Measurements.—Measurements of the type and average and extreme cranial measurements of 6 unsexed adults from Charleston, are, respectively, as follows: Total length, 1,330; length of tail, 470; length of hind foot, 170; length of ear, 34; occipitonasal length, 128.2, 134.3 (142.1-129.5); basilar length, 112.4, 117.2 (128.2-113.2); mastoid breadth, 62.5, 64.3 (68.9-60.2); interorbital breadth, 27.9, 26.0 (26.9-26.0); zygomatic breadth, 91.3, 93.8 (105.8-90.8); length of nasals, 44.9, 43.9 (51.3-41.5); breadth of nasals, 24.5, 23.7 (25.7-22.0); alveolar length of upper molariform teeth, 28.3, 30.9 (32.5-28.7).
Comparisons.—From topotypes and near topotypes of Castor canadensis taylori, C. c. rostralis differs as follows: Color: Darker on upper parts owing to darker underfur, guard hairs actually lighter. Skull: Longer; nasals shorter and wider (breadth of nasals averages 54 per cent of length of nasals as opposed to 47 per cent); extension of nasals posterior to premaxillae less; rostrum shorter, broader and deeper; dorsal surface of lacrimal bone smaller; zygomatic breadth relative to basilar length greater; mastoid breadth relative to zygomatic breadth less; coronoid process shorter; coronoid and condyloid processes farther apart and space between them shallower.
From one topotype and two specimens from the Colorado River at Yuma, Yuma County, Arizona, of Castor canadensis repentinus, C. c. rostralis differs as follows: Size larger; tail longer. Color: Darker throughout. Skull: Longer; nasals shorter and wider (breadth of nasals relative to length of nasals averages 54 per cent as opposed to 47 per cent); extension of nasals posterior to premaxillae less; rostrum shorter, deeper and wider; zygomatic breadth relative to basilar length greater; mastoid breadth actually as well as relatively greater; dorsal surface of lacrimal bone smaller; coronoid and condylar processes farther apart and space between them shallower.
From specimens of Castor canadensis baileyi, from 20 miles north northeast of Elko, Elko County, Nevada, C. c. rostralis differs as follows: Size larger; tail and hind foot longer. Color: Darker throughout. Skull: Larger in all measurements taken; nasals markedly wider (breadth of nasals relative to length of nasals averages 54 per cent as opposed to 41 per cent); extension of nasals posterior to premaxillae less; dorsal surface of lacrimal bone smaller; mastoid breadth relative to zygomatic breadth less.
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. rostralis differs as follows: Color: Guard hairs lighter; underfur darker (blackish as opposed to brownish). Skull: Longer and narrower; nasals broader and shorter (breadth of nasals averages 54 per cent of length of nasals as opposed to 48 per cent); dorsal surface of lacrimal bone smaller; distal end of meatal tube smaller; distal end of angular process rounded rather than pointed; coronoid process shorter; coronoid and condylar processes farther apart and space between them shallower.
Among known kinds of Castor canadensis, C. c. rostralis is most like Castor canadensis duchesnei, from which the former subspecies differs as follows: Tail and hind foot longer. Color: Darker throughout. Skull: Nasals shorter and wider (breadth of nasals averages 54 per cent of length of nasals as opposed to 46 per cent); nasals less arched transversely; rostrum shorter, deeper and broader; ventral surface of rostrum less concave dorsally; dorsal surface of lacrimal bone smaller.
For comparison with Castor canadensis pallidus, see account of that subspecies.
Remarks.—Animals from Kamas, in the drainage of the Weber River, are intergrades between C. c. rostralis and C. c. duchesnei, but their short, wide nasals and wide rostra make them referable to C. c. rostralis.
The available specimens of C. c. rostralis are all from streams which ultimately empty into Great Salt Lake, which is in the northern part of the basin of Pleistocene Lake Bonneville. Some streams drain into the Lake Bonneville Basin without emptying into Great Salt Lake proper. Beavers from these streams, we suspect, when they become known, will be found to be related to C. c. rostralis.
Specimens examined.—Total, 16 (2 skins and skulls, 14 skulls only), distributed as follows: Summit County: Kamas, 5,500 ft., 6. Salt Lake County: Red Butte Canyon, Fort Douglas, 5,000 ft., 1; Millcreek Canyon, 6 mi. above mouth, 7,000 ft., 1. Wasatch County: Charleston, Heber Valley, 5,500 ft., 8.