EODEXTIA OR GNAWERS. 467 



Eodents are characterised by having two long curved incisor 

 teeth in each jaw (fig. 222, i); the crowns of these are continually 

 being worn down, whilst growth as rapidly takes place at their 

 roots. These teeth are employed in gnawing, and are more 

 readily worn down behind than in front, so that they always 

 present a sharp edge. This is due to the front having a plate 

 of very hard enamel, whilst the back is composed of soft dentine. 

 The lower jaw has never more than two incisors, the upper may 

 have four. Canines are never present ; the molars and pre- 

 molars are seldom more than four in number on each side of 

 the jaw. The molars have fiat crowns, the enamelled surfaces 



Fig. 222. — Hkad of Rodent. The Hare {Lepns eiirnpceus). 

 i, incisor.s ; jim, premolars ; m, mol.irs. 



being in transverse ridges running across the teeth. The hind- 

 feet are longer than the fore-feet, thus giving them the curious 

 springing gait ; the feet have usually five toes furnished with 

 claws, but they may be reduced to four on each foot. The 

 eyes are large and directed laterally. The brain has no convo- 

 lutions, being nearly smooth. Many rodents have curious 

 lateral " cheek- pouches " in which food can be stored for some 

 little time ; when this food is required, the animal presses the 

 pouches with its fore-feet. Most rodents are small animals, and 

 feed upon vegetation. They are endowed with great repro- 

 ductive faculties. The fcetus is enclosed in a dcciduate dis- 



