IN THE LOWER ANIMALS, ETC. ' 317 



spectively commence to penetrate the gum in the 

 sixth, twentieth, and fiftieth year ; and the two 

 former are shed in the twenty-fifth and sixtieth 

 years ; while the sixth molar is supposed to remain 

 during the continuation of the animal's existence. 

 The perfection of a grinding apparatus is attained 

 in the internal structure of these molar teeth, the 

 three constituent elements, which we have already 

 noticed in the other graminivorous mammals, 

 being developed in a remarkably distinct manner. 

 A molar tooth is composed of a series of transverse 

 perpendicular plates, each of which consists of a 

 central table of dentine, invested by a layer of 

 enamel ; and these separate plates are at length 

 soldered together by the filling-up of the spaces 

 with the cement, which retains the several plates 

 in due relative position, until by the further 

 development of their bases, they unite to form a 

 solid mass of dentine in the more deeply implanted 

 portion of the tooth. 



Fig. 46. — Molar Teeth of Indian Elephant, showing seven of the 

 transverse plates already in use, the posterior portion of the tooth not 

 yet completely formed. 



"E. The oblong ring of enamel. H. The enclosed plate uf dentine. 



C. The cementum uniting the plates. 



