In the absence of ossified lateral processes my two specimens differ from bacula of Microtus (Arvicola) terrestris figured by Didier (1943:79, 1954:245, 247, 248) and by Ognev (1950:591). The median process relative to the size of the shaft is smaller, and the shaft relative to its length is wider in M. richardsoni than in M. terrestris. The stalk of M. (Arvicola) amphibius figured by Didier is like that of M. richardsoni in its greater breadth and median notch on posterior border.

The relationship of the New World water rat, M. richardsoni, to the Old World water rats (genus Arvicola of some European authors) is uncertain. Miller (1896:66) placed all of them in the subgenus Arvicola. Subsequent authors, stressing differences in the teeth, have placed M. richardsoni in the subgenus Aulacomys of Rhoads. Zimmerman (1955) has shown that teeth in some Arvicola approach the more complex pattern of M. richardsoni. He argues also that Arvicola is generically distinct from Microtus on the grounds that the two groups have separate origins, Arvicola having descended from the genus Mimomys and Microtus from some other group of microtines. This argument also was advanced by Hinton (1926:47-48). Pending further studies of the possible polyphyletic origin of other subgenera of the genus Microtus, I refer both M. richardsoni and M. terrestris to the subgenus Arvicola.

The evidence afforded by the bacula available is not conclusive as to relations of Old World and New World water rats. No general agreement on the number of species in this Palaearctic group has been reached, and bacula of only three or four of the numerous Old World subspecies have been figured. I have examined none.

Specimens examined: Two, from Wyoming; 42454 (31 mi. N Pinedale, 8025 ft., Sublette Co.), 37903 (23-1/2 mi. S, 5 mi. W Lander, 8600 ft., Fremont Co.).

Microtus (Chilotus) oregoni (Bachman)

Fig. 45

Baculum: Stalk broad, greatest length (2.2 mm.) 1¾ times greatest breadth, 3½ times greatest depth; three well-developed ossified processes; median process ²/₅ length of stalk, rounded or tapered terminally, proximal end opposed to tip of stalk and flattened obliquely; lateral processes ²/₃ length of median process, deeper than wide, curved; tuberosities of stalk well developed, confluent medially, visible in dorsal view; in end-view dorsal concavity narrow, moderately deep, rounded, ventral concavity wide, deep, flattened; base wider ventrally than dorsally; shaft tapering more or less uniformly, terminally inflated.

In the relative sizes, to each other and to the stalk, of the three digitate ossifications M. oregoni resembles closely the Old World representative of the same subgenus, M. (Chilotus) socialis, as figured by Argyropulo (1933b:181). In M. oregoni the greatest width of the baculum is more proximal on the stalk than in the M. socialis figured by Argyropulo but closely resembles the baculum of the M. socialis figured by Didier (1954:242). In possessing a shallow emargination in the base of the stalk and in possessing a median process that is smaller than the lateral processes, M. socialis, as figured by Didier, differs from M. oregoni. The baculum figured by Argyropulo (loc. cit.) of Sumeriomys colchicus schidlovskii [ = Microtus (Chilotus) socialis schidlovskii according to Ognev, 1950:392] differs from other Chilotus that have been studied in having an unusually elongate median process and a more distal placement of the widest part of the stalk.

Specimens examined: Three, of the subspecies M. oregoni oregoni, from 5 mi. N Orick, Humboldt Co., California, 3-C-248, collection of W. B. Quay; from Mary's Peak, Benton Co., Oregon, 66, collection of F. W. Sturges; and from Sec. 3, T. 11S, R. 5W, Benton Co., Oregon, 79183.

Microtus (Stenocranius) gregalis (Pallas)

Fig. 34

Baculum: Length of stalk (2.4 mm.) 1¾, times greatest breadth, 4¹/₃ times greatest depth; median ossified process well developed, more than ¹/₃ length of stalk, higher than wide, slightly bowed, closely appressed to terminus of shaft; basal tuberosities of stalk moderately developed, confluent medially, posterior profile of medial apex rounded in dorsal view, lateral indentations present, hence trilobate outline; in proximal end-view base wider ventrally, ventral concavity broader than dorsal concavity but of equal depth, medial constriction ²/₃ greatest depth; shaft slender in distal part, inflated terminally, and wider than high at mid-point of stalk; lateral profile a smooth slope of gradually decreasing curvature from point of greatest width to near distal end.

The baculum of this species figured by Ognev (1950:461) differs in having lateral ossified processes, and a more rounded base of the stalk. Resemblance to the New World Stenocranius is discussed below.

Specimen examined: One from "Eastern Europe," 8059.

Microtus (Stenocranius) miurus Osgood

Figs. 32 and 33

Baculum: Length of stalk (2.8 mm.) 1½ times greatest breadth, 3½ times greatest depth; median process ossified, ²/₅ to ³/₅ length of stalk, laterally compressed, sometimes arched in dorsoventral plane; lateral processes cartilaginous, slender; basal tuberosities well developed, averaging less enlarged than shown in Figure 32, but more angular in lateral outline than shown in Figure 33; tuberosities confluent posteriorly; posterior profile smoothly rounded to trilobate, curvature at point of greatest breadth usually acute; in proximal end-view base wider dorsally, deep dorsal concavity, shallow ventral concavity, medial constriction ³/₅ of greatest depth; shaft slender anteriorly, at mid-point of stalk twice as wide as high, at tip higher than wide, laterally inflated; lateral profile in most specimens abruptly curved anterior to point of greatest breadth.

The single specimen of the Old World M. (Stenocranius) gregalis examined resembles the New World M. (Stenocranius) miurus in the angular lateral profile at the point of greatest breadth of the stalk, slender shaft in comparison to broad base of stalk, and presence of a single well-developed laterally compressed median process. The base of the stalk in the baculum of M. gregalis is less well developed and smaller than in the baculum of M. miurus.

Specimens examined: Nine, all of the subspecies Microtus miurus muriei, from the Brooks Range, Alaska; 51077 (Lake Schrader, 145°09'50", 69°24'28", 2900 ft., Romanzof Mts.); 51151, 51152, 51154, 51164, 51166, 51169 (last 6 from Wahoo Lake, 69°08', 146°58', 2350 ft.); 51210, 51213 (last 2 from Porcupine Lake, 68°51'57", 146°29'50", 3140 ft.).

Microtus (Chionomys) nivalis Martins

Fig. 47

Baculum: Greatest length of stalk (2.7 mm.) 2¼ times greatest breadth, 4½ times greatest depth; three digitate processes, lateral processes mostly cartilaginous in single adult examined; median process well ossified, approximately ¹/₃ length of stalk, basally notched, not arched, laterally compressed distally; base of stalk broad and flat, basal tuberosities well developed, separate; posterior profile in dorsal view rounded, convex except for medial notch separating tuberosities; dorsal and ventral concavities deep, broad, equal; medial constriction less than ½ greatest depth; in dorsal view shaft tapering gradually from widest point, terminally rounded;