MEGATHERIUM. Ii9 
of the Beaver and other Rodentia,* supplied the 
loss that was 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 mechanism of dentition, which constitutes a 
more powerful engine 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 w ork. 
Lower Jaw. 
The lower jaw (PI. 5, 1. d.) is very large and 
weighty in proportion to the rest of the head ; the 
object of this size being to afibrd deep sockets 
* The incisors of the Beaver, and other Rodentia, and tusks of 
the Hog and Hippopotamus, which require only an external cut- 
ting edge, and not a grinding surface, are constructed on the 
same principle as the cutting edge of a chisel or an adze ; viz. a 
plate of hard enamel is applied to the outer surface only, of the 
ivory of these teeth, and in the same manner as the outer cutting 
edge of the chisel 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. 
t PI. 5, Fig. 11, represents the section of the cavity contain- 
ing this pulp. 
