THE MIOCENE OF EUROPE, ASIA, AND NORTH AMERICA 277 



periods in Europe; nor, again, can we at present sharply define either the 

 beginning or the close of the American IVIiocene by comparison witli Eu- 

 rope, because the similar animal forms are but few. Nor is there a divi- 

 sion of the American Miocene into an older and a newer fauna such as 

 occurs in Europe (p. 249), yet we can make a broad faunistic correlation 

 between the two countries, as follows: 



The older Miocene in both countries marks the disappearance or absence of 

 some of the most characteristic Oligocene ungulattit;, such as the entelodonts, 

 or giant pigs, and the dicerathere rhinoceroses. It marks the continued 

 prevalence of browsing herbivorous quadrupeds with short-crowned teeth. 



The newer Miocene marks the workl-wide prevalence of the first grazing 

 horses and other quadrupeds with long-crowned grinding teeth; also of 

 mastodons with trilophodont or three-crested intermediate molars, as well 

 as of mastodons with tetralophodont or four-crested intermediate molars. 



Periods Formations Horse Zones 



Upper Miocene Ogallala, "Loup Fork," Clarendon, Madison Hipparion 



Valley Protohippus 



Middle Miocene Deep River, Pawnee Buttes Merychippus 



Lower Miocene Arikaree ( = ' Upper Harrison ' and ' Upper Rose- Parahippus 



bud') 



Thus the transition in North America from Oligocene to Miocene times 

 is still undetermined, and the line of demarcation adopted in this volume 

 is provisional. The rate of evolution does not help us, because some groups 

 of American mammals evolve more rapidly than corresponding Old World 

 forms; for example, the canids, mustelids, leporids, and the equines. On 

 the other hand, the American geologic stage we have here selected as Lower 

 Miocene does not surely contain either the mastodons, the teleocerine 

 rhinoceroses, or the earliest horned deer so distinctive of the Lower Miocene 

 of Europe; our interpretation is that these animals during Lower Miocene 

 times were slowly migrating and spreading from Europe and Asia into 

 North America, so that they first surely appear in what we call the Middle 

 Miocene. The line of transition between the American Oligocene and 

 Lower Miocene is that indicated as follows: 



Transition to the Lower Miocene in the Arikaree ( = ' Upper Harrison,^ ' Upper Rose- 

 bud ') Formations 



Mammals characteristic of the Upper Oli- New maynmals characteristic of the 

 gocene, absent or undiscovered here Lower Miocene 



Entelodonts Mastodon, evidence doubtful 



Hypertragulus Blastomeryx, a cervid 



Syndijoceras Merycochoerus, a Miocene oreodont 



Steneofiber Merychyus, a Miocene oreodont 



(last appearance) Phlaocyon, the first procyonid 

 Dicer atheriu7n 



