The specimen of _Clethrionomys rutilus_ figured by Ognev (1950:120) is essentially like the North American specimens examined by me in the relative sizes of the ossifications and the general shape of the stalk. _Specimens examined_: Four, of one subspecies; _C. r. dawsoni_, west bank Gakona River, 1700 ft., 5 mi. NNE Gulkana, Alaska, 42865, 42866; SW end Dezadeash Lake, 2400 ft., Yukon Territory, 42910, 42921. Clethrionomys gapperi (Vigors) Fig. 10 Baculum: Stalk elongate, greatest length (2.8 mm.) 1-3/4 times greatest breadth, and 3-3/4 times greatest depth; proximally enlarged, greatest depth 1/2 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 (5-1/2 mi. E, 12 mi. S Collbran), 60022 (8 mi. E, 1/2 mi. S Skyway). Clethrionomys occidentalis (Merriam) Fig. 12