ELEPHANTS, HORSES, AND CAMELS 



179 



Orohippus, the next genus to appear, followed Eohippus in the 

 Eocene and shows only slight changes. The vestigial bones of 

 the first and fifth digits of the hind limb had disappeared, the fifth 

 digit of the fore limb was shorter, 

 and the third premolar was molar- 

 iform. It included somewhat larger 

 species than its predecessors. 



Epihippus, a third genus, occurred 

 later in the Eocene. Complete 

 skeletons have not been found, but 

 the last two premolars were molari- 

 form, the first still persisted, and 

 the teeth were somewhat higher 

 crowned than in the older genera. 



Mesohippus, an Oligocene genus, 

 included several species which 

 varied in size but scarcely attained 

 the size of a sheep. The upper 

 incisors differed from those of earlier 

 species in the "mark," an enamel 

 ridge behind the cutting edge, 

 which is characteristic of true 

 horses. The premolars were molar- 

 iform with the exception of the 

 fh'st, which shows a tendency to 



disappearance. The teeth had not yet acquired the high-crowned 

 form of the true horses (Fig. 106). The eye was set farther 

 back than in Orohippus, and its orbit was closed behind, unlike 

 the preceding forms. The bones of the feet of Mesohippus are 



still further reduced (Fig. 107). 

 All four feet had three functional 

 digits, and only the anterior pair 



Fig. 105.-Upper teeth of Eohippus. stained a vestige of the fifth. 



Premolars visibly smaller and sim- The ulna of the fore limb and 



pier than molars. (From Lull, after fibula of the hind limb were very 



^ ^^ slender, but complete. The same 



epoch produced Miohippus, a genus of similar forms. 



In his recent review of the evolution of the horse Matthew 



records changes in the relationships of the carpal and tarsal bones 



which are of importance in the development of the one-toed foot. 



Fui. 104. — Feet of Eohippus ven- 

 iicolus. (From Matthew.) 



