Clethrionomys gapperi (Vigors)
Fig. 10
Baculum: Stalk elongate, greatest length (2.8 mm.) 1¾ times greatest breadth, and 3¾ times greatest depth; proximally enlarged, greatest depth ½ greatest breadth; three well-developed ossified processes; length of stalk 2 1/3 times length of median process; median process arched in dorsoventral plane, with basiventral protuberence or spine and lateral lobes; lateral processes as large as median process, flattened distally, arched; basal tuberosities of stalk well developed, medially confluent; posterior profile in dorsal view trilobate or convex throughout with a rounded posterior apex; dorsal concavity well developed, ventral surface but slightly concave, or in some cases slightly convex; medial constriction of base 3/5 greatest depth; shaft straight, slender, at mid-point of stalk twice as wide as high; basal tuberosities dorsally placed relative to axis of shaft; lateral profile in dorsal view abruptly curved anterior to point of greatest width; slender stalk distinct from angular enlarged base.
The most noticeable difference between the baculum of C. rutilus and C. gapperi is size. The proportions of the four ossifications are approximately the same. Ventral vanes on the lateral processes are not developed in C. gapperi. C. gapperi and C. rutilus are more nearly alike in their bacula than any other two species of Clethrionomys examined. Clethrionomys occidentalis, the other New World species, is also much like C. gapperi and C. rutilus. The differences are of a magnitude comparable to those between the bacula in subspecies of Microtus montanus (Figs. 19-21) for example, or in subspecies of Lagurus curtatus (Dearden, 1958:542).
Specimens examined: Nine, of two subspecies; Clethrionomys gapperi athabascae, British Columbia, 42922 (Indian Creek, Mile Post 234 of Alaskan Highway), 64281 (West bank Racing River, 89 mi. W Muskwa), 64287 (North bank Tetsa River, 56 mi. W, 11 mi. S Muskwa), 64290 (44 mi. W, 9 mi. S Muskwa), 64310 (32 mi. W, 2 mi. S Muskwa); Clethrionomys gapperi galei, 31 mi. N Pinedale, Sublette Co., Wyoming, 42108; Grand Mesa, Delta Co., Colorado, 60014 and 60015 (5½ mi. E, 12 mi. S Collbran), 60022 (8 mi. E, 1/2 mi. S Skyway).