MAMMALIA, 571 



SUB-ORDER IV. — PERISSODACTYLA. 



A good deal has already been said concerning the 

 Perissodactyla in the chapter upon the Horse and Ox, in 

 which this sub-order is contrasted with that of the Artio- 

 dactyla (page 509). 



The main structural features of the sub-order are as 

 follows: — (i) The molar teeth are bilophodont, or with 

 complex crowns derived from the bilophodont condition. 



(2) Dorso-lumbar vertebrae, usually twenty-three in number. 



(3) The femur has a third trochanter. (4) In the skull the 

 nasals are large and there is an alisphenoid canal. (5) The 

 carpus and tarsus are alternate and the toes are never more 

 than four, mostly three or one, but in all cases the main 

 axis of support passes through tibia, astragalus, navicular, 

 and third toe. (6) Stomach simple. (7) Diffuse placenta 

 and mammae inguinal. 



The molar teeth pass, in the group, from the simple 

 brachydont bilophodont condition (derived, as shown, page 

 462, from the tubercular type) to the complex hypsodont 

 type with cement added. 



The third trochanter is preserved in this group from the 

 early condylarthrous ancestors, and the disappearance of 

 the toes can be traced upwards within the group. No 

 modern Perissodactyla have five toes, but the tapir has four 

 in the fore-foot, the pollex being lost, the rhinoceros has 

 three and the horse merely the one. The main axis passing 

 through tibia,, astragalus, navicular and third toe, it naturally 

 follows that the fibula is reduced or at least loses its articu- 

 lation with the calcaneum, and the astragalus has nearly all 

 its distal articular surface attached to the navicular. In the 

 front-limb the os magnum becomes more and more pro- 

 minent as the third toe usurps the functions of the others. 



In the simplicity of the stomach and the diffuse placenta 

 the Perissodactyla appear to present more primitive char- 

 acters than the Artiodactyla, (As has been noticed, there 

 has been a great deal of parallel evolution in these two sub- 

 orders. The common characters thus acquired form a basis 

 for the institution of the group Ungulata Vera containing 

 these two sub-orders, in contrast with the three preceding 

 sub-orders as Sub- Ungulata. Such a classification, based 

 upon parallel evolution, must, however, be unnatural.) 



