SUCCESSIVE MAMMALIAN FAUNAS 229 



2. Miocene 



North America. — Upper Miocene beds cover extensive 

 areas of the Great Plains region and are scattered from Mon- 

 tana far into Mexico. The rich fauna is an outgrowth and 

 development of that of the middle Miocene, with but few im- 

 migrant additions and, on the other hand, passes so gradually 

 into that of the lower Pliocene, that any line of separation 

 between them is very difficult to draw. The rodents, numerous 

 as they are among the fossils, are almost certainly very incom- 

 pletely represented in the collections ; the families are almost 

 all still in existence, but nearly every genus is extinct, and thus 

 the vernacular names used to designate them must be under- 

 stood in a broad sense. Hares, mice, pocket-gophers, squirrels, 

 marmots, beavers and the extraordinary fmylagauUds were 

 all abundant. 



In even more strongly marked sense must the broad mean- 

 ing for the vernacular names of the other mammals be em- 

 phasized, for we have to deal almost exclusively with extinct 

 genera, which differed much from their modern descendants. 

 Many of the Carnivora have been obtained; there were 

 numerous dogs, some rivalhng the largest of existing bears in 

 size, true felines and fsabre-tooth tigers, which were smaller 

 and lighter animals than the great beasts of the Pleistocene; 

 weasels, martens, otters and raccoons, but no bears. The bears, 

 a family of Old World origin, are not certainly known in America 

 before the Pleistocene, but had probably reached this continent 

 in the Pliocene. 



As is so very generally true, the commonest and best- 

 preserved of the fossils are those of the hoofed animals. The 

 fmastodons were of the four-tusked kind ('\Gomphotherium or 

 "fTrilophodon), the skull and teeth of which differed so markedly 

 from those of the true elephants. The relatively small, low- 

 crowned and simple grinding teeth were common to aU the 

 fmastodons, but the tusks were different from those of the 



