354 



THE AGE OF MAMMALS 



'Republican River' stage above described and of the Middle Pliocene, 

 'Blanco' stage, of Texas. 



The exposures in which this rich fauna occurs lie along a sand hill region, 

 or crest of the divide between the Niobrara and Platte rivers in Nebraska, 

 near the headwaters of Snake Creek, which gives the name to this formation. 

 It is an outlyer of the 'Ogallala' of Darton, a formation which is typically 

 composed of clean sand with a considerable amount of gravel; the Snake 

 Creek is to be regarded as a local facies of the Ogallala. The bones occur 



Fig. 162. — On the plains of western Nebraska. Miocene (Sheep Creek beds) overlaid by 

 a Pliocene (Snake Creek) formation. Photograph by American Museum of Natural History 

 expedition of 1908. 



apparently in an old river channel, in such vast numbers as in places to 

 form a veritable bone bed several feet thick, in which, owing to the scatter- 

 ing influence of river action, complete skulls and skeletons are very rare. 



All the species and mutations are more advanced than those of the Re- 

 publican River, or Peraceras Zone. Among the hoofed Herbivora all the 

 Lower Pliocene tj^es of rhinoceroses, however, still occur, including remains 

 which are attributed to Teleoceras, Aphelops, and to a still simpler brachyo- 

 dont rhinoceros probably belonging to the persistent brachyodont phylum 

 mentioned under the Republican River (p. 348). 



Among the dying-out members of the fauna 

 Prevailing Mammals ^^^ ^^^^ examples of the Miocene oreodont 



Mastodons, longirostral Merychyus. 



Tetralophodon The most important and unexpected fea- 



fTrilophodon (Florida) ture of this assemblage is the evidence of the 



Horses, several phyla presence of the bovid division of the Cavicornia. 



