TEETH OF DIPHYODONTS. 299 



out, as it were, the characteristic structure of the molars of the 

 Anoplothere and Ruminants, &c. 



The transverse section of the molar of the Water-vole, fig. 237, 

 shows that modification of the grinding surface in which the folds 



237 



Structure of the molar of the Water-vole (Arvicula amphibia), magnified. 



of enamel, e, extend like promontories, some outward, the others 

 inward, into the substance of the crown ; a like section of the 

 Beaver's molar exhibits islands with a promontory of enamel. 

 The transverse section of the crown of the molar of Lagostomus 

 displays not fewer than five islands of enamel, which hard sub- 

 stance is so thick that it enters more abundantly into the compo- 

 sition of the tooth than the dentine itself. The pulp, after the 

 formation of a certain thickness of tubular dentine, becomes 

 converted into osteo-dentine in both the rooted and rootless 

 molars of the Rodents. This fourth substance is exhibited at o, 

 fig. 237, which shows the four different dental tissues, viz. cement, c, 

 enamel, e, dentine, d, and osteo-dentine, o, entering in more equal 

 proportions into the formation of the crown than in other Mam- 

 malian teeth. When the crown is worn by mastication down to 

 the place of the section figured, the four substances appear in the 

 same proportions on the grinding surface, contributing to its effi- 

 ciency as a triturating organ by the inequalities consequent on 

 their various degrees of density and resistance to the abrading 

 forces. 



The molars are not numerous in any Rodent ; the Hare and 

 Rabbit (Lepus) have f:f, i.e. six molars on each side of the 

 upper jaw, fig. 238, and five on each side of the lower jaw, vol. ii. 

 fig. 233. The Pika (Lagomys) has f:f. The Squirrels have |:f. 

 The families of the Dormice, the Porcupines, the Spring-rats 

 (Echimyidce), the Octodonts, the Chinchillas, and the Cavies, figs. 

 235, 236, have |:£ molars. In the great family of Rats (Muridce 



