784 MAMMALIA. 



along the margins of the jaws ; but from the intermaxillary bone of the 

 left side of the face there projects a single tusk of great strength, which 

 sometimes attains the length of eight or ten feet. This formidable 

 weapon is fully developed only upon one side of the body ; nevertheless 

 the corresponding tooth exists in a rudimentary condition, enclosed in 

 the opposite intermaxillary bone. 



(2261.) In the Elephant (a creature which so obviously forms a 

 connecting link between the gigantic Cetacea and terrestrial quadrupeds) 

 tusks, more ponderous even than that of the Narwal, project from 

 both intermaxillary bones : but these, as well as the tusks of other 

 PACHYDEKMATA, grow upon a simple pulp, such as that which forms the 

 teeth of the Bottle-nose Whale ; are composed of ivory, without any 

 enamel ; and their growth is only limited by the abrasion to which they 

 are subject. 



(2262.) In by far the greater number of quadrupeds the teeth present 

 a more complex structure, and consist of two distinct substances of 

 very different texture the one analogous to the ivory of the simple 

 teeth described in the last paragraph, the other, called enamel, of cry- 

 stalline texture and such extreme density as to withstand being worn 

 away by acting upon the hardest materials used as food. Teeth of this 

 description may be advantageously divided into two principal groups : 

 first, those whose growth is continuous during the entire lifetime of the 

 animal ; and second, those which are completed at an early period, and 

 then cease to grow. 



(2263.) The first division includes the incisor teeth of the Rodentia, 

 or denies scalprarii, as they have been termed. Such teeth are, in fact, 

 chisels of most admirable construction, destined to gnaw the hardest 

 kinds of food, and yet never to all appearance wearing away or becoming 

 blunted by use. 



(2264.) The annexed figure (398) represents a section of the incisor 



.. Fig. 398. 



Growth of incisor tooth in the Porcupine. 



tooth and of the left ramus of the lower jaw of a Porcupine (Hystrix 

 cristata)-, and from this example the structure of such teeth will be 

 readily understood. The bulk of the tooth consists of solid ivory (a), 



