THE TEETH OE VERTEBRATES 53 



form predominates, as the teeth in these low 

 types are modifications of the simple cone. This 

 is the primitive typal form of tooth from which 

 all later and complex forms were derived. There 

 is considerable variation of the cone, however, in 

 these classes, some fishes, as the Kays, etc., having 

 plates or a pavement of teeth of flattened shape. 

 Others are of cylindrical or prism-like outline, bnt 

 the majority of fishes and reptiles exhibit modi- 



Fig. 16. — Roentgenogram of the mandible of a Squirrel (Sciurus niger), 

 showing the incisor of continuous growth and brachydont molars and pre- 

 molars. 



fications of the simple cone, which is employed for 

 prehension only. 



In the Mammalia there is a greater variety and 

 more complex forms of teeth. These are formed 

 by evolution of the primitive typal cone, by 

 duplication and modification of the cone to form 

 bicuspid, tritubercular, quadritubercular, etc., 

 forms, as of the molars. Teeth are developed 

 from the primitive cone for various functions : 

 thus the incisors are molded from a single cone 



