ORDERS OF MAMMALIA. 311 



are placed behind (Fig. 155, B). The result of this is, that 

 as the tooth is used in gnawing, the softer parts behind wear 

 away more rapidly than the hard 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 prolitio. 

 They subsist principally, 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 hybernate («'. e., remain torpid through- 

 out the winter). They are very generally distributed over the 

 whole world. 



The order Eodentia comprises a large number of families, 

 of which little more than the names can be mentioned. The 

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

 poridcB, comprising the Hares and Rabbits. The Hares gen- 

 erally occur in temperate regions, but some are African, and 

 one species occurs in the Arctic regions, while the common 

 American Hare (Lepus Americanus) extends from Canada to 

 Mexico. 3. Cavidm, comprising the Capybaras, Guinea-pigs, 

 etc. The Capybara 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 

 {Dasyprocta) of South America and the West Indies, and the 

 Paoas of South America. 3. Hystricidoe, comprising the Por- 

 cupines, and characterized 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 their habits, but some are furnished with prehen- 

 sile tails, and live in trees. 4. Gastoridae or Beaver family, 

 comprising the Beaver, Musquash, and Cojrpu. The Beaver 

 has webbed feet and a scaly tail, and the fur is an article of 

 considerable value. It inhabits both North America and 

 Europe. The Musquash resembles the Beaver in many re- 

 spects, and is also a native of Northern America; but the 

 Coypu is South American. 5. Muridoe, comprising the Mice, 

 Rats, Hamster (Fig. 156), Lemmings, etc. The Rats and 

 Mice are too well known to require more than merely to be 

 mentioned. 6. DipodidcB, comprising the Jerboas of the Old 

 World, and the Jumping Mice of America. 7. Myoxidm, com- 

 prising the Dormice, which must not be confounded with the 

 true Mice on the one hand, or with the Shrew-mice on the 

 other hand. 8. Sciuridce, comprising the Squirrels, Flying 

 Squirrels, and Marmots. The Flying Squirrels do not really 



