POSITION AND STRUCTURE OF HORSE 19 



been developed by the reduction of the lateral toes 

 of a nearly allied tridactyle relative. In this case 

 the specialisation was even greater than in the 

 horse group, as the splint-bones were reduced to 

 mere nodules of bone on either side of each cannon- 

 bone. 



If Thoatherium had been a near relative of the 

 horse, there would be no cause for surprise in its 

 having attained the same remarkable and final stage 

 of foot-development. As a matter of fact, it 

 belongs, however, to a totally different and much 

 more primitive group of ungulates, which appears 

 to have been always restricted to South America, 

 and although presenting certain structural resem- 

 blances to the Perissodactyla, is in other respects so 

 distinct that it is ranked, under the name of Lito- 

 pterna, as an equivalent subordinal group of the 

 great order Ungulata. The most remarkable thing 

 connected with Thoatherium is that, despite the 

 specialised toes, its carpus and tarsus are of an 

 exceedingly primitive type. 1 



Several noteworthy features occur in the skull 

 of the horse and its existing relatives. In the first 

 place, it differs from the skulls of all other living 

 perissodactyles namely, tapirs and rhinoceroses 

 in the complete closure of the rim of the socket of 

 the eye by means of a bridge of bone extending 



1 See W. B. Scott, "The Litopterna," Rep. Princeton Univ. 

 Exped. to Patagonia, vol. vii. pt. i. 1910. 



