HISTORY OF THE PERISSODACTYLA 



307 



lost, then the fifth, then the second and fourth were reduced 

 to dew-claws and finally to splints. Thus the pentadactyl 

 horses of the lower Eocene 

 were transformed into the 

 monodactyl species of the 

 Pliocene and Pleistocene. 



In South America the 

 story of the horses was a brief 

 one, for they were among the 

 immigrants from the north 

 and did not reach the south- 

 ern continent till the Plio- 

 cene, probably late in that 

 epoch, for none of the three- 

 toed genera have been found 

 in South America. So far 

 as known, these southern 

 equines were small and me- 

 dium sized animals, with large heads, relatively short feet 

 and somewhat ass-like proportions. There were two w^ell- 

 defined groups of these animals : (1) species of the genus 

 Equus, which thus, at one time or another, inhabited every 

 one of the continents, Australia excepted; (2) three gen- 

 era peculiar to South America and developed there from 

 northern ancestors, probably ■f Pliohippus . Two of these 

 genera {\Hijppidion and ■\Onohippidium) displayed curious 

 modifications of the nasal bones, which were extremely slender 

 and attached to the skull only at their hinder ends, instead 

 of being, as is normally the case, supported for nearly their 

 whole length by lateral articulation with other bones. What 

 can have been the significance and function of these excessively 

 slender, splint-like nasals, it is difficult to conjecture. The 

 third genus (fHyperhippidium) was a small mountain-horse, 

 with extremely short feet, which were well adapted to climbing. 



This is the merest outline sketch of a most wonderful series 



Fig. 157. — Right 

 manus and left pes 

 of ^Mesohippus. 



Fig. 158. — Right 

 manus and pes of 

 \Eohippus. 



