TEETH OF DIPHYODONTS. 297 



The differences in the mode of implantation of the molar teeth 

 relate to the differences of diet. The Rodents, which subsist on 

 mixed food, and which betray a tendency to carnivorous habits, as, 

 e.g., the true Rats, or which subsist on the softer and more nutri- 

 tious vegetable substances, such as the oily kernels of nuts, suffer 



2.36 



Cranium and upper teeth of the Patagonian Cavy (Doliehotis). 



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 economy of nature is manifested in the smallest parti- 

 culars as well as in her grandest operations, no more dental sub- 

 stance is developed after the crown is formed, than is requisite for 

 the firm fixation 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 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 reproductive force is active, the molar tooth is implanted, 

 like the incisor, by a long undivided continuation of the crown. 

 The rootless and perpetually growing molars are always more or 

 less curved, fig. 236, p, m ; they derive from this form the same 

 advantage as the incisors, in the relief of the delicate tissues of 

 the active vascular matrix from the effects of the pressure which 

 would otherwise have been transmitted more directly from the 

 grinding surface to the growing base. 



The complexity of the structure of the crown of the molar 

 teeth, and the quantity of enamel and cement interblended with 



