X.] EXTINCT MAMMALS. 375 



Ancodus, common to the Tertiaries of both hemispheres 1 , and 

 were represented by a large number of generic types, have 

 crescentic columns to the short-crowned cheek-teeth, the upper 

 molars usually carrying four such columns ; while the lower canine 

 is approximated to the incisors, its usual form and function being 

 assumed by the first pre-molar. The last upper pre-molar is 

 simpler than the molars ; and while the feet have usually four toes 

 each, in the typical genus Cotylops a rudiment of the thumb is 

 retained in the front pair, as in Ancodus. In Cotylops and most 

 of the other genera the molars of the upper jaw have but four 

 columns, but in Protoreodon there are five ; a feature serving to 

 connect the family with the Anthracotheriida, from which group 

 the oreodonts are probably descended. A nearly-allied but more 

 specialised family is that of the Agriochceridce, as represented by 

 the genus Agriochcerus* , of the upper and middle Oligocene, in 

 which the toes were developed into claws, instead of being 

 incased in hoofs. Here it may be mentioned that while the 

 peccaries (Dicotylida) are now exclusively New World types, and 

 the pigs (Sutdce) restricted to the Old World, the Tertiaries of both 



and Eocene. Introducing the former term, the series may be approximately 

 classified as follows, viz. : 



PLISTOCENE. Equus Beds. Eqtius^ Elephas primigenius. 



PLIOCENE (Upper). Blanco Series. Pliauchenia. 



(Lower). Loup-Fork. Protohippus, Hipparion. 



MIOCENE. Deep River. Anchitherium^ First Mastodons. 



OLIGOCENE (Upper). John Day. Miohippus, Ancodus. 



(Middle). White River. Agrwchcerus, Titanotherium. 



(Lower). Uinta. Amynodon, Mesonyx. 

 EOCENE (Up. and Mid.). Bridger. Pachynolophus, Paltzosyops. 



(Lower). Wahsatch. Hyracotheritim, Coryphodon. 



(Lowest). Puerco. Neoplagiaulax , Poly mastodon. 



In this series the Deep River beds are identified with the European 

 Miocene (supra^ p. 117) by the presence of Anchitheritim, and the first appear- 

 ance of Mastodon ; while the existence of Ancodus in the John Day and Upper 

 White River beds correlates them with the Upper and Middle Oligocene, 

 Hyracotherium and Coryphodon serving to identify the Wahsatch beds with the 

 Lower Eocene. 



1 Supra, p. 161. 



2 Equal Artionyx. 



