Baculum: In the single specimen examined, less mature than that figured by Hamilton (1946:384), the digitate processes are cartilaginous, the basal tuberosities are less well developed, and the shaft is slenderer throughout. The cartilaginous processes are of the same proportions as ossified processes in the figure mentioned. The shaft is also convex ventrally in lateral profile. The view of the side here considered to be anatomically the ventral side (adjacent to the urethra) is labelled dorsal view in Hamilton's specimen. _Specimen examined_: One, from Reserve, Brown Co., Kansas, 72405. Microtus (Herpetomys) guatemalensis Merriam Figs. 42 and 43 Baculum: Stalk moderately elongate, greatest length (3.5 mm.) 2-1/3 times greatest breadth, spatulate, flattened throughout, greatest thickness 1/3 millimeter; three ossified processes; median process having three cornered base, curved dorsally, wider than high, 1/4 to 1/5 greatest length of stalk; each lateral process bent at middle, as long as median process, compressed laterally; base of stalk curved dorsally, tuberosities marginal, hence narrow, lateral excavations of tuberous margin not confluent medially; in end-view ventral concavity broad, no dorsal concavity, medial constriction but slightly less than greatest thickness (not depth); shaft wider than high throughout, at mid-point more than 3 times as wide as high; tip of shaft slightly inflated both laterally and dorsoventrally; lateral profile gradually sloping anteriorly from widest point of stalk. Specimen number 65921 (Fig. 43) differs from number 65895 (Fig. 42) described above. Terminus of shaft of number 65921 has lateral lobes from which arise lateral cartilaginous processes; median terminal ossification irregular in shape, smaller, imbedded in terminally bilobate cartilage. In the spatulate flattened stalk these two specimens are much alike. An immature specimen, number 65908, is smaller (length of stalk 2.6 mm.) also flattened and spatulate, has the terminal processes cartilaginous, the lateral processes bent medially, and proportions as in the adult. The baculum shows no noteworthy resemblance to that of any other species of North American _Microtus_; on the other hand the differences between _M. guatemalensis_ and some other species are no greater than the differences between certain species included in the subgenus _Microtus_. The baculum neither strengthens nor weakens the case for subgeneric rank for _M. (Herpetomys) guatemalensis_. _Specimens examined_: Three from Guatemala; 65895 (2 mi. S San Juan Ixcoy), 65908, (3-1/2 mi. SW San Juan Ixcoy), 65921 (10 mi. E, 4 mi. S Totonicapán). Microtus (Arvicola) richardsoni (DeKay) Figs. 38 and 39