308 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



is formed a tooth, and thus there is a magazine of teeth 

 of all sizes, which are successively brought forward and 

 attached as the old tooth drops off (Fig. 193). 



Origin of Mammalian Teeth.— A characteristic 

 of mammalian teeth is that they are differentiated into 

 three groups, distinct in form and in function, viz., incisors. 

 for cutting, canines for seizing, and molars and premolars 

 for masticating. The complex structure of these last 

 is especially significant. Now in some early extinct rep- 

 tiles of the Permian and Triassic periods, immediately 

 before the appearance of mammals, the teeth were already 



Fig. 194. — Head of Cynognathus, a Triassic reptile, showing teeth similar 

 to those of mammals. (From Woodward.) 



differentiated into the three groups, and the jaw teeth 

 were already become molariform (Fig. 194). Indeed, a 

 complete series may be traced from the simple conical 

 prehensile teeth of ordinary reptiles to the most com- 

 plex grinders of ruminants. It is certain, therefore, that 

 mammalian teeth have come by gradual modification 

 from simple prehensile teeth of reptiles. 



Fishes.— The teeth of fishes are of three kinds — viz., 

 conical, lancet-shaped, and pavement teeth. The conical are 

 the commonest; they are prehensile only. The lancet- 

 shaped teeth are very characteristic of sharks. They are 



