298 



LAND MAMMALS IN THE WESTERN HEMISPHERE 



In the upper Miocene very much the same conditions pre- 

 vailed and, for the most part, the same genera of horses, with 

 different and somewhat less advanced species, were found as 

 rn the lower Pliocene, so that no particular accoimt of them is 



Fig. 150. — Three-toed, grazing horse (f Neohipparian whitneyi) of the upper Miocene. 

 Restored from skeletons in the American Museum of Natural History. 



needed. In the middle Miocene, however, there was a change, 

 the typically grazing horses being very rare or absent and those 

 with intermediate forms of teeth taking their place. Evi- 

 dently, it was about this time that the horses with more plastic 

 organization and capable of readjustment to radically different 

 conditions began to take to the grazing habit, while other 

 phyla, less capable of advance, retained the ancient, low- 

 crowned type of grinding teeth and, after persisting, as we have 

 seen, into the lower Pliocene, became extinct before the middle 

 of that epoch. It is of great interest to observe that in the 

 genus {^Mery chip-pus) intermediate between the browsing and 

 grazing types, the milk-teeth retained the older and more prim- 

 itive character of low crowns without covering of cement, 



