MAMMALS. 



393 



while in the ungulata vera they alternate. The subungulata 

 also present considerable differences in the placental arrange- 

 ments, allusion to which will be made below. 



Professor Cope, utilizing the characters presented by the 

 carpal and tarsal bones, has proposed to divide the ungulates 

 into five divisions, Taxeopoda, Toxodontia, Proboscidia, Am- 

 blypoda and Diplarthra, his Taxeopoda, including not only 

 forms usually recognized as ungulates, but the primates as well. 



SUB-ORDER i. CONDYLARTHRA (MESODACTYLA). 



Extinct ungulates with five-toed, plantigrade feet ; carpalia in straight 

 rows, not alternating ; femur with third trochanter, molars bunodont. 



The condylarthra are the most primitive of ungulates. From them have 

 doubtless descended the ungulata vera, and, if the views of Cope be correct, 

 the carnivores and primates as well. The group 

 appears in the lowest eocene, and is especially 

 well developed in the lower tertiaries of the 

 western U. S. Four families, PERIPTYCHID.E, 

 PHENACODID^E, MENISCOTHERIID/E, and PLEUR- 

 ASPIDOTHERIID^E recognized, the latter from 

 the eocene of France. 



SUB-ORDER 2. PERISSODACTYLA 

 (SOLIDUNGULA). 



Unguligrade ungulates with the middle toe 

 well developed, forming the axis of the foot, 

 carpals alternating ; astragalus with pulley-like 

 surface for tibial articulation ; placenta diffuse. 



The perissodactyls, which walk upon the 

 very tips of the toes, have the feet, as a rule, with 

 the toes three or four in front and three behind ; 

 but frequently only the third toe on either foot 

 comes to complete development, the others be- 

 ing very rudimentary. The dentition is usually 

 complete, the molars being lophodont or rarely 

 bunodont, while the premolars tend to resemble 

 the molars. The femur (except in Chalicothe- 

 riuiri) has a third trochanter, and the fibula does 

 not usually reach the calcaneum. The stomach 

 is simple; the intestine has large caecum, and a 



gall bladder never occurs. The mammas, few in number, are inguinal in po- 

 sition. The living perissodactyls present three very distinct types, horses, 

 tapirs, and rhinoceroses, but in the tertiary period many other forms occurred 



FIG. 364. Fore foot of 

 two-horned rhinoceros, Ate- 

 lodus bicornis. 



