TEETH AND THEIR VARIATIONS. 219 



last tooth of certain species, but they have also been 

 so much increased in height and narrowed in width 

 that they assume the form of thin plates, which may 

 be six or eight inches in height, and the sides of which 

 are almost parallel. The valleys between these plates, 

 as they may now be called, have likewise become thin 

 and deep slits, which are completely filled to their 

 very summits with the cement. Thus, comparing Fig. 

 77 with Fig. 76, it will be apparent that each of the 

 elongated discs seen in the former, which consists of a 

 layer of enamel surrounding a strip of dentine, corre- 

 sponds to the transverse ridges of the latter ; while the 

 spaces between the discs in Fig. 77, which are composed 

 of cement, represent the open valleys of Fig. 76. 



The surface of such an Elephant's tooth forms, 

 indeed, a millstone most perfectly adapted for grinding 

 vegetable substances, consisting as it does of parallel 

 ridges composed of elements of different degrees of 

 hardness. Such a tooth, with its height of nearly 

 eight inches in some species, takes many years to wear 

 away ; and with a succession of six of these teeth 

 gradually increasing in size and complexity from the 

 first to the last, we are well able to understand how 

 the Indian Elephant can live fully to the age of a 

 century. It is also equally evident that the more 

 simple and lower- crowned teeth of the Tertiary Mas- 

 todons must have been worn away at a far more rapid 

 rate ; so that we are justified in saying that these 

 animals could not have attained anything like the 

 length of life enjoyed by their modern descendants. 



