STRUCTURE OF THE VERTEBRATES 89 



of the leg passing between them. This order and the following 

 were formerly classified together as the ungulata, but their evo- 

 lutionary history shows them to have been separate groups since 

 very early in the history of mammals. 



Order Perissodactyla. These are the odd-toed herbivorous 

 mammals. The axis of the leg passes through the third digit, 

 with the weight resting upon it. The rhinoceros, horse, zebra, 



Tapir (Perissodactyla) 



Rhinoceros (Perissodactyla) 



Fig. 37. Order Perissodactyla. The order includes the horse, ass, zebra and 



the above forms. 



tapir and ass are members of the order. The horse is a very 

 highly specialized member of the group, the number of digits 

 having been reduced to one in the adult. There is, perhaps, no 

 evolutionary history better worked out than that of tlie horse. 

 Order Proboscidea. The proboscidea are animals with a great 

 elongation of the nose forming a proboscis, and the living mem- 

 bers of the order are usually included in one genus, Elephas. 

 The evidence warrants the separation of the African elephants 

 as a different genus from the Asiatic elephant. The elephants 



