PLANT-EATING MAMMALS 



175 



a sharp edge is maintained by similar means, for the front side 

 of each incisor is covered with a thick coat of hard enamel, at 

 the back of which comes a softer mass of dentine. Hence the 

 effect of wear is to produce and maintain a bevelled or chisel- 

 like edge. As these teeth are continually growing it is necessary 

 that they should be kept worn down by constant use, and should 

 a rodent be so unfortunate as to lose one of its front teeth, its 



Fig. 422. Hares ^Lepns timidus) 



fellow will have nothing to do to wear it down, and, continuing 

 to grow, will first prove a hindrance to feeding, and then stop 

 it altogether, sometimes even piercing the skull of its owner. 

 The same thing happens (fig. 423) if these teeth are misplaced. 

 In Rabbits, Hares, and their immediate allies, but not in re- 

 maining Rodents, two very small reduced incisors are present 

 behind the large ones, but it is doubtful if these are of much 

 use, and they may be looked upon as structures which are on 

 the downward path of degeneration. The Rabbit's cheek-teeth 

 are prismatic in form, and their broad grinding crowns are 

 crossed by enamel ridges which, as in many other cases, slant 

 differently above and below, so that when the upper and lower 

 teeth are brought together the opposed ridges cross one another. 

 A further adaptation to the plant-eating habit is found in the 



