Fig. 4. Castor canadensis concisor, male, adult, no. 2090, Mus. Nat. Hist., Univ. Kansas, from Trappers Lake, Garfield County, Colorado, obtained by L. L. Dyche, October 22, 1891.
Comparisons.—From topotypes and near topotypes of Castor canadensis taylori, C. c. duchesnei differs as follows: Color: Guard hairs lighter, underfur darker. Skull: Nasals narrower; rostrum narrower; mastoid breadth relative to zygomatic breadth less; zygomatic breadth relative to basilar length greater; tympanic bullae narrower and smaller; cheek teeth narrower.
Figs. 5-7 Lateral views of left side of skulls of Castor canadensis. × 1/2
Fig. 5. Castor canadensis rostralis, male, young adult, no. 5199 (holotype), Mus. Zoöl., Univ. Utah.
Fig. 6. Castor canadensis pallidus, female, adult, no. 719 (holotype), Mus. Zoöl., Univ. Utah.
Fig. 7. Castor canadensis duchesnei, male, young adult, no. 4625 (holotype), Mus. Zoöl., Univ. Utah.
From specimens of Castor canadensis baileyi, from 20 miles north northeast of Elko, Elko County, Nevada, C. c. duchesnei differs as follows: Size larger; tail and hind foot longer. Color: Guard hairs lighter, underfur darker. Skull: Larger in all measurements taken; nasals broader and longer (breadth of nasals averages 46 per cent of length of nasals as opposed to 41 per cent); rostrum broader and longer; mastoid breadth relative to zygomatic breadth less; tympanic bullae larger.
From one specimen of Castor canadensis concisor, from Trappers Lake, Garfield County, Colorado, and from the original description of that subspecies (Warren and Hall, 1939: 358), C. c. duchesnei differs as follows: Color: Lighter throughout. Skull: Nasals more convex transversely; rostrum narrower; ventral border of rostrum more concave dorsally, especially immediately behind upper incisors; distal end of meatal tube smaller; angular process shorter and rounded rather than pointed; cheek teeth smaller.