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. pallidus differs as follows: Size smaller. Color: Markedly lighter throughout. Skull: Smaller, narrower; nasals shorter and wider (breadth of nasals averages 54 per cent of length of nasals as opposed to 48 per cent); rostrum shorter; zygomatic breadth relative to basilar length less; mastoid breadth relative to zygomatic breadth greater; tympanic bullae narrower and smaller; jugals narrower; distal end of meatal tube smaller; coronoid process shorter and wider; angular process shorter and rounded rather than nearly pointed; cheek teeth narrower.
From the type and near topotypes of Castor canadensis rostralis, C. c. pallidus differs as follows: Size smaller; tail and hind foot shorter. Color: Markedly lighter throughout. Skull: Smaller and narrower; rostrum shallower and narrower; posterior end of nasals more constricted and barely projecting posteriorly beyond premaxillae; zygomatic breadth relative to basilar length less; mastoid breadth actually as well as relatively greater; dorsal surface of lacrimal bone larger; tympanic bullae narrower; coronoid process higher and wider; angular process not projecting so far caudad; cheek teeth narrower.
From the type and near topotypes of Castor canadensis duchesnei, C. c. pallidus differs as follows: Size smaller; tail and hind foot shorter. Color: Lighter throughout. Skull: Shorter, narrower and less massive; nasals shorter and wider (breadth of nasals averages 54 per cent of length of nasals as opposed to 46 per cent); nasals barely projecting posteriorly beyond premaxillae; rostrum shorter and narrower; zygomatic breadth relative to basilar length less; mastoid breadth actually as well as relatively greater; tympanic bullae narrower and smaller; coronoid process higher and wider; angular process not projecting so far caudad; cheek teeth narrower.
Remarks.—The Raft River Mountains of extreme northwestern Utah, where C. c. pallidus occurs, are the only mountains of the state within the drainage of the Snake River. The Snake River proper lies 50 miles to the northward in Idaho and contains another kind of beaver, C. c. taylori (Davis, 1939: 273). Although occurring within the same drainage as C. c. taylori, C. c. pallidus is as distinct from it as from any other named kind. The relationships of C. c. pallidus, as indicated by the short rostrum and short, wide nasals, are rather more with C. c. rostralis of the Wasatch Mountains, than with C. c. taylori.
The pale color of the animals belonging to C. c. pallidus was noted at the time of capture, and is the same in the young specimen (625 mm. total length) as in the type, an adult.
Specimens examined.—Total, 2, distributed as follows: Boxelder County: Raft River, 5 mi. S Yost, Raft River Mountains, 6,000 ft., 1; Lynn Canyon, Raft River Mountains, 7,500 ft., 1.
Castor canadensis rostralis new subspecies
Type.—Male, young adult, skin and skull, number 5199, Museum of Zoölogy, University of Utah; Red Butte Canyon, Fort Douglas, 5,000 ft., Salt Lake County, Utah; October 13, 1947; collected by Harold S. Crane and Clifton M. Greenhalgh, original number 446 of Crane.
Range.—Known from the western streams of the Wasatch Mountains; probably occurs in all streams draining westward into the basin of Pleistocene Lake Bonneville.
Diagnosis.—Size large; tail and hind foot long (see measurements). Color (type): Upper parts Snuff Brown, purest on head; underfur Brownish Black (2); base of tail Cinnamon Buff; hind feet Carob Brown; ears Blackish Brown (2); underparts Auburn, grading posteriorly to Cinnamon Buff; underfur Light Drab. Skull: Large, massive; nasals short and broad (breadth averaging 54 per cent of length) and moderately convex transversely; rostrum deep and broad; ventral surface of rostrum moderately concave dorsally; dorsal surface of lacrimal bone small; frontal region generally flat; zygomatic arches robust and widely spreading (zygomatic breadth averaging 82 per cent of basilar length).