Baculum: Stalk broad, greatest length (2.6-3.2 mm.) 1-1/3 to 1-2/3 times greatest breadth, 3-1/3 to 3-2/3 times greatest depth; median process ossified, with ventral depression, process 1/4 to 1/3 length of stalk, appressed to tip of shaft, wider than high proximally, relatively broad terminally; lateral processes cartilaginous, small, attenuate; posterior profile of stalk in dorsal view broadly rounded, bilobate, or trilobate, median lobe formed by posterior projection of dorsal shelf between enlarged lateral tuberosities that form outer lobes, posterolateral faces of these tuberosities visible in dorsal view of stalk; in end-view dorsal surface slightly concave, ventral concavity broad and deep, median constriction 1/2 greatest depth; shaft flattened except tip that is more terete, and bowed dorsally; at mid-point, stalk twice as wide as high; shaft relatively slender terminally, narrower than median ossification. The baculum of _M. quasiater_ is the largest and has the best developed base and median process of the three American species of the subgenus _Pitymys_. The three species closely resemble each other in basic form. _Specimens examined_: Five, all from Veracruz; Teocelo, 4500 ft., 30709, 30711; 4 km. N Tlapacoyán, 1700 ft., 24466; 5 km. N Jalapa, 4500 ft., 19869, 19878. Microtus (Pitymys) fatioi (Mottaz) Fig. 26 The baculum of a single specimen (KU 67103) of _M. fatioi_ from Zermatt, Valais, Switzerland, was examined. The baculum is immature, as evidenced by its small size, slender stalk and absence of ossified processes, therefore no characterization is included. The baculum of another Old World species of the subgenus _Pitymys_, _M. pyrenaicus_ from France, figured and described by Didier (1954:242-243), differs from all New World _Pitymys_ examined in processing ossified lateral processes. The status of _Pitymys_, as a genus or as a subgenus, is uncertain. Hall and Cockrum (1953:448) considered the North American _Pitymys_ and _Pedomys_ as subgenera of _Microtus_. They did not state specifically the basis for this point of view, but mention the fact that these two subgenera (_Pitymys_ and _Pedomys_) closely resemble each other cranially. These authors did not study nor comment upon the status of the Old World _Pitymys_. It may be asked whether the Old World and New World _Pitymys_ have developed as fossorial _Microtus_ independently, or from an ancestor common to both groups and not common to any other _Microtus_. Matthey (1955:202) found 62 chromosomes (2N) in both the New World _Pitymys pinetorum_ and the Old World _Pitymys duodecimcostatus_. This suggests, but does not prove, common ancestry. Neofiber alleni True Fig. 49