234 



THE AGE OF MAMMALS 



noteworthy that Agriochoerus is ap- 

 parently extinct. 



As we are now again observing the 

 fauna of the Great Plains, it is not 

 surprising to discover the excessively 

 delicate and graceful cursorial camelid 

 Stenomylus gracilis. 

 On the whole, however, browsing, slow-moving, river and forest-border 

 and actual forest-living types prevail over open-plains and cursorial types, 



Characteristic Mammals 

 (continiced) 

 Geomyids 

 Pro-Felids 

 Pro-Machserodonts 

 Canids (varied) 

 Mustelids 



Fig. 118. — Summit of the Oligocene or Lower Miocene, Lower Harrison beds, Sioux 

 County, Nebraska. A view of the Stenomylus quarry in the Promerycochoerus Zone. Photo- 

 graph by American Museum of Natural History, 1908. 



SO far as we know the fauna of this region at this time. Undoubtedly an 

 upland or plains and cursorial fauna existed in this western region, but it 

 has not become fully known. 



Physiographic conditions. — The geologic conditions of the Lower 

 Arikaree, Lower Rosebud, and Lower Harrison are peculiarly interesting. 

 Near the summit of the Lower Harrison occur the Dsemonelix beds of 

 Barbour, named from the giant spirals or corkscrews of harder rock, which 

 resist erosion, held together by fibrous material, and at many points stand 

 out prominently against the sloping bluffs of the Upper Niobrara River. 

 These were interpreted originally as representing the spiral roots of some 



