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 
strface hairy; second toe of hind foot usually furnished with a double 
claw, the supplemental one being placed transversely beneath the true 
one; under fur 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, approaching grayish. The Beaver is usually 
darker to the northward, occasionally even nearly black; albinistic spe- 
cimens creamy white to wholly white. 
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, nasalg 
extending farther backward, and dorsal surface of interorbital 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 ZAPODIDA. 
1857. < Sub-family Dipodine, Baird, M. N. A., 1857, 428 (Muridz). 
1368. == Group Jaculina, Carus, Handb. Zodl., i, 1868, 101. 
1872. = Family Jaculide, Gill, Arrang. Fam. Mamm., 1872, 20. 
1875. == Family Zapodidx, Coues, Bull. U. S. Geolog. and Geogr. Survey 
Terr., 1875, 2nd ser., No. 5, 253. 
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