1380 



CLASS MAMMALIA. 



(fig. 1253) is characterised by its vertically grooved premolars. 

 Other genera from the North American Eocene are Hexodon, Ecto- 

 conus, Am'sonchus, Hemithlceus, Haploconus, and Zetodon ; of which 

 the fourth may be not separable from the third. From this family 

 Dr Schlosser regards the bunodont Artiodactyla as derived, and 

 thinks a direct relationship can be traced from Periptychus to Achcen- 

 odo?i, and thus to the other Choeroftotamidce. The humerus is 

 short, and much expanded distally. 



Family Phenacodontid/E. — In this family the brain (fig. 1254) 

 is characterised by the extremely small size of the hemispheres, 



-Inferior, superior, and left lateral aspects of the brain of Pkenacodus primcevus ; 

 from the Lower Eocene of North America. (After Cope.) 



which are only one-fourth longer than the cerebellum, and thus 

 indicate a very low type of organisation. The family is readily 

 distinguished from the preceding by having a proximal trochlea to 

 the astragalus, by the longer neck, and less simple premolars, which 

 are, however, different from those of the following family. The 

 type genus Phenacodus (fig. 1255) includes several species from the 

 Puerco and Wasatch Eocene of North America, varying from the 

 size of a small terrier to that of a leopard. The dental series com- 

 prises the full typical number ; and although the crowns of the upper 

 true molars are of a bunodont structure, yet they could be readily 

 modified into the lophodont type of Hy?-acotherium, and we must 

 probably regard the latter as a direct descendant of the present 

 genus, with perhaps the intervention of Systemodon. Professor 

 Riitimeyer has described some upper molars from the Upper 

 Eocene of Switzerland which he refers to Phenacodus, although 

 they are much more of a lophodont type than in the American 

 species. 



Professor Cope remarks of one of the species of Phenacodus that " the 

 size of the animal is about that of a Bull-dog, but the head is smaller, 



