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. One specimen shows a break, perhaps resulting from injury, in the shaft (Fig. 14). One specimen has a posteromedian spine on the median digital ossification (Fig. 16). Comparison with _M. agrestis_ is included with the description of _M. agrestis_. _Specimens examined_: Thirteen, of six subspecies; _Microtus pennsylvanicus alcorni_, 20 mi. NE Anchorage, Alaska, 43043; _Microtus pennsylvanicus finitus_, Laird, Yuma Co., Colorado, 68544; _Microtus pennsylvanicus modestus_, 5 mi. N, 26 mi. W Saguache, 9500 ft., Saguache Co., Colorado, 42306; 3 mi. N, 16 mi. W Saguache, 8500 ft., Saguache Co., Colorado, 42416, 42417, 42418; 1 mi. S, 2 mi. E Eagle Nest, 8100 ft., Colfax Co., New Mexico, 42430, 42439; _Microtus pennsylvanicus pennsylvanicus_, 2 mi. S, 3 mi. E Ft. Thompson, 1370 ft., Buffalo Co., South Dakota, 42379; Vermillion, Clay Co., South Dakota, 37070; _Microtus pennsylvanicus pullatus_, 12 mi. S, 5 mi. E Butte, Silver Bow Co., Montana, 57501, 57503; _Microtus pennsylvanicus uligocola_, Muir Springs, 2 mi. N, 2-1/2 mi. W Ft. Morgan, Morgan Co., Colorado, 75082. Microtus agrestis (Linnaeus) Fig. 18 Baculum: Greatest length of stalk (2.9 mm.) twice greatest breadth, 4-1/2 times greatest depth; stalk well developed, shaft not flattened dorsoventrally; large median ossified process, minute lateral ossifications in single specimen examined; length of stalk 2-1/2 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 3/4 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.