Microtus pennsylvanicus (Ord)
Figs. 14, 15, 16 and 17
Baculum: Stalk heavy, broad, greatest length (2.2 to 3.0 mm.) 11/3 to 12/3 times greatest breadth, up to 3¾ 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 ½ 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 ½ 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.
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, 2½mi. W Ft. Morgan, Morgan Co., Colorado, 75082.