2o8 ANIMAL LIFE PAST AND PRESENT. 



visible in Fig. 65 having now developed into one of 

 nearly equal importance with the others. The most 

 remarkable feature connected with this type of tooth 

 is, however, that all the cones have been compressed 

 from side to side, and have acquired a crescent-like 

 curvature, so that the worn surfaces of dentine are 

 crescent-shaped, and the cones themselves should more 

 properly be described as crescents. Hence this type 

 of tooth has been described as the selenodont (Gr. selenc, 

 the moon, and hence a crescent) modification. It will 

 be observed that in this form of tooth the crescents 

 are still low, and the valleys separating them quite 

 shallow ; and it is further evident that the compli- 

 cated grinding surface thus produced is one better 

 adapted for the thorough mastication of substances 

 like grass than that of the teeth shown in Fig. 65, thus 

 enabling its owner to obtain a better sustenance out 

 of such herbage. With. Fig. 68 we 

 come to the tooth of another extinct 

 animal of this group, in which the 

 fifth crescent has been lost, while 

 the four main crescents have be- 

 come decidedly more deeply curved 

 FIG. 68.-A right upper and somewhat higher, although the 

 cheek-tooth of an extinct bottom of the intervening valleys 



animal intermediate be- 

 tween the Pigs and the [ s s ^{\\ more or less distinctly 



Deer. 



visible. The worn surfaces of den- 

 tine, it will be noted, are quite V-shaped. 



Thus far we have succeeded in tracing the gradual 

 modification of the bunodont tooth of a true 



