402 LAND MAMMALS IN THE WESTERN HEMISPHERE 



series, though it is very possible that fuller knowledge and 

 more complete material would enable us to distinguish more 

 than one. This monophyletic condition continued through 

 the Eocene and most of the Oligocene, but in the upper por- 

 tion of the latter and, more markedly in the lower Miocene, 

 the two families branched out, each in its own fashion. 



Of course, there were differences in the development of 

 the camels and horses, some conditioned by the fundamental 

 distinction between artiodactyl and perissodactyl, such as the 

 didactyl foot as the possible minimum and the formation of 

 cannon-bones in the camels. Other differences are character- 

 istic of the latter family, such as the great elongation of the 

 neck and the peculiar structure of its vertebrae, the formation 

 of pads on the feet and concomitant reduction of the hoofs. 

 In a general way, the two families kept quite an even pace in 

 their advance from the more primitive to the more specialized 

 condition and, though the camels were the first to acquire 

 certain modifications, the horses ultimately surpassed them. 



Even more close was the parallelism in evolution between 

 the camels and the true ruminants (suborder Pecora) , and this 

 case is of particular importance as clearly demonstrating the 

 development, in two independent but related lines, of similar 

 structures not derived from a common ancestry. This com- 

 parison must naturally await the description of the Pecora. 



7. \HypertragulidcE. fHypertragulids 



This was a very peculiar family, of exclusively North 

 American distribution and of doubtful systematic position, the 

 known history of which extended from the upper Eocene into 

 the lowest Miocene and then abruptly terminated. None of 

 its members attained to considerable size, the largest hardly 

 surpassing a sheep, and some were extremely small. In view 

 of its comparatively brief career, this family was surprisingly 

 ramified, and no less than four phyla may be distinguished 

 within its limits. 



