THE TEETH OF ARTIODACTYLE UNGULATA. 329 



In antero-posterior extent the third molar equals the first 

 and second true molars and the third and fourth premolars 

 (the whole number of teeth of the molar series possessed by 

 the animal) together. 



When a little worn its surface presents about thirty islands 

 of dentine, surrounded by rings of enamel, the interspaces 

 and the exterior of the whole being occupied by cementum. 

 Of course, prior to the commencement of wear, each of these 

 islands was an enamel-coated cusp. 



The Wart-hog's dentition has, however, another instructive 

 peculiarity ; the first true molar is in place early, and be- 

 comes much worn down (this is true, in a less degree, of the 

 common pig, and indeed^of most Ungulata). Eventually it 

 is actually shed ; the same fate later befalls the third pre- 

 molar and second true molar, so that the dentition in an 

 aged specimen is reduced to the fourth premolar and the 

 third true molar alone, and eventually to the last true 

 molars alone. Thus, both in the great complexity of the 

 back molars and the fact that the anterior teeth are worn 

 out and then discarded, the Wart-hog affords a parallel to 

 the anomalous dentition of the elephant. 



As has already been noticed, the upper canines in the 

 boar turn outwards and finally upwards, so as to pass out- 

 side the upper lip ; this peculiarity in direction, yet more 

 marked in Phacochcerus, attains its maximum in the Sus 

 babirussa. 



This creature, strictly confined to the Malay Archipelago, 

 where it frequents woody places, has (in the male) the upper 

 and lower canines developed to an enormous extent. The 

 upper canines are turned upwards so abruptly that they 

 pierce the upper lip, instead of passing outside it as in 

 other Suidce, preserve a nearly upright direction for some 

 little distance, and then curve backwards, so that their 

 points are directed almost towards the eyes. 



The lower canines are less aberrant in direction and in 



