408 



INTRODUCTION TO ZOOLOGY. 



tine," which constitutes the principal component; the "ena- 

 mel," which is a much harder substance; and the "cement," 

 which is a softer one, and serves to unite the plates of which 

 the tooth is composed. The unequal density causes the sur- 

 face to wear away in an unequal manner, and hence the pro- 

 perty which makes a mill-stone most valuable is secured. The 

 arrangement to make good what a mechanic would call " the 

 wear and tear" of the apparatus is not less effectual. The 

 teeth are ever growing, not as in the Eodentia (ante, p. 388), 



Fig. 322.— Indian Elephant. 



by a deposit of new matter at the base, but by the develop- 

 ment of new teeth. We are accustomed to see a new tooth 

 come forth from the mouth of a child from the place where the 

 former tooth had been shed ; but in the young Elephant the 

 plan of development and succession is altogether ditFerent. 

 Each tooth is formed in a membranous bag, enclosed in a 

 chamber of bone, forming part of the massive jaw. They ai'e 

 successively developed, so that an Elephant may have in each 

 jaw not less than six of these enormous molar teeth in the 

 com'se of its life, or twenty-four in all, although never more 

 than two are seen in each jaw at the same time. As the 



1 



