Fig. 18
Baculum: Greatest length of stalk (2.9 mm.) twice greatest breadth, 4½ times greatest depth; stalk well developed, shaft not flattened dorsoventrally; large median ossified process, minute lateral ossifications in single specimen examined; length of stalk 2½ times length of median ossification which is higher than wide, slightly decurved, sagittate in dorsal view, with three-cornered base; basal tuberosities of stalk moderately well developed, medially joined; posterior profile in dorsal view evenly rounded; ventral concavity broader than, but of comparable depth to, dorsal concavity in end-view, base of stalk wider ventrally, constriction ¾ greatest depth; at mid-point of stalk shaft is but slightly wider than high; pronounced terminal inflation of shaft; lateral profile in dorsal view sloping abruptly from widest point of stalk anteriorly onto stalk which then tapers more gradually to terminal inflation.
From the baculum of its New World counterpart, namely Microtus pennsylvanicus, my specimen of Microtus agrestis and the specimen figured by Didier (1954:239) differ in their minute lateral processes, relatively larger median processes, and more elongate, less dorsoventrally flattened shafts.
The specimen of M. agrestis figured by Ognev (1950:320), in dorsal view has lateral concavities producing a somewhat trilobate outline in the base of the stalk, and the lateral processes are well developed; the median process is larger and bulbous, wider distally than proximally. Without larger numbers of bacula of M. agrestis I am unable to reconcile these differences. The differences between M. agrestis and M. pennsylvanicus seem comparable to the differences between some other species of Microtus.
Specimen examined: One, from Gryon, Switzerland, 67102.
Microtus (Pedomys) ochrogaster (Wagner)
Fig. 31
Baculum: Stalk broad, greatest length (3.2-4.0 mm.) 12/3 to 2 times greatest breadth, 2½ to 4 times greatest depth; median process ossified, relatively small, less than 3/10 length of stalk; lateral processes arising from subterminal part of stalk, cartilaginous or with small ossifications; posterior profile in dorsal view broadly rounded or slightly angular, widest point of stalk 1/6 to ¼ the length of stalk from base; basal tuberosities well developed and medially confluent, in end-view dorsally convex, or at least less deeply concave than ventrally; shaft straight, base bent ventrally or more commonly dorsally; at mid-point of stalk wider than high, often twice as wide as high; viewed from above, lateral profile from point of greatest breadth to middle of shaft a gradual sigmoid curve; slight terminal inflation of shaft.
Specimens examined: Forty-one, of three subspecies; Microtus ochrogaster haydeni, Muir Springs, 2 mi. N, 2½ mi. W Ft. Morgan, Morgan Co., Colorado, 74995, 74998, 74999, 75002; 1 mi. W Laird, Yuma Co., Colorado, 57304, 76833; 2 mi. N, 2 mi. W Haigler, Dundy Co., Nebraska, 75016; 2 mi. S Franklin, Franklin Co., Nebraska, 75043, 75044; Atwood, Rawlins Co., Kansas, 75020, 75023, 75025, 75027, 75028; 1 mi. N, 2 mi. E Oberlin, Decatur Co., Kansas, 75030, 75032, 75034, 75035, 75036; 1½ mi. N, ¼ mi. E Norton, Norton Co., Kansas, 68327; 1 mi. SW Norton, Norton Co., Kansas, 75037; 2 mi. S, 1 mi. W Norton, Norton Co., Kansas, 75038; M. ochrogaster ochrogaster, Rydal, Republic Co., Kansas, 75047-75053, 75060, 75062, 75063, 75066, 75070, 75071, 75073; 1 mi. N, 1 mi. W Holton, Jackson Co., Kansas, 75077; 2 mi. W Court House, Lawrence, Douglas Co., Kansas, 76832; Univ. Kansas Natural History Reservation, Douglas Co., Kansas, 68536; M. ochrogaster taylori, Meade County State Park, Kansas, 68539, 68542.