272 LAND MAMMALS IN THE WESTERN HEMISPHERE 



commonest of the genera in the middle and lower Bridger was 

 ^PalcBosyops, which was hornless, while in the upper part of 

 the beds are found genera (e.g. \Manteoceras and ]Dolicho- 

 rhinus) in which the horns were just beginning to appear. 

 Another extinct family, the fLophiodontidae, which was very 

 abundant in the European Eocene, formed a very subordinate 

 element in this fauna and included a number of small tapiroid 

 genera {e.g. ^Helaletes). 



The horses {'\Orohippiis) were very small and primitive 

 creatures, no bigger than a fox, with four toes in the front foot 

 and three in the hind. So completely different in appearance 

 and proportions were these little animals from any of the 

 modern horses, that it requires an effort of the imagination 

 to think of them as belonging to the same family, and it is only 

 by employing the family to designate a genetic series that such 

 a classification can be justified. The fhyracodonts, or cursorial 

 rhinoceroses, were very abundantly represented by a number 

 of small and medium-sized animals {\Hyrachyus) which had 

 less specialized teeth, shorter neck and limbs than their upper 

 Eocene and Oligocene successors, and four toes in the front 

 foot ; one genus {^Colonoceras) had a pair of nasal horns, 

 but would seem to have died out without leaving descendants. 

 In the upper part of the beds is found the Uinta genus \Triplo- 

 pus, with three-toed fore foot ; and in the same division occurs 

 another Uinta genus, \Amynodon, the most ancient known 

 species of the supposedly aquatic rhinoceroses. True rhi- 

 noceroses, that is animals which were directly ancestral to the 

 modern members of the family, have not been identified and 

 may not have been present in North America; that is still 

 an open question. Tapirs, all of them quite small, were rel- 

 atively common, but are still very incompletely known. The 

 earliest known members of the clawed fchalicotheres were of 

 Bridger date. 



It is worth remarking that, except a single genus in the 

 upper and later portion of the stage (fTriplopus), all of the 



