666 CLASS XVII. 



The Rodents. The incisors are covered by a plate of enamel on 

 the anteiior surface only, which in many species is coloured yellow or 

 ruddy-brown (by a very thin layer of cement) \ Not only the enamel, 

 but also the anterior dentine is harder than the posterior of the 

 incisors; hence a greater wearing down of these teeth is effected 

 at the back part by use, and their crowns acquire a chisel-shape 

 with a surface declining from the sharp anterior margin backward. 

 The condyle of the lower jaw is longitudinal, and slides forward 

 and backward. 



The rodents live principally, or exclusively, on vegetable food, 

 often on hard parts of plants, as bark of trees, roots, &c. They are 

 commonly small, and exceedingly prolific. The species are very 

 numerous (compai-e p. 605) ; about one-fifth of these are found in 

 North America, where especially many species of the genus Arc- 

 tomys, Sciurus and Lepus occur. 



Comp.are on this order Pallas Novce specks Quadrupeduni e Gliriiini 

 ordine, Erlangte, 1778, 4to; of the anatomical peculiarities Rymer Jones 

 has given a compendious account in his article Rodentia, Todd's Cydo- 

 pmdia, IV. pp. 368 — 396. 



Family XXII. Duplicidentata. Upper incisors four, duplicate, 

 with two thin teeth placed behind the anteror and normal. Molars 

 destitute of roots, formed of two coalesced laminse, upper six or five 

 on each side, lower six. Fore feet pentadactylous, hind feet tetra- 

 dactylous with hairy soles. Claws elongate, compressed. Teats 

 several (4 — 10). Tail short or none. 



Lej)us L. Ears elongate. Tail recurved, short. Hind legs 



^ — ^ 

 much longer than fore legs. (Molar teeth ^ . Dental formula 



2—2 3—3 3—3 ^—0 



Owen, i. :r — z- , p. ^ — ^ , m. r^ — « = 28, the last upper molar 



small, simple. Clavicles imperfect.) 



The hares are distinguished from all the other rodents by the two small 

 upper incisors placed behind the usual ones. When the mouth is closed 

 the inferior molars are situated within the margin of the superior molars, 

 as in the ruminantia. Hence for the due production of chewing a large 

 lateral motion is required, which is therefore facilitated by the free articu- 

 lation of the hemispherical articular condyle of the lower jaw and the 

 shallow articular cavity. By this motion the surface of the molars is un- 

 equally worn dovm, and presents projecting lines of enamel with intervening 



^ Owen's Odvntofjraplnj, p. 399. 



