_Specimens examined_: Thirteen, of four subspecies; _Microtus mexicanus mexicanus_, Las Vigas, Veracruz, 30692; Nevada de Toluca, México, 63101; Valle de Bravo, México, 63094; _Microtus mexicanus mogollonensis_, Mt. Taylor, Valencia Co., New Mexico, 63298, 76830; Park Well, Mesa Verde National Park, Montezuma Co., Colorado, 69448, 69453; Upper Nutria, McKinley Co., New Mexico, 69997, 70000; _Microtus mexicanus phaeus_, Sierra Patamba, 9000 ft., Michoacán, 63085; _Microtus mexicanus subsimus_, 2 mi. E Mesa de Tablas, Coahuila, 58916; 13 mi. E San Antonio de las Alazanas, Coahuila, 68019, 68021. Microtus californicus (Peale) Fig. 37 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. Microtus pennsylvanicus (Ord) Figs. 14, 15, 16 and 17 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.