RODENTIA. 



383 



duces 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 upon the back of a chisel ; whence the 

 name dentes scalprarii given to the incisors 

 of the Rodentia. 



The varieties to which these incisors are 

 subject 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 coypa they are 

 as broad, and in the Cape mole rats 

 (Bathyergus and Orycteromys) broader than 

 the molars. 



In the coypa, 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, meriones, gerbilla, 

 hydrochaerus, lepus, and lagomys, the anterior 

 surface of the upper incisors is indented by a 

 deep longitudinal groove. This character 

 seems not to influence the food or habits of 

 the species ; it is often present in one genus 

 and absent in another of the same natural 

 family; in mostRodents 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 lateral series converging anteriorly in 

 both jaws ; but they present a striking con- 

 trast 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 her- 

 bivorous genera of other orders of Mammalia. 



In some Rodents the molar teeth are root- 

 less, like those of the wombat, the toxodon, 

 and elasmothere; some have short roots 

 tardily developed, like the molars of the horse 

 and elephant ; and some soon acquire roots 

 of the ordinary proportional length. 



The Rodents which have rootless molars 

 comprise the families of the hares, chin- 

 chillas, Chili rats, and cavies ; most of the 



voles, the houtias (Capromys)^ and the Cape 

 jerboa (Helamys). 



The genera which have molars, with short 

 or incomplete roots, developed late, are 

 Castor (beaver), Rystrix (porcupine), .Ccelogenys 

 (spotted cavy), Dasyprocta (agouti), Spalax 

 (blind rat), Myopotamns (coypa), Euryotis, 

 Accomys, 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 sub- 

 stance, as the oily kernels of nuts, suffer less 

 rapid abrasion oif 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 economy 

 of nature is manifested in the smallest par- 

 ticulars as well as in her grandest operations, 

 no more dental substance is developed after 

 the crown is formed than is requisite for the 

 firm implantation of the tooth in the jaw. 



Rodents that exclusively subsist on vege- 

 table substance, especially 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 propor- 

 tion as its calcified constituents, forming the 

 exposed working part of the tooth, are worn 

 away. So long as this reproductive force is 

 active the molar tooth is implanted, like the 

 incisor, by a long undivided continuation of 

 the crown ; when the force begins to be ex- 

 hausted the matrix is simplified by the sup- 

 pression of the enamel organ, and the dentinal 

 pulp continues to be reproduced only at 

 certain points of the base of the crown, which 

 by their elongation constitute the fangs. The 

 beaver and other Rodents in the second cate- 

 gory of the order, according to the implanta- 

 tion of the molar teeth, exemplify the above 

 condition ; but in the capybara, dolichotis, 



Fig. 267. 



Lower jaw of the Porcupine (Hystrix cristata). 



i, incisor tooth ; m, the molar teeth, implanted in the jaws by means of fangs ; i* t pulp at the base of 

 incisor tooth ; p, anterior molar. 



