QUALITIES OF IVORY 221 



of these, two in the upper and two in the lower jaw, are 

 developed and in use at the same time, but at more or less reg- 

 ular intervals these are renewed, the new teeth not growing 

 beneath the old ones, but being developed alongside of them 

 in the jaw, and pushing them out sideways. There are thus 

 six double pairs of molars grown in the course of a normal 

 period of life, the first set appearing at the age of two weeks 

 and lasting until the second year, when the teeth are re- 

 placed by the second set which is shed when the animal is 

 six years old; a third set follows, lasting until the ninth year, 

 and a fourth, which remains longer, until the elephant is 

 from twenty to twenty-five years old; then comes the fifth 

 set, only shed in the sixtieth year; and lastly the sixth set, 

 which may serve for forty to sixty years longer, as the ele- 

 phant sometimes reaches the age of one hundred or even 

 one hundred and twenty years or more. In structure the 

 teeth differ from those of any other mammal, being com- 

 posed of vertical plates of dentine surrounded by enamel 

 and connected by a cement. When worn down by use the 

 surface of these large molars shows alternating layers of 

 cement, dentine, and enamel.* 



That portion of the animal framework that is exposed 

 and projected, and is not protected by covering of any sort, 

 is harder and finer textured than the material properly 

 known as bone. Exposed as the former material is to 

 blows and friction, its hardness and toughness take the 

 place of the protection that covering affords to bone. This 

 applies in a general way to tusks and teeth of all kinds. 



Bone is the framework of the animal. It braces, holds, 

 carries, and binds together the flesh, blood, and organs by 

 which it is covered and surrounded; and the necessity for 



♦Richard S. Lull, "The Evolution of the Elephant," Annual Report (1908) of the Smith- 

 sonian Institute, Washington, 1909, pp. 645, 646. Reprinted from Amer, Jour. Sc, Vol. 

 XXV. March, 1908. 



