HISTORY OF THE PERISSODACTYLA 333 



upper Miocene and lower Pliocene. The middle Miocene 

 species {'\T. medicornutus) would seem to have been descended 

 from \T. aurelianensis of the lower Miocene of France; the 

 two species agreed not only in having a small horn on the tip 

 of the nose, but also in the presence of a still smaller one on 

 the forehead. 



In the lower Miocene but two phyla of rhinoceroses have 

 been found, both of which were the comparatively little changed 

 descendants of Ohgocene ancestors ; and there was thus a notable 

 difference from the rhinoceroses of the middle Miocene and sub- 

 sequent stages, which were decidedly more modern in character. 

 One of these phyla was constituted by those rhinoceroses 

 {^Diceratherium, Fig. 129, p. 239) whichhad a transversely placed 

 pair of horns on the nose, not one behind the other, as in all 

 of the subsequent two-horned species, of which North America 

 had but the one middle Miocene form {]T. medicornutus) 

 mentioned above. The lower Miocene species of ]Dicera- 

 therium was a very small animal, and smaller than any mem- 

 ber of the family from later formations. The fdiceratheres 

 originated in North America, and the stages of their develop- 

 ment may be clearly made out ; they also migrated to the east- 

 ern hemisphere and have been found in France, though it is 

 possible that the genus was not truly monophyletic and arose 

 independently in both hemispheres. 



The second phylum is that of the hornless forms {'fCcenopus) 

 which were so abundantly represented in the Oligocene and 

 persisted with little change into the Pliocene. 



In the upper Oligocene, or John Day, the fdiceratheres 

 are the only rhinoceroses certainly yet obtained, and of these 

 there were several species, large and small. The hornless 

 forms may have been present in Oregon, but this has not been 

 clearly demonstrated. That they continued to exist some- 

 where during that stage is hardly open to question, for they 

 reappeared in the lower Miocene. 



From the White River, or lower Oligocene, many well- 



