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.
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.
Baculum: Stalk massive, greatest length (4.7 mm.) 1-3/4 times greatest breadth, 4 times greatest depth; ossification in digitate processes variable; in one (KU 27123) of two specimens examined lateral processes ossified and median process unossified, as in two specimens examined by Hamilton (1946:379) from "southern Florida"; in my other specimen (KU 27268) that is possibly more mature, median process ossified although less deeply stained than lateral ossifications or stalk; posterior profile in probable dorsal view roughly rounded; in end-view probable dorsal concavity deep, ventral concavity broad but shallow, and with center convex; median constriction 3/5 greatest depth; shaft heavy, least depth 2/3 greatest depth of base; stalk, at mid-point, slightly wider than deep and more than 1/3 width of base; lateral profile in dorsal view sharply incurved distal to point of greatest breadth, shaft therefore relatively distinct from basal part of stalk; slight subterminal constriction; tip less reduced in the two specimens examined than in two figured by Hamilton. In preparation, the tissues that make it possible to distinguish with certainty the dorsal and ventral surfaces of the baculum were removed in both specimens.