SUB-FAMILY CASTORINAE. 



Molars or , rootless. No post-orbital process. Ante-orbital foramen "small, rounded, very far forward, or in the 

 base of the zygoma ; fore feet with five distinct toes and claws. 



The two genera, Castor and Aplodontia, have long proved stumbling blocks in the way of a 

 systematic arrangement of the rodents. Partly in consequence of the difficulty in getting 

 specimens, and partly in consequence of the actual combination of characters of other forms, 

 these genera have been placed, by each different author, in some new relationship, although 

 there has of late been a decided tendency to place them near or among the Sciuridae. In this 

 view I am disposed to concur, although there still remains the question, whether the two are 

 not typical of as many different sub-families, themselves forming a family of full rank? 

 Waiving this question until fuller series of entire specimens may serve to decide, I shall, for 

 the present, unite them as above. 



The two genera, then, Castor and Aplodontia, are confined to the northern hemisphere, the 

 latter, indeed, restricted to a very narrow region on the west coast. The two are readily dis 

 tinguished by the flat scaly tail, webbed feet, and the complicated molars ~ of Castor, and the 

 very short and hairy tail, unwebbed feet, and simple molars ^ of Aplodontia. 



APLODONTIA, Rich. 



Aplodontia, RICHARDSON, Zool. Jour. IV, Jan. 1829, 334. IB. F. Bor. Am. I, 1829, 210. 



AUD. and BACH. N. Am. Quad. Ill, 1853, 98. 

 Ilaplodon, WAOLER, Syst. Amphib. 1831, 23. 



WAGNEK, Suppl. Schreb. Ill, 1843, 395. 



Apluodontia, RICH. Rep. British Assoc. for 1836, V, 1837, 187. 

 Uaploodon or Hdpludon, BRANDT, Beit. Kennt. Saugt. Russl. 1855, 150. 

 Anisonyx, RAFIKKSQCE, Am. Monthl. Mag. H, 1817, 45. (In part.) 



Ears moderate, distinctly visible ; muffle as in Sciuridae. Fore claws much larger than hinder ones ; the short thumb 

 with a well developed claw ; soles and heels naked. Tail very short, almost rudimentary. Skull very much depressed, 

 triangular. Molars ^, prismatic ; the anterior very small. 



The systematic position of this genus has long been a matter of uncertainty, owing to the 

 fact that the single species has only been known from the specimen described first by Richard 

 son, and subsequently by Audubon and Bachman. In many respects, however, it bears a very 

 close resemblance to the spermophiles, particularly to Cynomys ludovicianus ; having a similar 

 muffle, fore feet with five claws, rather larger ears, still shorter tail, &c. The outline and 

 some other characteristics of the skull continue the analogy, which fails, however, in the root 

 less molars, the absence of post-orbital processes, and the presence of a rounded foramen in the 

 base of the zygoma. In the rootless molars and absence of post-orbital process, it resembles 

 Castor. In the absence of information respecting the skeleton, and internal anatomy generally, 

 it is impossible to say where it should be placed ; in a linear series, however, it would seem to 

 connect Castor with the squirrels, through the spermophiles. Of any affinities with Myoxlnae 



