class="bump3">(Microtus) arvalis (Pallas)

1 (Microtus) orcadensis Millais 1 (Microtus) guentheri Danford and Alston 1 (Microtus) fortis Büchner 2 (Microtus) montanus (Peale) 15 (Microtus) townsendii (Bachman) 3 (Microtus) oeconomus (Pallas) 10 (Microtus) mexicanus (Saussure) 13 (Microtus) californicus (Peale) 2 (Microtus) pennsylvanicus (Ord) 13 (Microtus) agrestis (Linnaeus) 1 (Pedomys) ochrogaster (Wagner) 41 (Pitymys) pinetorum (LeConte) 2 (Pitymys) parvulus (Howell) 1 (Pitymys) quasiater (Coues) 5 (Pitymys) fatioi Mottaz 1 Genus: Neofiber True, 1884 Neofiber alleni True 2 Genus: Lagurus Gloger, 1841 Lagurus curtatus (Cope) 7 Total number examined 184

ACCOUNTS OF SPECIES

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.

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

Baculum: Stalk elongate, greatest length (2.7 to 2.8 mm.) 2 ¹/₃ to 2½ times greatest breadth, 4 to 5 times greatest depth; three processes ossified or lateral processes unossified, ossifications relatively small (in 78380, median ossification less than ¼ as large as lateral ossifications although median cartilaginous process is larger), length of median process ¹/₅ to ¹/₆ of length of stalk, cartilaginous part of median process larger; posterior profile in dorsal view convex throughout or bilobate; tuberosities moderately developed, deflected dorsal to axis of shaft; in end-view medial construction ³/₅ greatest depth of tuberosities; shaft tapered from point of greatest width, slightly inflated terminally.

The specimen (KU 13716) figured by Hibbard and Rinker (1942:29) has been restudied. It was first cleared and stained to soften the dry cartilage binding the digital processes together and to differentiate bone and cartilage. The lateral processes are small and cartilaginous (Fig. 6) and seem intact. The differences between this specimen and others examined by Hamilton (1946:381), Dearden (1958:542), and myself, namely the relatively larger median ossification, the absence of ossification in lateral processes, and the distinctly bilobate base and larger size, may represent geographic differences, or individual variation. The proportions of length, width, and depth of the stalk,