Baculum: Stalk heavy, broad, greatest length (2.8 mm.) in mature individuals (Fig. 2) as little as 1-1/3 times greatest breadth, greatest length no less than 2-2/3 times greatest depth of base; three ossified processes, median one from as long as to 1/2 longer than the lateral processes, and approximately 2/3 wider and twice as deep as lateral processes; length of median process almost 3-1/2 times its breadth, approximately 1/2 length of stalk; basal fossae broadly confluent; posterior profile in dorsal view evenly rounded; in end-view ventral concavity deeper than dorsal concavity, constriction as little as 1/2 greatest depth in mature specimens; shaft straight, bluntly rounded, or slightly decurved and laterally inflated terminally; lateral profile in dorsal view a gradual slope from widest point of stalk anteriorly onto shaft; in younger individuals stalk slenderer, otherwise as described above. Five specimens examined by me differ from one figured and described by Hamilton (1946:379) in that stalk is better developed, larger relative to size of processes, length of stalk in my specimen (Fig. 2) 2.8 as opposed to 2.1 mm. in Hamilton's specimen; median process shorter, 1.5 as opposed to 1.8 mm., proximal end rounded rather than concave, not partially enclosing tip of shaft; proportion of and relative sizes of median and lateral processes approximately same as in Hamilton's _Lemmus helvolus_ [ = _Lemmus trimucronatus helvolus_]. A specimen figured by Dearden (1958:542) has a basally trilobed median process. The baculum of the Asiatic _Lemmus lemmus_ figured by Ognev (1948:413) agrees with my specimens in the ossification of three processes, the relative sizes of these processes to each other and to the stalk, the well-developed base of the stalk and heavy bluntly rounded shaft; the baculum of _Lemmus lemmus_ differs in greater anterolateral extent of basal tuberosities, in proximal notch seemingly separating these tuberosities, and in median process being slenderer. _Specimens examined_: Five, of two subspecies; _Lemmus trimucronatus alascensis_, Point Barrow, Alaska, numbers 50591, 50678, 50731, 50758; _Lemmus trimucronatus subarcticus_, Wahoo Lake, 69°08', 146°58', 2350 ft., Brooks Range, Alaska, 50948. Synaptomys cooperi Baird Figs. 5 and 6