Part 4

Subfamily: Microtinae Number of Specimens Tribe: Lemmi Dicrostonyx groenlandicus (Traill) 4 Lemmus trimucronatus (Richardson) 6 Synaptomys cooperi Baird 5 Tribe: Microti Genus: Clethrionomys Tilesius, 1850 Clethrionomys rutilus Pallas 4 Clethrionomys gapperi (Vigors) 9 Clethrionomys occidentalis (Merriam) 1 Clethrionomys glareolus Schreber 1 Clethrionomys rufocanus Sundevall 1 Genus: Phenacomys Merriam, 1897 Phenacomys intermedius Merriam 5 Genus: Ondatra Link, 1795 Ondatra zibethicus (Linnaeus) 1 Genus: Microtus Schrank, 1798 (Herpetomys) guatemalensis Merriam 3 (Arvicola) richardsoni (DeKay) 2 (Chilotus) oregoni (Bachman) 3 (Stenocranius) gregalis (Pallas) 1 (Stenocranius) miurus Osgood 9 (Chionomys) longicaudus (Merriam) 6 (Chionomys) nivalis Martins 2 (Microtus) arvalis (Pallas) 1 (Microtus) orcadensis Millais 1 (Microtus) guentheri Danford and Alston 1 (Microtus) fortis Buechner 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 2-1/2 times greatest breadth, and 4-1/2 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 deg.08', 146 deg.58'), 50539 (Porcupine Lake, 68 deg.51'57", 146 deg.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 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 deg.08', 146 deg.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 2-1/2 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 1/4 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, and the appearance in lateral view do not differ greatly from others examined by Hamilton, by Dearden (1958:546), and by me.

Specimens examined: Five, representing four subspecies; S. cooperi gossii, 6 mi. N Midway, Holt Co., Nebraska 78379, 78380; S. cooperi relictus, 5 mi. N, 2 mi. W Parks, Dundy Co., Nebraska, 72601 (immature); S. cooperi saturatus, 3 mi. S Demotte, Jasper Co., Indiana, 3-C-454, collection of W. B. Quay; S. cooperi paludis, Meade County State Park, Kansas, 13716.

Clethrionomys rutilus Pallas

Fig. 11

Baculum: Stalk elongate, and proximally enlarged, greatest length (2.7 mm.) 2 times greatest breadth; less than 4 times greatest depth; three well-developed ossified processes; length of stalk 2-1/3 times length of median process; median process with basal (and ventral) protuberence and lateral lobes, arched in dorsoventral plane; lateral processes as large as median process, flattened distally, having ventromedial vane on distal half; basal tuberosities of stalk well developed, medially confluent; posterior profile in dorsal view trilobate or convex throughout with rounded posterior apex; dorsal concavity well developed, ventral surface but slightly concave, medial constriction of base as little as 1/2 greatest depth; shaft straight, slender, at mid-point of stalk but slightly wider than high; basal tuberosities largely dorsal to axis of shaft in lateral view; lateral profile in dorsal view with an abrupt curvature separating the gently sloping sides of the shaft from the basal part at its greatest breadth.

The specimen of Clethrionomys rutilus figured by Ognev (1950:120) is essentially like the North American specimens examined by me in the relative sizes of the ossifications and the general shape of the stalk.

Specimens examined: Four, of one subspecies; C. r. dawsoni, west bank Gakona River, 1700 ft., 5 mi. NNE Gulkana, Alaska, 42865, 42866; SW end Dezadeash Lake, 2400 ft., Yukon Territory, 42910, 42921.

Clethrionomys gapperi (Vigors)

Fig. 10

Baculum: Stalk elongate, greatest length (2.8 mm.) 1-3/4 times greatest breadth, and 3-3/4 times greatest depth; proximally enlarged, greatest depth 1/2 greatest breadth; three well-developed ossified processes; length of stalk 2-1/3 times length of median process; median process arched in dorsoventral plane, with basiventral protuberence or spine and lateral lobes; lateral processes as large as median process, flattened distally, arched; basal tuberosities of stalk well developed, medially confluent; posterior profile in dorsal view trilobate or convex throughout with a rounded posterior apex; dorsal concavity well developed, ventral surface but slightly concave, or in some cases slightly convex; medial constriction of base 3/5 greatest depth; shaft straight, slender, at mid-point of stalk twice as wide as high; basal tuberosities dorsally placed relative to axis of shaft; lateral profile in dorsal view abruptly curved anterior to point of greatest width; slender stalk distinct from angular enlarged base.

The most noticeable difference between the baculum of C. rutilus and C. gapperi is size. The proportions of the four ossifications are approximately the same. Ventral vanes on the lateral processes are not developed in C. gapperi. C. gapperi and C. rutilus are more nearly alike in their bacula than any other two species of Clethrionomys examined. Clethrionomys occidentalis, the other New World species, is also much like C. gapperi and C. rutilus. The differences are of a magnitude comparable to those between the bacula in subspecies of Microtus montanus (Figs. 19-21) for example, or in subspecies of Lagurus curtatus (Dearden, 1958:542).