542 



MANUAL OF ZOOLOGY. 



G/ires, comprising the Mice, Rats, Squirrels, Rabbits, Hares, 

 Beavers, c. 



The Rodentia are characterised by the possession of two 

 long curved incisor teeth in each jaw, separated by a wide in- 

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

 two of these incisors, and the upper jaw very rarely ; but some- 

 times there are four upper incisors. There are no canine 

 teeth, and the molars and praemolars are few in number (rarely 

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

 furnished with five toes each, all of which are armed with claws ; 

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

 other digits. The testes pass periodically from the abdomen 

 into a temporary scrotum, and the placenta is discoidal and 

 deciduate. 



The most characteristic point about the Rodents is to be 

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

 continuous gnawing hence the name of Rodentia. The in- 

 cisor teeth are commonly two in each jaw, and they grow from 

 persistent pulps, so that they continue to grow throughout the 

 life of the animal. They are large, long, and curved (fig. 208, 

 B), 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 more rapidly than the hard 

 enamel in front. The result of this is that the crown of the 

 tooth acquires by use a chisel-like shape, bevelled away be- 

 hind, and the enamel forms a persistent cutting edge (fig. 208). 



Fig. 208. A, Skull of the Beaver (after Owen). B, Diagram of the incisor teeth of 

 a Rodent, showing the chisel-shaped point : a Enamel ; d Dentine. 



The gnawing action of the incisors is assisted by the articu- 

 lation of the lower jaw, the condyle of which is placed longi- 

 tudinally and not transversely, so that the jaw slides backwards 



