We recognize the need for caution in proposing new names for American beavers, because the transplanting of these animals from one watershed to another may have permitted the animals of a given area to change genetically, say, through hybridization, and may also have altered the geographic distribution of the several kinds. The officials of the Utah State Fish and Game Commission have assured us that such transplants have not occurred in the areas where these three new kinds are found, and further that nowhere in the state have transplants been made from one major drainage system to another; such transplants as have been made were only within the same major drainage system.
The capitalized color terms used in this paper are after Ridgway, Color Standards and Color Nomenclature, Washington, D. C., 1912. All measurements are in millimeters. We are indebted to the officials of the United States National Museum for the loan of comparative materials.
Castor canadensis pallidus new subspecies
Type.—Female, adult, skin and skull, number 719, Museum of Zoölogy, University of Utah; Lynn Canyon, 7,500 ft., Boxelder County, Utah; September 7, 1932; collected by W. W. Newby.
Range.—Known only from the Raft River Mountains.
Diagnosis.—Size small; tail and hind foot short (see measurements). Color (type): Pale, upper parts uniformly Ochraceous-Buff; underfur Snuff Brown; underparts uniformly Light Buff, grading to Light Ochraceous-Buff at base of tail; underfur Light Drab; front and hind feet Light Ochraceous-Buff. Skull: Rostrum short; nasals broad (breadth averaging 54 per cent of length), constricted posteriorly and barely projecting posteriorly beyond premaxillae; zygomatic arches robust, but not widely spreading (zygomatic breadth 77 per cent of basilar length); mastoid breadth 73 per cent of zygomatic breadth; anterolateral margin of orbit narrow (6.2); occipital condyles visible from dorsal view; condylobasal length greater than occipitonasal length; upper incisors narrow (Orange Chrome in color); coronoid processes high and wide; cheek teeth narrow.
Measurements.—Measurements of the type are as follows: Total length, 1040; length of tail, 380; length of hind foot, 157; length of ear, 35; occipitonasal length, 129.1; basilar length, 116.6; mastoid breadth, 65.6; interorbital breadth, 23.6; length of nasals, 43.3; zygomatic breadth, 89.7; breadth of nasals, 23.4; alveolar length of upper molariform teeth, 30.4.
Comparisons.—From topotypes and near topotypes of Castor canadensis taylori, C. c. pallidus differs as follows: Size smaller; tail and hind foot shorter. Color: Markedly lighter throughout. Skull: 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; zygomatic breadth relative to basilar length less; mastoid breadth actually as well as relatively greater; interorbital breadth greater; occipitonasal length shorter rather than longer than condylobasal length; tympanic bullae smaller; coronoid process higher and wider; cheek teeth narrower.
From specimens of Castor canadensis baileyi, from 20 miles north northeast of Elko, Elko County, Nevada, C. c. pallidus differs as follows: Body smaller; tail longer; hind foot shorter; ears shorter: Color: Markedly lighter throughout. Skull: Larger; nasals shorter and wider (breadth of nasals averages 54 per cent of length of nasals as opposed to 41 per cent); nasals barely projecting posteriorly beyond premaxillae; rostrum broader; zygomatic breadth relative to basilar length less; mastoid breadth actually as well as relatively greater; occipitonasal length less rather than greater than condylobasal length; tympanic bullae smaller; coronoid process higher and wider; cheek teeth narrower.
From one topotype and two specimens of Castor canadensis repentinus, from the Colorado River at Yuma, Yuma County, Arizona, C. c. pallidus differs as follows: Tail and hind foot shorter. Color: Lighter throughout. Skull: Narrower; nasals shorter and wider (breadth of nasals averages 54 per cent of length of nasals as opposed to 47 per cent); nasals barely projecting posteriorly beyond premaxillae; rostrum shorter; zygomatic breadth relative to basilar length less; mastoid breadth actually as well relatively greater; tympanic bullae narrower and smaller; coronoid process higher and wider; cheek teeth narrower.