MAMMALIA. 667 



In by far the greater number of quadrupeds the teeth pre- 

 sent a more complex structure, and consist of two distinct sub- 

 stances of very different texture : the one analogous to the ivory of 

 the simple teeth described in the last paragraph ; the other called 

 enamel, of crystalline 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. 



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. 



The annexed figure (312) represents a section of the incisor 

 tooth, and of the left ramus of the lower jaw of a Porcupine 



Fig. 312. 



(Hystrix cristatd), and from this example the structure of such 

 teeth will be readily understood. The bulk of the tooth consists 

 of solid ivory (a), which in its texture and mode of growth 

 resembles that of a simple tusk, being continually growing from 

 behind by the addition of new matter produced from the vas- 

 cular pulp (c), so that, were such a tooth not worn away constantly 

 at the point, it would curl up over the face like the tusk of the 

 Babiroussa ; and if by accident the opposing tooth in the upper 

 jaw should be broken off, this circumstance in fact really takes 

 place. 



But, besides the ivory-forming pulp (c), there is a vascular 

 membrane (e) which exists only upon the anterior surface of the 



