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 (5-1/2 mi. E, 12 mi. S Collbran), 60022 (8 mi. E, 1/2 mi. S Skyway).
Baculum: Stalk elongate, greatest length (2.8 mm.) 2-1/2 times greatest breadth, 6 times greatest depth; three well-developed ossified processes; median process larger than lateral processes, 1/2 the length of stalk, curved, basally broad, ventrally keeled, trilobate posteriorly; lateral ossifications large, flattened distally, curved; posterior profile of stalk posteriorly slightly emarginate, thus bilobate in outline; in end-view dorsal concavity deeper than ventral, constriction less than 1/2 greatest depth, tuberosities confluent, visible in dorsal view at each side; shaft slender, especially in depth, straight; at mid-point of stalk almost twice as wide as deep, slight terminal inflation.
The general proportions of the stalk and the relatively large, uniquely shaped processes, are characteristic of most specimens of the genus _Clethrionomys_ examined.
_Specimen examined_: _C. occidentalis californicus_, one from Mary's Peak, Benton Co., Oregon, 30, F. W. Sturges' collection.
Baculum: Stalk elongate, greatest length (2.9 mm.) twice the greatest breadth in the specimen examined, flattened proximally, greatest length almost 6 times greatest depth of base; three well-developed ossified processes; median process arched in a dorsoventral plane, with basal notch and lateral lobes; lateral processes as long as median process, bowed in dorsal view, flattened distally, with ventromedial vane; basal tuberosities of stalk weakly developed, medially confluent; posterior profile in dorsal view evenly rounded; in end-view dorsal concavity shallow in comparison to most species but deeper than ventral concavity, constriction 3/4 greatest depth; shaft straight, at mid-point slightly wider than high, elongate, widest point of stalk less than 1/4 of total length from proximal end, slight lateral inflation at tip; lateral profile in dorsal view sloping at first abruptly and then gradually from widest point of stalk anteriorly onto shaft.