MEGATHERIUM. 
147 
same principle is applied by tool-makers for the 
purpose of maintaining a sharp edge in axes, 
uythes, bill-hooks, &c. An axe, or bill-hook, is 
ut made entirely of steel, but of one thin plate 
steel, inserted betwmen two plates of softer 
iron and so enclosed that the steel projects be- 
yon the iron, along the entire line of the cut- 
ing edge of the instrument. A double advan- 
tage results from this contrivance; first, the in- 
strument IS less liable to fracture than if it were 
entirely made of the more brittle material of 
stee ; and secondly, the cutting edge is more 
easi y ept 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- 
ing e ges are produced on the crown of the 
molar teeth of the Megatherium. (See PI. h, 
• X. Y. Z. and PI. . 5 , Figs. 6 — 10.) * 
surface fzl ataH V across the grinding 
face z) and forms he cutting edges of two parallel wedges, 
'''Ory (c) whi h' iT is a central mass of 
enamel ^’a wE tl.e 
*ay, would ‘'^"sUucted of materials of unequal deu- 
Uian the h * ®°her parts, (a c), worn down more readily 
an the harder plates of enamel, (b b). 
ducina "h ^ u'^ety of mechanical contrivance, for pro- 
un maintaining two transverse wedges upon the surface 
