336 



PALEONTOLOGY. 



ScelidotJierium and Platyonyx are from the sub-apennine 

 stage of the Pampas ; and Ccelodon and Bplienodon are found 

 in the caverns of Brazil. 



FIG. 246. Chincella lanigera 



5th Order. RODENTIA (or Gnawers). Form a natural 

 order of unguiculated animals, whose dentition consists of 

 incisor and molar teeth. The ca- 

 nines are absent, and there is a 

 considerable vacant space between 

 the incisors and the molars. (Fig, 

 247.) The incisors, generally , 

 are long, arched, and deeply im- 

 planted in the jaw ; the front of 

 the tooth is thickly coated with 

 enamel ; the cutting-edge is chisel- 

 shaped; and the tooth is con- 

 stantly growing and advancing in the jaw, to supply the 

 wear and tear occasioned by the gnawing habits of the 

 rodents. The molars have large crowns; the dentine is 

 variously intersected with seams of enamel. The lower jaw 

 is articulated to the skull by a longitudinal condyle, which 

 permits a backward and forward movement in this organ, 

 by which they are enabled to gnaw and chisel away very hard 

 bodies. The rodents are in general small animals, and we 

 observe a considerable disproportion between the front and 

 the hind extremities (fig. 246) . We know many fossil genera, 

 as the Archceomys, from the falunian stage of Auvergne ; 



FIG. 247. 

 Skull of a Squirrel. 



