129 



short, rounded, furred, and nearly hidden in the pelage ; tail broad and 

 flat, covered with horny, blackish scales; fore feet short and weak, un- 

 webbed; hind feet large, fully palmate; soles of all the feet naked, upper 

 surface hairy; second toe of hind foot usually furnished with a double 

 claw, the supplemental one being placed transversely beneath the true 

 one ; under far soft, dense and grayish ; overlying hairs coarse, shining, 

 chestnut. Length of body about thirty inches; of tail about ten; weight 

 of adult, about forty-five to fifty pounds, ranging to upward of sixty 

 pounds. General color above reddish brown, lighter or darker in different 

 specimens; below lighter, approachipg grayish. The Beaver is usually 

 darker to the northward, occasionally even nearly black ; albinistic spe- 

 cimens creamy white to wholly whita. 



The Beaver is several years attaining its growth, even increasing in 

 size long after mature dentition. Two-year-old Beavers average thirty- 

 five to forty pounds ; old ones sometimes attain a weight of sixty and 

 even sixty-three pounds. The skull increases in size apparently nearly 

 through life, and in weight by the thickening and increased density of 

 the bones. 



The Old- World form has the basilar cavity deeper and larger, nasa^ 

 extending farther backward, and dorsal surface of interqrbital region 

 broader than in var. canadensis ; these variations probably result from 

 long separation of the race and accidental causes rather than to original 

 diversity of the stock ; the same conclusion applies to their habits and 

 instincts. 



The carminative substance, castoreum, exuding from two pairs of 

 glands, one in each groin, into the preputial and ano-preputial pass- 

 ages of this animal, is a somewhat complex substance, composed of the 

 grayish-colored and somewhat offensive secretion of the upper glands, 

 mixed with the yellow, viscid, and musky matter from the lower glands; 

 these mixed and dried secretions constitute the castor of commerce, which 

 evidently varies with the climate, food, etc. The castor from the Ameri- 

 can Beaver has a much lower commercial value, and on chemical analysis 

 is found to contain less volatile oil, castorin and resin, and much more 

 carbonate of lime than that of the Russian Beaver. 



FAMILY ZAPODID^. 



1857. < Sub-family Dipodinas, Baird, M. N. A., 1867, 428 (Muridas). 

 1868. = Group Jaculina, Carus, Handb. Zool., i, 1868, 101. 

 1872. = Family JacuUdai, Gill, Arrang. Fam. Mamm., 1872, 20. 

 1875. = Family Zapodidas, Coues, Bull. U. S. Geolog. and Geogr. Survey 

 Terr., 1875, 2nd ser., No. 5, 253. 

 9 



