CHISELS THAT DO NOT REQUIRE GRINDING. 353 
parallel; the descending ramus ‘is bent, so as to 
be exactly horizontal, behind, the postero-inferior 
edge being a straight line, nearly perpendicular 
to the vertical plane of the skull’s axis. The 
conformation of the incisor-teeth is admirably 
adapted to the purposes they have to fulfil; no 
carpenter’s gouging chisels are more effective 
tools than are these exquisitely-constructed teeth. 
It is essential that they should always have a 
sharp-cutting edge, in order to nip through the 
tough vegetable fibre on which the animal sub- 
sists; at the same time, strength and durability 
are indispensable. The Aplodontia has no whet- 
stone or razor-grinder, to sharpen his tools when 
they grow blunt; but an Allwise Providence has 
so fashioned these wondrous chisels in all rodents, 
that the more they are used the sharper they keep; 
the contrivance is simple as it is beautiful. The 
substance of the tooth itself is composed of tough 
ivory, but plated on the outer surface with ene. 
mel as hard as steel. The ivory, being the 
softer material, of course wears away faster than 
the enamel; hence the latter, plating the front of 
the tooth, is always left with a sharp-cutting 
edge. 
The position this genus should occupy, in a 
systematic arrangement of the rodents, has always 
VOL wl. AA 
