ART10DACTYLA. 755 



nature of the tarsal bones, the tendency that the stomach 

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

 oortant 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 crowias 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 ^ * 43 . 



The stomach has three chambers ; there is no caecum, 

 Suidae. The Old World boars and pigs, characterised by the mobile 

 snout and terminal nostrils. There are four well-developed 

 digits on the narrow feet, but the second and fifth do not reach 

 the ground in walking. The incisors are rooted ; the upper 

 canine curves outwards or upwards. The stomach is almost 

 simple, but has more or less of a cardiac pouch and several 

 short blind saccules ; there is a caecum. 



Examples. Sus, ^3; Babirusa, 2 -^, the male with remarkable 



canines, the upper pair growing upwards from their base 

 through the skin, arching backwards as far as the forehead, 

 and sometimes forwards and downwards again, the lower pair 

 with a more or less parallel course; Phacochcerus^ the wart-hog. 

 Dicotylidae. The New World peccaries (Dicotyles], with a snout 

 like that of pigs, with four toes on the fore-feet, and three behind. 

 The incisors are rooted, the upper canines are directed down- 

 wards, the dental formula is ^^. The stomach is complex, and 



there is a caecum. 



Group 2. Tylopoda, comprising the family Camelidae the camels 

 of the Old World and the llamas of S. America. The limbs 



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