328 NATURE'S TEACHINGS. 



the stone must be re-faced, and sooner or later is worn out 

 altogether, and must be replaced with a new one. This, how- 

 ever, is not the case with the Elephant's molar teeth, which 

 not only keep their facing perfectly sharp, but have the faculty 

 of renewing themselves as fast as they are worn away. 



How these important objects are attained we shall now see. 



If the reader will refer to the upper left-hand figure of the 

 illustration, he will see that its surface is for the most part 

 round, with irregularly oval figures, close and thick at one 

 end, and almost disappearing at the other. These are the 

 " facings" of the Elephant's tooth, and they are formed as 

 follows : 



The tooth, which is of enormous size, is not solid, but is 

 composed of a number of plates laid side by side, like a pack of 

 cards when set on their edge. Each of these plates is com- 

 posed of a hard external layer of enamel, and an internal 

 layer of comparatively soft bony matter. A slice of badly 

 made toast affords a familiar parallel, the half-charred outside 

 representing the enamel, and the soft, sodden interior being 

 analogous to the bony matter. In order to show the arrange- 

 ment of these plates, a side view of part of the tooth is given 

 on the same illustration. Sometimes, when the teeth of fossil 

 elephants are discovered, these plates all fall asunder, the 

 material which connected them having been dissolved away in 

 the earth. 



When, however, we look upon the upper surface of a recent 

 tooth, we see it present the appearance which is shown in the 

 illustration. The elongated oval marks are the edges of the 

 hard enamel plates, while the spaces between them are filled 

 with the soft bony matter. It will be evident, then, that if two 

 teeth such as these be in opposite jaws, and perform the task 

 of grinding food, their surface will always be well " faced." 

 Owing to the different hardness and density of the enamel and 

 bony substance, the latter will wear away with comparative 

 rapidity, leaving the former to project slightly, and thus to 

 preserve the facing of the natural mill. 



This is, indeed, but a modification of the beautiful animal 

 mechanism which keeps the teeth of a rodent animal always 

 sharp, and always bevelled off at the proper angle. If we 

 could invent some plan whereby, in our millstones, we could 



