Notes on sows Small Rodents found in North America. 115 



narrower. The molars in both jaws are situated much further 

 back than is usual ; the centre of the skull being about oppo- 

 site to the meeting of the second and third. The lower jaw is 

 very singularly shaped, the inner edges of the molars on oppo- 

 site sides being parallel ; the descending ramus is bent, so as 

 to be exactly horizontal behind ; the postero-inferior edge, being 

 a straight line, nearly perpendicular to the vertical plane of the 

 skull's axis. The conformation of the incisor teeth is admirably 

 adapted to the purposes they have to fulfil ; no carpenter's 

 gouging chisels are more effective tools than are these exqui- 

 sitely constructed teeth. It is essential that they should always 

 have a sharp, cutting edge, in order to nip through the tough 

 vegetable fibre on which the animal subsists; at the same time 

 strength and durability are indispensable. The Aplodontia 

 has no whetstone, or quadrupedal razor-grinder, to sharpen his 

 tools when they grow blunt; but an all- wise foreseeing Provi- 

 dence has so fashioned these wondrous chisels in all Rodents, 

 that the more they are used the sharper they keep ; the con- 

 trivance is as simple as it is beautiful. The substance of the 

 tooth itself is composed of tough ivory, but plated on the outer 

 surface with enamel as hard as steel. The ivory being the softer 

 material of course wears away faster than the enamel, hence the 

 latter, plating the front of the tooth, is always left with a sharp 

 cutting edge. Where could we find a more striking evidence 

 of design and forethought! 



The position this genus should occupy in a systematic ar- 

 rangement of the Rodents has always been a stumbling-block 

 and a matter of doubt, in great measure attributable to the fact 

 that but a single species of the genus is known, and very few 

 specimens have hitherto been obtained. A fine male specimen 

 has recently been set up in the British Museum collection, that 

 I caught near my camp on the prairie. 



In many particulars the Aplodontia very nearly resembles 

 the Sperm ophiles, particularly the Prairie Dog (Oynomys Ludo- 

 vic'dana), but differs, as in the true squirrels, in the rootless 

 molars and the absence of post orbital processes. In this 

 respect it is allied to the beaver. It is quite impossible to 

 assign it a well defined and settled position, until a greater 

 number of specimens are procured, from which more minute 

 and careful examination of the bony and internal anatomy can 

 be made. At present, however, it would appear to connect 

 the beavers with the squirrels, through the Spermophiles. All 

 hitherto known about its habits . is the quotation from the 

 journal before mentioned. 



The name Lewis and Clark gave this animal, Sewellel, is 

 evidently a corruption of an Indian word. The Chinook Indians 

 are a powerful tribe living near the mouth of the Columbia, and 



VOL. VII. — NO. II. I 



