and the appearance in lateral view do not differ greatly from others examined by Hamilton, by Dearden (1958:546), and by me.

Specimens examined: Five, representing four subspecies; S. cooperi gossii, 6 mi. N Midway, Holt Co., Nebraska 78379, 78380; S. cooperi relictus, 5 mi. N, 2 mi. W Parks, Dundy Co., Nebraska, 72601 (immature); S. cooperi saturatus, 3 mi. S Demotte, Jasper Co., Indiana, 3-C-454, collection of W. B. Quay; S. cooperi paludis, Meade County State Park, Kansas, 13716.

Clethrionomys rutilus Pallas

Fig. 11

Baculum: Stalk elongate, and proximally enlarged, greatest length (2.7 mm.) 2 times greatest breadth; less than 4 times greatest depth; three well-developed ossified processes; length of stalk 2¹/₃ times length of median process; median process with basal (and ventral) protuberence and lateral lobes, arched in dorsoventral plane; lateral processes as large as median process, flattened distally, having ventromedial vane on distal half; basal tuberosities of stalk well developed, medially confluent; posterior profile in dorsal view trilobate or convex throughout with rounded posterior apex; dorsal concavity well developed, ventral surface but slightly concave, medial constriction of base as little as ½ greatest depth; shaft straight, slender, at mid-point of stalk but slightly wider than high; basal tuberosities largely dorsal to axis of shaft in lateral view; lateral profile in dorsal view with an abrupt curvature separating the gently sloping sides of the shaft from the basal part at its greatest breadth.

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.) 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).

Clethrionomys occidentalis (Merriam)

Fig. 12

Baculum: Stalk elongate, greatest length (2.8 mm.) 2½ times greatest breadth, 6 times greatest depth; three well-developed ossified processes; median process larger than lateral processes, ½ 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 ½ 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.

Clethrionomys glareolus Schreber

Fig. 13

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 ¾ greatest depth; shaft straight, at mid-point slightly wider than high, elongate, widest point of stalk less than ¼ 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.

The specimen of Clethrionomys glareolus figured by Ognev (1950:31) in dorsal view as I interpret it, resembles my specimen in the rounded base; in the elongate, distally inflated shaft; in the initially abrupt slope of the lateral profile in dorsal view from the greatest width of stalk anteriorly; and in the presence of three well ossified processes. Ognev's specimen differs from mine in the median process being more elongate relative to its width, and rounded proximally, lacking lateral lobes and basal notch; in lateral processes being less curved; in the greater terminal inflation of the shaft; and in the closer approximation of the terminal processes to the shaft. The baculum of Clethrionomys glareolus as described and figured by Didier (1954:243-244) resembles my specimen in general proportions, but is more pointed proximally and more curved in dorsoventral plane. Didier states that the baculum is rather variable in form in this species, in different regions, but that a large number of specimens must be examined to assess the geographic nature of this variation.

Specimen examined: One from Zermatt, Valais, Switzerland, 67100.

Clethrionomys rufocanus Sundevall

Fig. 9

Baculum: Base of stalk broad but relatively flattened