_Measurements._--Measurements of the type and average and extreme cranial measurements of 9 unsexed adults, from Currant Creek, are, respectively, as follows: Total length, 1,176; length of tail, 458; length of hind foot, 165; length of ear, 33; occipitonasal length, 123.6, 132.1 (138.5-122.3); basilar length, 98.6, 114.4 (125.8-99.2); mastoid breadth, 60.4, 65.1 (67.2-64.1); interorbital breadth, 23.0, 25.1 (26.1-23.7); zygomatic breadth, 88.3, 94.2 (99.7-89.5); length of nasals, 46.1, 48.4 (51.5-46.2); breadth of nasals, 20.5, 22.5 (24.5-18.8); alveolar length of upper molariform teeth, 28.9, 29.9 (32.2-26.5). [Illustration: FIGS. 1-4 Dorsal views of skulls of _Castor canadensis_. x 1/2] FIG. 1. _Castor canadensis rostralis_, male, young adult, no. 5199 (holotype), Mus. Zool., Univ. Utah. FIG. 2. _Castor canadensis pallidus_, female, adult, no. 719 (holotype), Mus. Zool., Univ. Utah. FIG. 3. _Castor canadensis duchesnei_, male, young adult, no. 4625 (holotype), Mus. Zool., Univ. Utah. 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. [Illustration: FIGS. 5-7 Lateral views of left side of skulls of _Castor canadensis_. x 1/2] Fig. 5. _Castor canadensis rostralis_, male, young adult, no. 5199 (holotype), Mus. Zool., Univ. Utah. Fig. 6. _Castor canadensis pallidus_, female, adult, no. 719 (holotype), Mus. Zool., Univ. Utah. Fig. 7. _Castor canadensis duchesnei_, male, young adult, no. 4625 (holotype), Mus. Zool., 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.