HISTORY OF THE PERISSODACTYLA 



299 



while the permanent grinders had much higher, cement-covered 

 and complex crowns. In the lower Miocene, the variety of 

 horses was much diminished and all had the low-crowned, 

 cement-free, browsing type of teeth. Reversing the statement, 

 we see that in the middle and still more in the upper Miocene 



J i 



Fig. 151. — Skeleton of ^ Neohipparion whitneyi, American Museum. 



the primitive and more or less distinctly homogeneous phylum 

 branched out into several series, like a tree, some of the branches 

 continuing and further subdividing through the Pliocene and 

 Pleistocene, while others, less progressive and less adaptable, 

 underwent but little change and had died out before the middle 

 Pliocene. 



The Oligocene horses deserve more particular attention, 

 for they were almost the half-way stage of development in the 

 long backward ascent to the earliest known members of the 

 family in the lower Eocene. We may pass over the John Day 

 horses {'\Miohippus), which were somewhat larger than those 

 of the White River, but otherwise very like them, merely noting 

 the presence of a slightly different genus {]Anchitherium) 



