MEGATHERIUM. 119 



enamel, (b) rubbing upon the ivory, (c ;) and the enamel, 

 (b',) upon the crusta petrosa, (a,) of the two teeth opposite 

 to it. Hence the act of mastication formed and perpetually 

 maintained a series of wedges, locking into each other like 

 the alternate ridges on the rollers of a crushing-mill ; and 

 the mouth of the Megatherium became an engine of prodi- 

 gious power, in which thirty-two such wedges formed the 

 grinding surfaces of sixteen molar teeth ; each from seven 

 to nine inches long, and having the greater part of this length 

 fixed firmly in a socket of great depth. 



As the surfaces of these teeth must have worn away with 

 much rapidity, a provision, unusual in molar teeth, and simi- 

 lar to that in the incisor teeth of the Beaver and other Ro- 

 dentia,* supplied the loss that M^as continually going on at 

 the crown, by the constant addition of new matter at the 

 root, which for this purpose remained hollow, and filled with 

 pulp during the whole life of the animal.f 



It is scarcely possible to find any apparatus in the me- 

 chanism of dentition, which constitutes a more powerful en- 

 gine for masticating roots, than was formed by these teeth 

 of the Megatherium ; accompanied also by a property, 

 which is the perfection of all machinery, namely, that of 

 maintaining itself perpetually in perfect order, by the act of 

 performing its woi'k. 



* The incisors of the Beaver, and other Rodentia, and tusks of the Hog 

 and Hippopotamus, whicli require only an external cutting edge, and not 

 a grinding surface, are constructed on the same principle as the cutting edge 

 of a chissel or an adze; viz. a plate of bard enamel is applied to the outer 

 surface only, of the ivory of these teeth, in the same manner as the outer 

 cutting edge of the chissel and adze is faced with a plate of steel, welded 

 against an inner plate of softer iron. A tooth thus constructed maintains its 

 cutting edge of enamel continually sharp, by the act of working against the 

 similarly constructed extremity of the tooth opposed to it. 



f FI. 5, Fig. 11, represents the section of the cavity containing this pulp. 



