Sus-Famiiy CASTORINAE. 
Molars it 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 Sciwridae. 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. d 
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 
4-5 
very short and hairy tail, unwebbed feet, and simple molars {=} of Aplodontia. 
AP LODON Diy Riek: 
Aplodontia, Ricuarvson, Zool. Jour. 1V, Jan. 1829, 334.—Is, F. Bor. Am. I, 1829, 210. 
Avp, and Baca. N. Am. Quad. TII,, 1853, 98. 
Haplodon, Wacirr, Syst. Amphib. 1831, 23. 
Waaner, Suppl. Schreb. LI, 1843, 395. 
Apluodontia, Ric. Rep. British Assoc. for 1836, V, 1837, 187. 
Haploodon or Hapludon, Branvr, Beit. Kennt. Saugt. Russl. 1855, 150. 
Anisonyx, Rariesque, Am. Monthl. Mag. II, 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 a= 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 proeesses, 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 Myowinae 
