SUCCESSIVE MAMMALIAN FAUNAS 257 



Of the horses there was no great variety and all the species 

 so far discovered are included in a single genus {'\Mesohippus), 

 though there was a decided increment in the size of the suc- 

 cessive species from the earUer to the later portion of the stage. 

 Looked at superficially, it seems absurd to call these Uttle 

 creatures "horses" at all and the term can be justified only as 

 implying that they were ancestral members of the family. 

 The largest of the White River species hardly exceeded a sheep 

 in size and all of them had comparatively short necks, long 

 and slender legs and three-toed feet. The low-crowned grind- 

 ing teeth show that they were browsers, not grazers. The 

 abundant Eocene family of the fLophiodontidae anade its last 

 appearance in the White River, where it was scantily repre- 

 sented by slender, long-legged animals {'\Colodon), with feet 

 singularly like those of the contemporary horses, except that 

 there were four toes in the front foot. Tapirs ('\Protapirus) 

 were very much less common than rhinoceroses or hordes and 

 were hardly half as large as the existing species of the family 

 and of relatively far more slender form; the development 

 of the proboscis had already begun. Lastly, the presence of 

 the clawed fchalicotheres has been reported from the lower 

 Oligocene of Canada, but the material is too fragmentary for 

 generic reference. 



Though the number of artiodactyl families yet identified 

 among the White River fossils is no larger than that of the 

 perissodactyl families, the artiodactyls greatly preponderated 

 in individual abundance. The peccaries, which were fairly 

 common, resembled those of the John Day, but were consider- 

 ably smaller. Of the camels, there were two series, one of 

 which {■\Eotylopus), lately described by Dr. Matthew, is of yet 

 unknown significance, while the other (^Poebrotherium) was 

 apparently the ancestor common to all the subsequent phyla 

 of camels and llamas. This extremely interesting genus had 

 species which ranged in size from a gazelle to a sheep, had two 

 toes in each foot, a moderately elongate neck and teeth which 



