RODENTS. 401 



The Rodents which have rootless molars, comprise the families 

 of the Hares, (1) Chinchillas, (2) Chili-rats, (3) and Cavies(4), most of 

 the Voles, (5) the Houtias (Capromys), and the Cape Jerhoa (Helamys). 



The genera which have molars with short or incomplete roots, 

 developed late, are Castor (Beaver), Hystrix (Porcupine), (6) Ccelogenys 

 (Spotted Cavy), Dasyprocta (Agouti), Spalax (Blind-rat), Myopota- 

 mus (Coypu), Euryotis, Ascomys, and Aplodontia. 



The families of the Squirrels, Dormice, Rats, and Jerboas have 

 rooted molars. 



The differences in the mode of implantation of the molar teeth 

 relate to differences of diet. The Rodents which subsist on mixed 

 food and which betray a tendency to carnivorous habits, as the true 

 Rats, or which subsist on the softer and more nutritious vegetable 

 substances, as the oily kernels of nuts, suffer less rapid abrasion 

 of the molar teeth ; a minor depth of the crown is therefore needed 

 to perform the office of mastication during the brief period of 

 existence allotted to these active little Mammals ; and, as the eco- 

 nomy of nature is manifested in the smallest particulars as well 

 as in her grandest operations, no more dental substance is deve- 

 loped after the crown is formed, than is requisite for the firm 

 implantation of the tooth in the jaw. 



Rodents that exclusively subsist on vegetable substances, espe- 

 cially the coarser and less nutritious kinds, as herbage, foliage, the 

 bark and wood of trees, wear away more rapidly the grinding surface 

 of the molar teeth ; the crowns are therefore larger, and their growth 

 continues by a reproduction of the formative matrix at their base 

 in proportion as its calcified constituents, forming the exposed 

 working part of the tooth, are worn away. So long as this repro- 

 ductive force is active the molar tooth is implanted, like the incisor, 



to their modes of implantation, in his excellent Paper on the Teeth of that order, in the Munich 

 Transactions, ' Abhandlungen der K. Bayerischen Akademie der Wissenschaften,' Bd. iii, 

 1841, p. 529. 



(1) Lepus, Lagomys. 



(2) Lagostomus, Lagotis, Chinchilla. 



(3) Abrocoma, Octodon, Schizodon, Poephagomys, Ctenomys. 



(4) Hydrochcerus, Dolichotis, (PI. 104, figs. 2 & 3), Kerodon, Cavia. 



(5) Lemmus, Arvicola, except A, riparia. 



(6) PI. 104, fig. 1. 



D D 



