 

 PalcBotlierhim, was derived from an antiodont Bunodont. Hence, 

 while the discrimination between opposite and alternate types is in 

 some cases most radical, in others its importance is but slight. 



L Antiodonts: Bunodont type AcTicenodon (Fig. 19). 



1. Selenodont type apjDroximated by the bunodont Hippopota- 

 muSi where the tubercles are com^^ressed, thus : the intervening 



Fig, 19. — AcTicenodon insolens. 



valleys are deepened, and the cusps wear readily into separate 

 crescents. Another intermediate form is seen in the genus An- 

 tliracotherium, where the tubercles of the mandibular teeth are 

 compressed, while they remain conic (Fig. 21) ; selenodont forms 

 of Omnivora present us with near approaches to these Bunodont 

 genera. Thus in Hyopotamiis and Anoplotheriiim, the crowns. 



