xrii PHYLUM CHORDATA 533 



The incisors of the upper jaw are vertical, those of the lower 

 greatly inclined forwards. The canines are greatly developed, 

 especially in the male, and grow from persistent pulps ; both upper 

 and lower are bent upwards and outwards and work against one 

 another in such a manner that the upper wears on its anterior and 

 external surface, the lower at the extremity of the posterior. The 



FIG. 1123. Left lateral view of the dentition of the Boar (Sus scrofa), the roots of the teeth 

 being exposed. (After Flower and Lydekker. 



pre-molars are compressed with longitudinal cutting edges, the 

 molars are provided with numerous tubercles or cusps arranged 

 for the most part in transverse rows. The formula of the milk 

 dentition is 



.31 3 



In the typical Ruminants there are no teeth on the pre-maxillae, 

 the incisors of the lower jaw and the canines, which resemble them 

 in shape, biting against a thickened callous pad on the opposed 

 surface of the upper jaw, and the upper canines are also usually 

 absent ; there are three pre-molars and three molars in both upper 

 and lower series, all characterised by the presence of column-like 

 vertical folds of enamel, the interstices between which may be 

 filled up with cement (Fig. 1112, V). In the Camels there are a 

 pair of upper incisors and a pair of large canines in each jaw. 



In the Perissodactyla the molars and pre-molars form a con- 

 tinuous series of large teeth with ridged or complexly- folded 

 crowns, the posterior pre-mclars often differing little in size 



