PART IL 



THE STEUOTUEAL EYIDE^OE OF 

 EYOLUTIOK 



VII. 



ON THE HOMOLOGIES AND OEIGIN OF THE TYPES 

 OF MOLAR TEETH OF THE MAMMALIA EDUCA- 



BILIA. 



THE TYPES OF MAMMALIAN" MOLARS. 



It has been already stated* tliat the transition from simple to 

 complex teeth is accomplished by repetition of the type of the 

 former in different directions. "In the cetaceans this occurs in 

 the Squalodonts ; the cylindric incisors are followed by flattened 

 ones, then by others grooved in the fang, and then by two-rooted, 

 but never by double-crowned teeth. This is the result of antero- 

 posterior repetitive acceleration of the simple cylindric dental type 

 of the ordinary toothed cetacean. Another mode of dental com- 

 plication is by lateral repetition. Thus the internal heel of the 

 superior sectorial tooth of a carnivore is supported by a fang 

 alongside of the usual posterior support of a premolar, and is the 

 result of a repetitive effort of growth-force in a transverse direc- 

 tion. More complex teeth, as the tubercular molars, merely ex- 

 hibit an additional lateral repetition, and sometimes additional 

 longitudinal ones. As is well known, the four tubercles of the 

 human molar commence as similar separated knobs on the [primi- 

 tive] dental papilla." 



Accordingly, the simple tubercle may be regarded as the least 

 specialized form of tooth. It may be low and obtuse, as in the 

 Chiromt/s, or the walrus ; more elevated and conic, as in the dol- 



* "Method of Creation," p. 10. Philadelphia, 187L 

 IC 



