172 R. S. Lull— The Evolution of the Horse Family. 



contains the type specimens of Eo/iippus pemix and E. validus 

 and numerous jaw, teeth, and limb bones referable to the genus. 

 During the succeeding Wind River age Protorohippus 

 appears, representing the so-called third stage in the evolution 

 of the horse. In stature somewhat larger than Eohippus, this 

 animal reached the height of fourteen inches, and while the 

 beginnings of speed are evident from the increasing length of 

 limb, the bodily contour is still primitive. The hand seems to 

 have lost the vestigial thumb and a shortening of the outer- 



/v 



Fig. 8. a, Fore foot and b, hind' foot of Orohippus agilis. 

 natural size. (After Marsh.) 



One-half 



most digit indicates that the tendency toward a three-toed con- 

 dition is already strong. The foot is much like that of its 

 predecessor with three functional toes. The dentition shows 

 progress in the further perfection of the crests, in the fact 

 that the fourth premolar is quite molariform, and that the 

 third is beginning to assume the shape of a molar. Protoro- 

 hippus had a very limited geographical range, being found 

 thus far only in Wyoming and Colorado. In the Yale collec- 

 tion the genus is represented by a cast of the skull of P. ven- 

 tricolus from the mounted specimen in the American Museum 

 of Natural History. 



