350 



YERTEBIIATE ANIMALS. 



timious gnawing. Tliey giviwfii.m peiwistent ]>ul|«, .-inil consequently 

 continue growing as long as the animal lives. Tliey are Uu'ge, long, 

 .anil curveil, and are covered in front with a layer of hard enamel, 

 so that the softer parts of the tooth are placed Ijcliind (fig. 2.')4, A). 

 The result of this is, that as the tooth is irscd in gnawing, the softer 

 parts behind wear ,i\vay more ra[jidly than the hard enamel in front, 

 and thus the crown i,i the tooth acquires liy use a chi.sel shape, 

 bevelled away behin<l, and the enamel forms a jjersi.stent cutting- 

 edge. The Kodents are almost all of small size, and ai-e very jjro- 

 litic. They subsist principally, if not entirely, on vegetable matters, 

 especially the harder ])arts of ])lauts, such as the bark and rcjots. 

 Many possess the ]iiiwer nf building very elaborate nests, and mo.st 

 of them hibernate (i.r., remain torjnd throughout the winter). They 

 are ver_y generally ilistributed over the whole world. 



The order Rodcntin conqirises a large number of families, of which 



-The Bcavrr (Castor fbrr). 



Httle njore than the names iif the princip:il ones can be mentioned. 

 The most im]ioitant families of Kodents are the following : 1. Lcpor- 

 idfe^ comin'i.sing the Hares and Ilabbits. The Hares generaJh' occur 

 in temperate regions, lint s(ime are African, and one species occurs 

 in the Arctic regions, while the common American Hare [Lepiis 

 Atiu'ririiiiitx) extenils from < 'anada, to Mexico. 2. Ciiriihi\ compris- 

 ing the (.'apyl)ar;is, ( Jnine.-i-piiis, &c. The t'apybara is the laigest 

 of living Itodents, .-inil is not unlike a small ])ig. It is a native of 

 South .America, and le.-ids •■in anipliibious life. Here also belong the 

 Agoutis {l),i.tiiiirnrt,i) of Sonlh .America and the West Indies, and 

 the I'aeas of Sonlh .\meriea. .'!. lliixlririihr^ comprising the Porcu- 

 pines, and eharaeti'riscd by the fact that the body is co\'ered with 

 lo)iger or slioiier spines or qnills nnxed \\\(\\ bristly liair.s. Most 

 of the I'oieupines li\e in buirows, and are nmch like the IJabbits in 



