OLIGOCENE OF EUROPE, NORTH AFRICA, AND NORTH AMERICA 221 



Chiefly Plains Chiefly Riverside 



Carnivora 

 Hyoenodon 

 Dinictis 

 Cijnodictis 



Hoplophoneus Hoplophorwus 



Daphcenus Daphcenus 



Rodentia 



Ischyromys 



Pakvolagus Steneofiber 



Eumys 



Gymnoptychns 



Insectivora 

 Leptictidae 



Of the same age are the 'Cedar Creek' beds of northeastern Colorado 

 and widespread similar exposures in southeastern Wyoming, South Dakota, 

 northwestern Nebraska, and far north in western Montana. These ex- 

 posures are renewed proofs of the existence of vast fertile and nearly level 

 flood plains, east of the Rockies, gently sloping eastward and traversed 

 by stream channels or rivers which are narrower than those of Titano- 

 therium times. The plain is chiefly forested along the river borders; but 

 the flora is entirely unknown. 



Approximate homotaxis with the Stampian or Middle Oligocene of 

 Europe is indicated by similar stages in the evolution of the anthracotheres 

 (Ancodus), of the amynodont rhinoceroses (Metamynodon, Cadurcotherium), 

 of the true rhinoceroses, or aceratheres and diceratheres. In both countries 

 the Middle Oligocene is the disappearing point both of the amynodonts 

 and of the archaic carnivores (hyaenodonts) . 



The Oreodon beds are the favorite fossil hunting grounds of the West, 

 because the fossils are or were extremely abundant. The mammalian 

 fauna is also very rich, more than 150 species of mammals having been found 

 in the Big Badlands of South Dakota alone. Since this level is much more 

 favorable for the smaller forms of life than the Titanotherium Zone, the 

 considerable number of new forms is partly attributable to this fact. Large 

 herds of the small browsing oreodonts took the place of the caenotheres in 

 Europe, and other ruminants abound in this level. 



The observer readily distinguishes the Oreodon stage not only by its 

 geologic differences, but by the abundance of oreodonts and the absence of 

 all traces of titanotheres. The Rodentia include the tree-living squirrels 

 (Prosciurus) , the ground-squirrel or semi-cursorial type {Eutypomys), and 

 the hares, or leporids (Palceolagus) ; the mice, or Murida?, now make their 

 first appearance (Eumys), while the peculiarly Eocene ischyromyids make 

 their last appearance. Among Insectivora the erinaceids appear for the 

 first time (Proterix), and the talpids or moles {Domnina), are represented. 

 The opossums appear in numerous species (Peratherium). 



