MEGATHERIUM. 147 
same principle is applied by tool-makers for the 
purpose of maintaining a sharp edge in axes, 
scythes, bill-hooks, &c. 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 ad- 
vantage 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 of 
exterior soft iron, than if the entire mass were of 
hard steel. By a similar contrivance, two cut- 
ting edges are produced on the crown of the 
molar teeth of the Megatherium. (See PL 6, 
W. X. Y. Z. and PL 5, Figs. 6-10.)^ 
* The outside of the tooth, like that of an axe, is made of a 
comparatively soft material, viz. thecrusta petrosa, (a a), inclos- 
ing a plate of enamel, (b b), which is the hardest substance, or 
steel of the tooth. This enamel passes twice across the grinding 
surface, (z), and forms the cutting edges of two parallel wedges, 
Y. b. b. : a longitudinal section of these wedges is seen, PL 6, 
V. w. X. Y. Within the enamel, (b b), is a central mass of 
ivory, (c), which, like the external crust, (a) is softer than the 
enamel. A tooth, thus constructed of materials of unequal den- 
sity, would have its softer parts, (a c), worn down more readily 
than the harder plates of enamel, (b b). 
We find a further nicety of mechanical contrivance, for pro- 
ducing and maintaining two transverse wedges upon the surface 
