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 er^ 

 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 



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