148 FOSSIL MAMMALIA. 
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 enamel, 
(b) rubbing upon the ivory, (c) ; and the enamel, 
(b), upon the crusta petrosa, (a), of the two teeth 
opposite to it. Hence the act of mastication formed 
and perpetually maintained a series of wedges, 
locking into each other like the alternate ridges 
on the rollers of a crushing-mill ; and the mouth 
of the Megatherium became an engine of pro- 
digious power, in which thirty-two such wedges 
formed the grinding surfaces of sixteen molar 
teeth ; each from seven to nine inches long, 
and having the greater part of this length fixed 
firmly in a socket of great depth. 
As the surfaces of these teeth must have worn 
away with much rapidity, a provision, unusual in 
molar teeth, and similar to that in the incisor teeth 
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 the 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 thicker and stronger 
transverse portions, (b b), and do not prevent the excavation of 
the furrow across the surface of the ivory, c. 
