128 THE STEUCTUEE OF THE HOESE. 



development the first set being preceded in their 

 places by other teeth of the milk, or deciduous series, 

 and the last set coming up behind the last of the 

 milk-teeth, without any predecessors the distinction 

 was thought to be of sufficient importance to give 

 them different names, the first being called "premo- 

 lars," or " false molars,'' and the last " true molars." 



In the horse there is no difference in form or size 

 between the premolars and true molars, and it is only 

 by the analogy of other animals, and by a knowl- 

 edge of their early history in the horse itself, that 

 we can divide them, and know that the great mill- 

 like mass of cheek-teeth consists of three premolars 

 and three molars. 



It is characteristic of a primitive condition of 

 dentition that premolars and molars should differ 

 in form. Such a condition is, doubtless, best adapted 

 for an omnivorous or generalized mode of feeding ; 

 but it is a specialty on the part of the Perissodactyles, 

 which was acquired very early in their history, and 

 is, no doubt, in accordance with their strictly vege- 

 tarian lif e, that the premolars have taken on the form 

 of the true molars, and have become as completely 

 adapted as the latter for the grinding function. 



Another and still more important deviation which 

 has taken place in the general condition of the molar 

 teeth from their primitive state is this. The crowns 



