118 FOSSIL MAMMALIA. 



&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 in- 

 strument. A double advantage resuUs from this contri- 

 vance ; 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 en- 

 tire mass were of hard steel. By a similar contrivance, two 

 cutting edges are produced on the crown of the molar teeth 

 of the Megatherium. (See PI. 6, W. X. Y. Z. and PI. 5, 

 Figs. 6—10.*) 



PI. 6, W. X. represents the manner in which each lower 

 tooth was opposed to the tooth above it, so that the hard 

 enamel of the one should come in contact only with the 

 softer materials of the other ; viz. the edges of the plates of 



* The outside of the tooth, like that of an axe, is made of a comparative!}' 

 soft material, viz. the crusta petrosa, (a a,) enclosing 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 parellei wedges, Y. b. b. : a longitudinal section of these vs^edges is seen, 

 PI. 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 density, would have its softer parts, (ac,) 

 worn down more readily than the harder plates of enamel, (b b.) 



We find a farther nicety of mechanical contrivance, for producing and 

 maintaining two transverse wedges upon the- surface of each tooth, in the 

 relative adjustment of the thickness, of the lateral and transverse portions of 

 the plate of enamel, which is interposed between the external crust, (a,) and 

 the central ivory, (c.) Had this enamel been of uniform thickness all round 

 the central ivory, the tooth would have worn down equally to a hori- 

 zontal surface. In the crown of the tooth, PI. 6, Z. the plate of enamel is 

 seen to be thin on the two sides of tiic tooth, whilst the transverse portions 

 of the same plate, (b. b.) are comparatively thick and strong. Hence the 

 weaker lateral portions of thin enamel wear away more rapidly, than the 

 tliicker and stronger transverse portions, (b b,) and do not prevent the exca- 

 vation of the furrow across the surface of tiic ivory, c. 



