RODENTIA. 



457 



persistent pulps, so that they continue to grow throughout the 

 life of the animal. They are large, long and curved (fig. 371, 



Fig. 371. A, Skull of the Beaver (after Owen). B, Diagram of the incisor teeth 01 

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



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 behind, 

 and the enamel forms a persistent cutting edge (fig. 371). 



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 

 and forwards. The molars, consequently, have flat crowns, the 

 enamelled surfaces of which are always arranged in transverse 

 ridges, in opposition to the antero-posterior movement of the 

 jaw. 



The earliest known traces of the Rodents in past time have 

 been discovered in the Eocene Tertiary. The order comprises 

 a large number of families, only the more important of which 

 can be noticed here. 



Fam. i. Leporidce. In this family are the Hares and Rabbits 

 (Lepus), and the Pikas (Lagomys), distinguished amongst the 

 Rodents by the possession of two small incisors in the upper 

 jaw, placed behind the central chisel-shaped incisors, so that 

 there are four upper incisors in all. The molars and prsemolars 

 are rootless, and the dental formula is 



. 2 2 o o T, ^ T, T, 



'!=?; *?%* *"$' *"-* 



The clavicles are imperfect. The fore-legs are furnished 



