THE TEETH OF ARTIODACTYLE UNGULATA. 327 



directed forwards, and its sides being nearly flat. Enamel 

 is confined to the internal and external anterior surfaces ; 

 the posterior surface, which plays against the upper canine, 

 is devoid of enamel ; the tooth is kept constantly pointed by 

 the obliquity with which its posterior surface is worn away. 

 The tusks of a boar are most formidable weapons, and are 

 capable of disembowelling a dog at a single stroke, but they 

 are greatly exceeded by those of the African wart-hog (Phaco- 

 chcerus), which attain to an immense size. 



In the domestic races the tusks of the boars are much 

 smaller than in the wild animal, and it is a curious fact 

 that, in domestic races which have again become wild the 

 tusks of the boars increase in size, at the same time that 

 the bristles become more strongly pronounced. Mr. Darwin 

 suggests that the renewed growth of the teeth may perhaps 

 be accounted for on the principle of correlation of growth, 

 external agencies acting upon the skin and so indirectly 

 influencing the teeth. 



As in most artiodactyles, the teeth of the molar series 

 increase in size from before backwards : thus the first pre- 

 molar or milk molar has a simple wedge-shaped crown, and 

 two roots ; the second and third by transitional characters 

 lead to the fourth premolar, which has a broad crown with 

 two principal cusps, and has four roots. 



The first true molar has four cusps divided from one 

 another by a crucial depression ; and the cingulum in front, 

 and yet more markedly at the back, is elevated into a 

 posterior transverse ridge. In the second molar the trans- 

 verse ridge is more strongly developed, and the four cusps 

 are themselves somewhat divided up into smaller accessory 

 tubercles. 



The last molar measures, from front to back, nearly twice 

 as much as the second ; and this great increase in size is 

 referable to a great development of the part corresponding 

 to the posterior ridge or cingulum of the second molar, 



