THE EOCENE PERIOD. 



235 



used for grasping or digging, as in the case of the carnivores, they gradu- 

 ally dwindled away. On the whole, the two-toed system seems to 

 have proved the best; at least the artiodactyls are now much the more 

 numerous. 



The evolution of the perissodactyls did not pass beyond the three- 

 toed form during the Eocene period. The three present types, the 

 tapir, the horse, and the rhinoceros, were, however, distinctly fore- 

 shadowed. The most undifferentiated of the early perissodactyls were 

 the lophiodonts, which seem to have graded almost insensibly into the 

 ancestral tapirs (Systemodon) , horses (Hyracotherium) , and rhinoceroses 

 (Hyrochinus). The first definite steps in the development of the horse, 

 which has become a classic example of evolution, appeared in the second 

 stage of the earlier Eocene (Wasatch), no traces having yet been found 

 of the equine line in the Puerco. The earliest recognized form was the 

 Hyracotherium (Fig. 431), whose equine characters are quite obscure. 



Fig. 431. — An early ancestor of the horse family, Hyracotherium, (Protorohippas) 

 venticolum, from the Lower Eocene (Wind River formation) of Wyoming, \ natu- 

 ral size. (Skeleton restored by Cope.) 



Pachynolophus represented a slight step in advance, and the Orohippus 

 (Epihippus) a more decided step. The latter was four-toed in front 

 (three functional) and three-toed behind, and the limbs and teeth were 

 slightly modified in the direction of the horse. These forms were about 

 the size of a small dog, and as nearly canine as equine in appearance. 

 The evolution continued through the remaining periods of the Tertiary, 

 the true horse only appearing in the Pliocene. The primitive Eocene 



