QUADRUPEDS. 



339 



altogether destitute of canines, and their incisors, which are 

 separated by a great blank space from the molars, are 

 furnished with enamel only 

 on one side. They project 

 from the front of each jaw in 

 a curve \ and, as they have 

 no roots, but spring from a 

 pulpy germ deeply embedded 

 in their sockets, they are Skull of Bodent. 



continually growing. These peculiarities have a direct 

 relation to the habits of the animals; for they live 

 upon food, usually hard and solid, which they gnaw 

 away atom by atom, with the tips of these projecting 

 teeth. For this work it is needful that the tips of the 

 teeth, which meet and play upon each other, should have 

 a sharp chisel-like edge, and this result follows from the 

 provision above mentioned, that the hard enamel is con- 

 fined to the front side of the tooth ; the bony portion of 

 the tip, being soft, wears away more rapidly than the 

 enamel of the front, which thus always presents a sharp 

 cutting edge. The constant growth, too, just balances the 

 ordinary wear of the teeth in eating, so that they are main- 

 tained in constant opposibility to each other. The perfec- 

 tion of this balance becomes manifest, when, by accident, 

 one of the incisor teeth is lost ; for, in this case, the oppo- 

 site tooth, having no wear, grows out to a monstrous length, 

 maintaining its original curve throughout, and becomes a 

 tusk, which in time presents a bar to the reception of food, 

 and death by starvation ensues. 



Strange as it may be thought, there is an affinity by no 

 means obscure, between these minute animals and the 



