SUCCESSIVE MAMMALIAN FAUNAS 233 



as was first pointed out by Kowalevsky, the explanation is 

 probably to be found in the spread of grassy plains at the ex- 

 pense of the forests. On account of the silica which they 

 contain, the grasses are very abrasive and rapidly wear the 

 teeth down. In adaptation to this new source of abundant 

 and nutritious food, many kinds of mammals developed a form 

 of tooth which was fitted to compensate by growth for the 

 loss through abrasion. 



The middle Miocene, small areas of which occur in Montana, 

 eastern Oregon and northeastern Colorado, has received various 

 local names, the typical one being the Deep River of Montana. 

 Very probably, these scattered areas are not strictly contem- 

 poraneous, but form a closely connected series. That a land- 

 connection with the eastern hemisphere existed, is made clear 

 by the appearance of several unmistakably Old World types 

 of animals and the beginnings of migration from South America 

 are perhaps also to be noted, though this cannot be positively 

 stated. The evidence for the South American connection 

 is the finding in the middle Miocene of Oregon of what are 

 believed to be the earliest remains of fground-sloths yet dis- 

 covered in North America, but the material is too scanty for 

 altogether certain determination. 



The smaller animals are not very well represented in the 

 middle Miocene faunas, as conditions appear to have been 

 unfavourable to their preservation ; something is known of 

 them, nevertheless. The very curious extinct family of rodents 

 known as the fMylagaulidae, the presence of which was noted 

 in the upper Miocene and lower Pliocene, first appeared here. 

 These fmylagaulids, which were distantly related to the modern 

 Sewellel (Aplodontia rufa), were characterized by the great 

 enlargement and complication of one of the grinding teeth 

 in each jaw and the consequent reduction of the others. One 

 genus of this family, as in the Pliocene, had the peculiarity, 

 unique among rodents, of developing a large horn upon the nose, 

 like a miniature rhinoceros. Among the Carnivora, we find a 



