696 MAMMALIA. 



In these typical Ungulates the feet are never plantigrade. 

 In modern types there are never more than four functional 

 toes. The os magnum of the carpus articulates freely with 

 the scaphoid. The brain is well convoluted. The testes 

 descend into a scrotum. The uterus is bicornuate. The 

 placenta is non-deciduate, and either diffuse or cotyle- 

 donary. 



Sub-Order Artiodactyla. Even-toed Ungulates. 



Pigs and Hippopotamus (Suina), Camels (Tylopoda), 

 Chevrotains (Tragulina), and Ruminants (Pecora) like 

 Cattle and Deer. 



The general characters of this sub-order have been stated 

 above in contrast to those of Perissodactyla. The equal 

 development of the third and fourth digits, the fact that the 

 premolars have a single lobe while the molars have two, the 

 nature of the tarsal bones, the tendency that the stomach 

 has to be complex (as in Camels and Ruminants), are im- 

 portant characteristics. There are others of less obvious 

 importance, such as the absence of the alisphenoid canal, 

 which in Perissodactyla encloses the external carotid artery 

 as it passes along the alisphenoid. 



There are primitive extinct Artiodactyla which connect 

 the four modern groups — Suina, Tylopoda, Tragulina, 

 and Pecora. Thus they unite the bunodont types, such 

 as pigs, with cone-like tubercles on the crowns of the 

 molars, and the selenodont types, such as cattle, with the 

 tubercles expanded from before backwards, and curved in 

 crescents. 



Group I. Suina — hippopotamus, pigs, and peccaries. The molars 

 are bunodont ; the third and fourth metacarpals and metatarsals are not 

 completely fused as " cannon bones." 



Hippopotamidae. — Huge African mammals, included in the single 

 genus Hippopotamus. They spend the day in the rivers and 

 lakes, swimming and diving well, but usually remaining concealed. 

 At night they come on land and browse on grass and herbage. 

 The skin is extremely thick, with a few hairs restricted to the 

 snout, head, neck, and tail. There are four toes on each foot, all 

 reaching the ground. The rootless incisors continue growing ; 



so do the large curved canines; the dental formula is 2 ^^ 43 . 



1-3, 143 

 The stomach has three chambers ; there is no csecum. 



