ELEPHANTS, HORSES, AND CAMELS 



181 



two significant genera, Merychippus (Fig. 108) and Protohippus. 

 The molars of Merychippus mark the transition from the browsing 

 to the grazing type, for paleontology records that the milk denti- 



FiG. 108. — Restoration of the prairie horse, Merychippus, from the Miocene. 



(From Lull.) 



tion is of the low crowned primitive type while the permanent 

 molars are high crowned grazing teeth (Fig. 109). The feet 

 were three-toed, but the middle toe was so much more highly 



developed than the others that it alone 

 supported the body, while the others 

 did not reach the ground. Protohippus 

 differed from Merychippus in the fact 

 that both milk and permanent molars 

 were high crowned. These genera in- 

 clude the first highly specialized grazing 

 horses. 



From the same stem another genus, 



Hipparion, originated during the late 



Fig 109.-lTpper premolars Miocene in North America, whence it 



of Merychippus. A, milk . -^^ • 



teeth, without cement; B, migrated mto Asia and Europe. During 



permanent teeth, with ce- the Pliocene it became extinct. Matthew 

 ment. (From Lull.) states that Hipparion whitneyi, from 



South Dakota, was a very slender and graceful horse, except for its 

 large head, and was adapted by the complexity of the enamel 

 ridges of its teeth to eat the harshest herbage. It is therefore 

 looked upon as a very fleet species, fitted to live in semi-desert 



