MAMMALIA, 



513 



Turning to the dentition there are sufficiently obvious 

 lifferences. In the ox there are no incisors nor canines 

 n the upper jaw, their place being taken by a horny pad. 

 In the mandible there are three pairs of chisel shaped 

 iiK isors and a pair of canines which resemble incisors in 

 ^licipe and size. In the horse, on the other hand, there are 

 three pairs of incisors in both upper and lower jaw which 

 are of a peculiar shape. They have their terminal surface 

 pushed in as a deep pit, partially filled with cement. On 

 being worn flat the surface of the tooth presents two con- 

 centric circles of enamel, the inner circle becoming narrower 

 with age. The canines 



are small and pointed Fig. 352.— Upper Jaw (left-half) 

 and are only rarely pre- ^^ young (A) and Old Horse (B). 

 sent m the female. 



As regards the molar 

 series we have seen that 

 there are considerable 

 resemblances in the two 

 types, and in each there 

 are six functional teeth 

 on each side, of which 

 three are premolars and 

 three are molars. Here 

 the resemblances end. 

 In most horses there is, 

 at least in the adolescent 

 stage, a very small first 

 premolar in each upper 



jaw, which usually falls out at maturity. Thus the full 

 dentition of a young horse may be given as 2143, but that of 



a mature mare is ?^. The dentition of the ox is ^231. 

 3033 . 3133 



Though the patterns of the enamel in the molars have 

 a general resemblance, a little study shows that they are 

 derived from different types. The horse starts from the 

 simple bilophodont type, found in the tapir, consisting of 

 a pair of transverse ridges : this is further complicated, as 

 in the rhinoceros, by a junction of the two ridges and by 

 their bending into a crescentic outline : in the horse these 

 ridges are still further twisted, the multiplication of enamel 

 ridges being the end in view. 



M. 34 



