214 THE MAMMALIA. 



series. Orohippus was but little larger than 

 Eohippus, and in most other respects very similar. 



' Near the base of the Miocene we find a third 

 closely allied genus (Mesohippus), which is about as 

 large as a sheep and one stage nearer the horse. 

 There are only three toes and a rudimentary splint 

 bone on the fore-feet and three toes behind. Two 

 of the premolar teeth are quite like molars. The 

 ulna is no longer distinct, or the fibula either, and 

 other characters show clearly that the transition is 

 advancing. In the Upper Miocene Mesohippus is 

 not found, but in its place a fourth form (Mio- 

 liippus) continues the line. The genus stands 

 close to the Anchitherium of Europe, but presents 

 several important differences. The three toes in 

 each foot are more nearly of a size, and a rudi- 

 ment of the fifth metacarpal bone (of the second 

 series) is retained. All the known species of this 

 genus are larger than those of Mesohippus, and 

 none pass above the Miocene. 



* The genus Protohippus of the Lower Pliocene is 

 still more equine, and some of its species equalled 

 the ass in size. There are still three toes on each 

 foot, but only the middle one, corresponding to the 

 single toe of the horse, comes to the ground. This 

 genus resembles most nearly the Hipparion of 



