DENTITION. 119 



appears to be peculiar to animals of great longevity, and 

 which subsist on vegetable substances containing a large pro- 

 portion of tough fibres, or other materials of great hardness; 

 and requiring for their mastication teeth so large as not to 

 admit of both the old and new tooth being contained, at the 

 same time, in the alveolar portion of the jaw. 



An expedient of a different kind has been resorted to in 

 the Rodentia, for the purpose of preserving the long chisel- 

 shaped incisors in a state fit for use. By the constant and 

 severe attrition to which they are exposed, they wear away 

 very rapidly, and would soon be entirely lost, and the ani- 

 mal would perish in consequence, were it not that nature 

 has provided for their continued growth, by elongation from 

 their roots, during the whole of life. This growth proceeds 

 in the same manner, and is conducted on the same princi- 

 ples, as the original formation of the simple teeth already 

 described: but, in order to effect this object, the roots of 

 these teeth are of great size and length, and are deeply em- 

 bedded in the jaw, in a large bony canal provided for that 

 purpose; and their cavity is always filled with the vascular 

 pulp, from which the continued secretion and deposition of 

 fresh layers, both of ivory and enamel, take place. The 

 tusks of the Elephant and of the Hippopotamus exhibit the 

 same phenomenon of constant and uninterrupted growth. 



In the Shark, and some other fishes, the same object is 

 attained in a different manner. Several rows of teeth are 

 lodged in each jaw, but one only of these rows projects and 

 is in use at the same time; the rest lying flat, but ready to 

 rise in order to replace those that have been broken or worn 

 down. In some fishes, the teeth advance in proportion as 

 the jaw lengthens, and as the fore teeth are worn away: in 

 other cases, they rise from the substance of the jaw, which 

 presents on its surface an assemblage of teeth in different 

 stages of growth: so that, in this class of animals, the great- 

 est variety occurs in the mode of the succession of the teeth. 



The teeth of the Crocodile, which are sharp-pointed hoi- 



