220 



THE AGE OF MAMMALS 



and the plains fauna contained in the clays of the Oreodon Zone.* The 

 only explanation is that the clay and sandstone fossils represent two co- 

 existent faunas of different local habitat.'- 



TiG. 108. — Skeleton of the Oligocene aquatic rhinoceros Metamynodon planifrons. (See Res- 

 toration, page ISO). In the American Museum of Natural History. After Osborn. 



Contrasting Plains and River-Border Mammals 



Chiefly Plains 



Mesohippus bairdi, meteulo- 



phus, etc. 

 Hyracodon 

 Colodon 



Leptauchenia 



Oreodon 



Eporeodon 



Poebrotherium 



Leptomeryx 



Hypertragidus 



Hypisodus 



Perissodactyla 



Artiodactyla 



Chiefly Riverside 



Mesohippus intermedins and Mio- 



hippus 

 Metamynodon 

 Ccenopus 

 Protapirus 

 Titanotherium 



Agriochoerus 



Perchoerus 



Entelodon 



Ancodns 



Anthracotherium 



Protoceras 



* Matthew, A Provisional Classification of the Fresh-Water Tertiary of the West. Bull. 

 Amer. Mus. Nat. Hist., Vol. XH, 1899, pp. 19-75. 



- Matthew, Fossil Mammals of the Tertiary of Northeastern Colorado. Mem. Amer. 

 Mus. Nat. Hist., no. 1, Pt. 7, Nov., 1901. 



