RODENTIA, CHEIROPTERA, AND INSECTIVORA. 405 



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. 



The gnawing action of the incisors is assisted by the 

 articulation of the lower jaw, the condyle of which is placed 



Pig. 6S2. — A, Side view of the skull of Sclurus (Cynomys) Ludovicianus ; b, Molar teeth 

 of the upper jaw of the Beaver {Castor fiber). (After Giebel.) 



longitudinally and not transversely, so that the jaw slides 

 backwards and forwards. The molars, consequently, have 

 flat crowns (fig 682, b), the enamelled surfaces of which 

 are always arranged in transverse ridges, in opposition to 

 the antero-posterior movements of the jaw. 



The Eodents make their first appearance in the Eocene 

 Tertiary, and abound at the present day. Very many fossil 

 forms are known, and many of these, even of the oldest, 

 belong to genera still in existence, though few are of special 

 interest. 



The order Rodcntia comprises a very large number of 

 families, of which only those containing important fossil 

 representatives can be noticed here. 



Fain. 1. Leporidm. — In this family are the Hares 

 {L&pus timidus) and Kabbits {Lcpus cuniculus), distinguished 

 amongst the Eodents by the possession of two small in- 

 cisors in the upper jaw, placed behind the central chisel- 

 shaped incisors, so that there are four upper incisors in 

 all. The molars and pra^molars are rootless, and the dental 

 formula is — 



