404 



CHAPTER XLVII. 



ORDERS OF MAMMALIA (Continued). 

 RODENTIA, ChEIKOPTEKA, AND IXSECTIVORA. 



Order XIII. Eodentia. — The tliiiteenth order of 3fam- 

 iiialia is tliat of the Rodentia, or Eodeiit Animals, often 

 spoken of as Glwes, comprising the Mice, Eats, Squirrels, 

 Eabbits, Hares, Beavers, &c. 



The Bodcntia are characterised by the possession of two 

 long curved incisor teeth in each ja^o, separated, hy a ivide in- 

 terval from the molars. The loiver jaw never has more than 

 two of these incisors, and the dipper jaw rarely ; hut some- 

 times there are four upper incisors. There are no canine teeth, 

 and the molars and prmmolars are few in number {rarely more 

 than four on each side of the jaw). The feet are usually fur- 

 nished tvith five toes each, all of which arc armed with claws ; 

 and the hallux, when present, does not differ in form from the 

 other digits. 



The most characteristic point al^out the Eodents is to be 

 found in the structure of the incisors, which are adapted for 

 continuous gnawing — hence the name of Rodcntia. The 

 incisor teeth are commonly two in each jaw, and they grow 

 from persistent pulps, so that they continue to grow through- 

 out the life of the animal. They are large, long, and curved 

 (fig. 682, a), and are covered anteriorly by a plate of hard 

 enamel. The back part of each incisor is composed only of 

 the comparatively soft dentine, so that when the tooth is 

 exposed to attrition, the soft dentine behind wears away 



