THE TEETH OF VERTEBRATES 63 



which are found between the canines and true 

 molars among mammals, and vary in number in 

 different species. They are the principal crush- 

 ing members of the dental series, and are placed 

 midway of the cutting and grinding teeth proper. 

 In man the upper premolars (or bicuspids) are of 

 simple form, being composed of two or three 

 cones united together. The lower premolars vary 

 from this form. In the higher Apes the pre- 

 molars are of the same form as in man, but are 

 coarser and larger. In the lower Quadrumana 

 they are reduced to simple crushing teeth with the 

 outer cusp enlarged and the inner reduced. In 

 the Carnivora they are highly specialized on ac- 

 count of the tubercles being raised into large cut- 

 ting blades. In the Herbivora they are similar 

 to the true molars in form, and are developed for 

 triturating purposes. In the Insectivora they are 

 very variable in shape, but possess, like the true 

 molars, long, sharp cusps for crushing insect cov- 

 erings. In the Rodentia they are entirely absent 

 in most of the species of this extensive order, as 

 there is a large vacant space between the incisors 

 and the grinding teeth. The true molars are 

 found alone in the Mammalia, and are highly 

 specialized teeth, being developed for the per- 

 formance of the function of mastication. In the 

 Bruta the teeth are all molars of a simple form, 

 for crushing purposes. Some low forms have 



