Dicrostonyx groenlandicus (Traill)
Figs. 3 and 4
Baculum: stalk elongate, greatest length (3.1 mm.) 2 1/5 to 2½ times greatest breadth, and 4½ times greatest depth; digitate processes usually cartilaginous, occasionally lateral processes partly ossified; basal tuberosities weakly to moderately developed, medially confluent; posterior profile in dorsal view rounded with rounded posterior apex or shallow notch; dorsal concavity in end-view shallower and not so wide as ventral concavity; median constriction approximately 2/3 greatest depth; ventral part of base in end-view wider than dorsal part; shaft straight or slightly curved; base of stalk placed dorsally relative to axis of shaft; stalk spatulate, sometimes with distal enlargement; at mid-point stalk wider than high; lateral profile in dorsal view sloping gradually without abrupt curvature anterior to point of greatest width.
The baculum of Dicrostonyx torquatus figured by Ognev (1948:476) agrees with that of D. groenlandicus in shape of stalk, and in lateral digitate processes that are small relative to size of median process; but differs in more elongate, terminally enlarged, bulbar shape of median process. None of my specimens showed ossification in the lateral processes, observed by Hamilton (1946:381) in Dicrostonyx rubricatus richardsoni [ = D. groenlandicus richardsoni]. In all of my specimens the cartilaginous median process was larger than that figured by Hamilton, or by Dearden (1958:542).
Specimens examined: Four from; Point Barrow, Alaska, 52524 (Barrow Village), 67264 (died in captivity); Brooks Range, Alaska, 50536 (Wahoo Lake, 69°08', 146°58'), 50539 (Porcupine Lake, 68°51'57", 146°29'50", 3140 ft.).
Lemmus trimucronatus (Richardson)
Fig. 2
Baculum: Stalk heavy, broad, greatest length (2.8 mm.) in mature individuals (Fig. 2) as little as 11/3 times greatest breadth, greatest length no less than 22/3 times greatest depth of base; three ossified processes, median one from as long as to ½ longer than the lateral processes, and approximately 2/3 wider and twice as deep as lateral processes; length of median process almost 3½ times its breadth, approximately ½ 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 ½ 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.