296 ANATOMY OF VERTEBRATES. 



but less thin, in some Rodents, upon the posterior and lateral 

 parts of the incisor. The substances of the incisor diminish in 

 hardness from the front to the back part of the tooth, not only in 

 so far as the enamel is harder than the dentine, but because the 

 enamel consists of two layers, of which the anterior and external 

 is denser than the posterior layer, and the posterior half of the 

 dentine is rendered by a modified number and arrangement of the 

 dentinal tubes less dense than the anterior half. 



The abrasion resulting from the reciprocal action of the upper 

 and lower incisors produces, accordingly, an oblique surface, sloping 

 from a sharp anterior margin formed by the dense enamel, like 

 that which slopes from the sharp edge formed by the plate of 

 hard steel laid on the back of a chisel ; whence the name ' scalpri- 

 form,' ( dentes scalprarii,' given to the incisors of the Rodentia. 

 In Leporidce the enamel is traceable to the back of the incisors : 

 with this exception, the varieties to which these incisors are sub- 

 ject in the different Rodents are limited to their proportional size, 

 and to the colour and sculpturing of the anterior surface. Thus in 

 the Guinea-pig, Jerboa, and Squirrel, the breadth of the incisors is 

 not half so great as that of the molars: whilst in the Coypu they 

 are as broad as the molars, and in the Cape Mole-rats (Bathyergus 

 and Orycteromys} are even broader. In the Coypu, Beaver, Agouti, 

 and some other Rodents, the enamelled surface of the incisors is 

 of a bright orange or reddish-brown colour. In some genera of 

 Rodents, as Orycteromys, Otomys, Merioncs, Ht/drochoerus, Lepus, 

 and Layomys, the anterior surface is indented by a deep longitu- 

 dinal groove. This character seems not to influence the food or 

 habits of the species : it is present in one genus and absent in 

 another of the same natural family. In most Rodents the 

 anterior enamelled surface of the scalpriform teeth is smooth and 

 uniform. 



The molar teeth are always few in number, obliquely implanted 

 and obliquely abraded, the series on each side converging ante- 

 riorly in both jaws ; but they present a striking contrast to the 

 incisors in the range of their varieties, which are so numerous that 

 they typify almost all the modifications of form and structure 

 which are met with in the molar teeth of the omnivorous and 

 herbivorous genera of other orders of mammalia. In some 

 Rodents e.g. Cavies, the molar teeth, fig. 236, /?, m, are rootless ; 

 others e.g. the Agouti, have short roots, tardily developed like 

 the molars of the Horse and Elephant ; others, again e.g. the 

 Rat and the Porcupine, soon acquire roots of the ordinary pro- 

 portional length. 



