350 



VEKTEBKATE ANLMALS. 



tiniious gnawiDg. They grow from persistent pulps, niul coiisecpiently 

 continue growing as long as the animal lives. They are large, long, 

 and curved, and are covered in front with a layer of hard enamel, 

 so that the softer parts of the tooth are placed behind (fig. 254, A). 

 The resvdt of this is, that as the tooth is i\sed in gnawing, the softer 

 parts behind wear away more I'apidly than the haid enamel in front, 

 and thus the crown of the tooth acquires by use a chisel shape, 

 bevelled away behind, and the enamel forms a persistent cutting- 

 edge. The Rodents are almost all of small .size, and are very pro- 

 lific. They subsist piincipally, if not entirely, on vegetable matters, 

 especially the harder parts of plants, such as the bark and roots. 

 Many possess the power of building very elaborate nests, and most 

 of them hibernate [i.e., remain torpid throughout the winter). They 

 are very generally distributed over the whole world. 



The order Rodentia comprises a large immber of families, of which 



Fig. 255.— The Beaver (Castor /!)er). 



little more than the names of the principal ones can be mentioned. 

 The most important families of Rodents are the following : 1. Lepor- 

 idce, comprising the Hares and Rabbits. The Hares generally occur 

 in temperate regions, but some are African, and one species occurs 

 in the Arctic regions, while the common American Hare {Lepus 

 America iius) extends from Canada to Mexico. 2. Cai-uhr, compris- 

 ing the C'apybaras, Guinea-pigs, &c. The Cajiybara is the largest 

 of living Rodents, and is not unlike a small pig. It is a native of 

 South America, and leads an amphibious life. Here also belong the 

 Agoutis (Dasi/pi-octa) of ,Soath America and the West Indies, and 

 the Pacas of South America. 3. Ifi/atriciihc, comprising the Porcu- 

 pines, and characterised by the fact that the body is covered with 

 longer or shorter spines or quills mixed with bristly hairs. Most 

 of the Porcupines live in burrows, and are much like the Rabbits in 



