1831 - 1841 .] TEETH OF MEGATHERIUM, 
131 
substantial, and so perforated with holes for the passage 
of nerves and vessels, that we may be sure it supported 
some organ of considerable size ; a long trunk was needless 
to an animal possessing so long a neck ; the organ was 
probably a snout, something like that of the tapir, suffi¬ 
ciently elongated to gather up roots from the ground ; such 
an apparatus would have afforded compensation for the 
absence of incisor teeth and tusks. Having no incisors, the 
megatherium could not have lived on grass ; the structure 
of the molar teeth shows that it was not carnivorous. 
u The composition of a single molar tooth resembles that 
of one of the many denticules that are united in the com¬ 
pound molar of the elephant; and affords an admirable 
exemplification of the method employed by nature, where¬ 
by three substances of unequal density, viz. ivory, enamel, 
and crusta petrosa, or csementum, are united in the con¬ 
struction of the teeth of graminivorous animals. The teeth 
arc about seven inches long, and nearly of a prismatic form. 
The grinding surfaces exhibit a peculiar and beautiful con¬ 
trivance for maintaining two cutting wedge-shaped salient 
edges in good working condition during the whole exist¬ 
ence of the tooth ; this is the principle of the mechanism 
which is adopted in the graminivorous animals. The 
various edges are always kept up ; inside and outside them 
are depressions, the consequence of which is that the state 
of the large tooth is in the state of a millstone kept sharp 
by doing its work ; therefore I say it is the perfection of 
machinery to keep itself in the highest order by doing the 
hardest work. The same principle is applied by tool- 
makers for the purpose of maintaining a sharp edge in 
axes, scythes, bill-hooks, etc. An axe or bill-hook is not 
made entirely of steel, but of one thin plate of steel inserted 
between two plates of softer iron, and so enclosed that the 
steel projects beyond the iron along the entire line of the 
cutting edge of the instrument: a double advantage 
results from this contrivance ; first, the instrument is less 
liable to fracture than if it were entirely made of the 
more brittle material of steel ; and secondly, the cutting 
edge is more easily kept sharp by grinding down a portion 
