RODENTIA. 



221 



upper jaw, and these he uses as a means of defence and 

 to assist in climbing on the ice. The walrus attains a 

 length of ten or twelve feet ; the seals are smaller, ex- 

 cepting the sea-elephant, which is said to grow as long 

 as thirty feet. 



CHAPTER XXIV. 

 Rodentia Edentata. 



RODENTIA. 



THE rodents are mostly terrestrial mammals, having 

 four clawed feet and incomplete dentition. They have 

 no canines ; the incisors have chisel-like edges, and are 

 curved in a circular arc, the convexity outward ; besides 

 they grow indefinitely during life, so that they are 

 pushed out at the base 

 as they are worn away FIG- 150. 



at the edge, and the 

 posterior surface wears 

 more rapidly than the 

 anterior, whose enamel 

 is harder. This struc- 

 ture indicates the diet 

 of the animals, organ- 

 ized for gnawing every- 

 where and continually, 

 and in whom the 

 gnawing instinct pre- 

 dominates nearly all others, 

 the incisors and the molars. 



19* 



SKULL OF COMMON PORCUPINE (Hystrix cris- 

 tata). The lower jaw partly in section to 

 show the lower incisor tooth. 



There is a space between 



