Baculum: Stalk elongate, greatest length (3.0 mm.) 2-1/3 times greatest breadth, 4-1/2 times greatest depth; median process ossified, 1/4 length of stalk, basally broadened, flattened and shallowly grooved ventrally to fit tip of shaft, to which the process is closely appressed; lateral processes cartilaginous; ends of stalk bowed upwardly; posterior profile of base of stalk rounded or slightly trilobate if posterolateral concavities form in tuberosities; moderate development of tuberosities, in end-view dorsal concavity slightly deeper and narrower than ventral concavity, both comparatively shallow, median constriction 4/5 greatest depth; shaft curved, more or less terete at mid-point of stalk, terminally inflated dorsally; lateral profile in dorsal view gradually curved from point of greatest width anteriorly onto shaft.
_Specimens examined_: Two, of two subspecies; _Microtus californicus californicus_, 1 mi. NE Berkeley, in Contra Costa Co., California, 76828; _Microtus californicus mohavensis_, 1/2 mi. SE Victorville, San Bernardino Co., California, 63745.
Baculum: Stalk heavy, broad, greatest length (2.2 to 3.0 mm.) 1-1/3 to 1-2/3 times greatest breadth, up to 3-3/4 times greatest depth; three ossified processes, median one largest, usually not twice so deep as lateral ossifications; median process usually distinctly widened basally, in length approximately 1/2 length of stalk; base broad, frequently angular laterally and basally, sometimes bilobate; basal tuberosities well developed, medially confluent; in end-view more or less uniformly biconvex or ventral surface more flattened than dorsal surface, medial constriction 1/2 to 2/3 greatest depth; shaft relatively heavy, at mid-point stalk almost twice as wide as deep and 1/3 as wide as base of stalk; shaft terminally rounded and sometimes slightly inflated; lateral profile in dorsal view abruptly or gradually curved anterior to point of greatest width and then gradually curved anteriorly.
Specimens examined averaged slightly smaller and were more variable than those described by Hamilton (1946:382). The greater variation may be in part geographic, as five subspecies are represented. Lateral processes are the last to ossify. One specimen (75082) with well-ossified median process lacks any lateral ossification. Four bacula of _M. pennsylvanicus_ (locality not specified) studied by Dearden (1958:547) agree in general with the description above.