6 SKETCH OF THE HISTORY OF MAMMALIA. 



that the Rodents resolve themselves into three great pri- 

 mary sections : first, the Murine section ; secondly, the 

 Hystricine section ; and thirdly, the Leporine section. 



Each of these sections embraces several families, each 

 of the latter comprehending- several genera. The prin- 

 cipal genera contained in the 71/^^?^we section are — Schuns, 

 Arctomys, SperinopJiilus, lamias, Myoxvs, Dipus, 3Ius, 

 Arvicola, Geomys, and Castor. The principal genera 

 contained in the Hi/stricine section are — Bathurgus, Oryc- 

 terus, Poephagomys, Oclodon, Abrocoma, Myopotamus, 

 Capromys, Ediimys, Aulacudus, Histrix, JDasyprocta, 

 Chinchilla, Cavia, and Hydroclicerus. The Leporine 

 section contains the genera Lepiis and Lagomys. 



Respecting a few genera, as Ctenodactylus, Helamys, 

 Otomys (Smith, not F. Cuvier), Ahodon, and Heteroniys, 

 Mr. Waterhouse has not been able to satisfy himself as 

 to their precise systematic classification ; and with re- 

 spect to the genus Aplodontia (Fig. 1, skull and teeth), 

 though he places it in the squirrel family {Sciuridce), 

 yet it differs, as he admits, in the absence of a post- 

 orbital process to the skull, and in the molar teeth being 

 i-ootless. We may here remark that the genus AjjIo- 

 dontia contains a Rodent, called by Lewis and Clark the 

 Sewellel {A. lepoiind), and which inhabits the neigh- 

 bourhood of the Columbia river (N. America), where it 

 lives in burrows, and associates in small companies. The 

 head is large, the nose is thick and obtuse, covered with 

 a dense coat of short fur ; eye very small ; ear resembling 

 the human in form. Body short, thick, and rabbit-like. 

 Legs very short, and covered down to the wrists and 

 heels with fur similar to that on the body : a little above 

 the wrist-joint, on the inner side, is a small tuft of stiff 

 white hairs. Fur like that of a rabbit out of season, 

 amber and chestnut-brown above, grayish or clove- 

 brown beneath ; lips whitish ; a rather large spot of pure 

 white on the throat ; some white hairs dispersed through 

 the fur. Tail slender, cylindrical, hardly half an inch 

 long. The figure (1) represents the skull, teeth, and 

 paws : 1, anterior half of skull with lower jaw, profile ; 

 2, anterior half of skull seen from below ; 3, the same 



