The Baculum In Microtine Rodents Part 2

_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 monta.n.u.s_ (Figs. 19-21) for example, or in subspecies of _Lagurus curtatus_ (Dearden, 1958:542).

_Specimens examined_: Nine, of two subspecies; _Clethrionomys gapperi athabascae_, British Columbia, 42922 (Indian Creek, Mile Post 234 of Alaskan Highway), 64281 (West bank Racing River, 89 mi. W Muskwa), 64287 (North bank Tetsa River, 56 mi. W, 11 mi. S Muskwa), 64290 (44 mi. W, 9 mi. S Muskwa), 64310 (32 mi. W, 2 mi. S Muskwa); _Clethrionomys gapperi galei_, 31 mi. N Pinedale, Sublette Co., Wyoming, 42108; Grand Mesa, Delta Co., Colorado, 60014 and 60015 (5-1/2 mi. E, 12 mi. S Collbran), 60022 (8 mi. E, 1/2 mi. S Skyway).

Clethrionomys occidentalis (Merriam)

Fig. 12

Baculum: Stalk elongate, greatest length (2.8 mm.) 2-1/2 times greatest breadth, 6 times greatest depth; three well-developed ossified processes; median process larger than lateral processes, 1/2 the length of stalk, curved, basally broad, ventrally keeled, trilobate posteriorly; lateral ossifications large, flattened distally, curved; posterior profile of stalk posteriorly slightly emarginate, thus bilobate in outline; in end-view dorsal concavity deeper than ventral, constriction less than 1/2 greatest depth, tuberosities confluent, visible in dorsal view at each side; shaft slender, especially in depth, straight; at mid-point of stalk almost twice as wide as deep, slight terminal inflation.

The general proportions of the stalk and the relatively large, uniquely shaped processes, are characteristic of most specimens of the genus _Clethrionomys_ examined.

_Specimen examined_: _C. occidentalis californicus_, one from Mary's Peak, Benton Co., Oregon, 30, F. W. Sturges' collection.

Clethrionomys glareolus Schreber

Fig. 13

Baculum: Stalk elongate, greatest length (2.9 mm.) twice the greatest breadth in the specimen examined, flattened proximally, greatest length almost 6 times greatest depth of base; three well-developed ossified processes; median process arched in a dorsoventral plane, with basal notch and lateral lobes; lateral processes as long as median process, bowed in dorsal view, flattened distally, with ventromedial vane; basal tuberosities of stalk weakly developed, medially confluent; posterior profile in dorsal view evenly rounded; in end-view dorsal concavity shallow in comparison to most species but deeper than ventral concavity, constriction 3/4 greatest depth; shaft straight, at mid-point slightly wider than high, elongate, widest point of stalk less than 1/4 of total length from proximal end, slight lateral inflation at tip; lateral profile in dorsal view sloping at first abruptly and then gradually from widest point of stalk anteriorly onto shaft.

The specimen of _Clethrionomys glareolus_ figured by Ognev (1950:31) in dorsal view as I interpret it, resembles my specimen in the rounded base; in the elongate, distally inflated shaft; in the initially abrupt slope of the lateral profile in dorsal view from the greatest width of stalk anteriorly; and in the presence of three well ossified processes. Ognev's specimen differs from mine in the median process being more elongate relative to its width, and rounded proximally, lacking lateral lobes and basal notch; in lateral processes being less curved; in the greater terminal inflation of the shaft; and in the closer approximation of the terminal processes to the shaft. The baculum of _Clethrionomys glareolus_ as described and figured by Didier (1954:243-244) resembles my specimen in general proportions, but is more pointed proximally and more curved in dorsoventral plane. Didier states that the baculum is rather variable in form in this species, in different regions, but that a large number of specimens must be examined to a.s.sess the geographic nature of this variation.

_Specimen examined_: One from Zermatt, Valais, Switzerland, 67100.

Clethrionomys rufoca.n.u.s Sundevall

Fig. 9

Baculum: Base of stalk broad but relatively flattened dorsoventrally, greatest length (3.2 mm.) less than 1-1/2 greatest width, 4 times greatest depth; three well-developed ossified processes; median process arched in dorsoventral plane, having basal notch and lateral lobes; lateral processes as long as median process, flattened distally, with ventromedial vane; basal tuberosities of stalk weakly developed, medially confluent; posterior profile in dorsal view convex with rounded posterior apex; dorsal surface of base almost flat, ventral concavity broad and shallow; constriction 3/4 greatest depth (not including an unusual irregularity on the ventral surface of the base); shaft straight, at mid-point of stalk distinctly wider than high, slender at distal end, widest point of stalk almost 1/3 of total length from proximal end, tip of shaft rounded; lateral profile in dorsal view gradually sloping from widest point anteriorly onto shaft.

The specimen of _Clethrionomys rufoca.n.u.s_ figured by Ognev (1950:97) resembles my specimen in the presence of three well ossified processes.

Ognev's specimen differs however in the lack of a proximal notch on the median process, the lesser proportion of the stalk included in the basal enlargement, the more posterior position of the point of greatest width, and the presence of a concavity in the posterior profile of the stalk in dorsal view. These differences in the stalk may be owing to a difference in age (my specimen perhaps being older).

_Specimen examined_: One from 1 mi. NW Oho-ri, 6 M., Korea, 60438.

Phenacomys intermedius Merriam

Figs. 7 and 8

Baculum: Stalk slender, greatest length (2.9 mm.) 2-1/4 to 2-1/2 times greatest breadth, 4 times greatest depth; three well-developed ossified processes, median one almost 1/2 length of stalk, curved, broad basally and slightly larger in all dimensions than either lateral process; lateral processes flattened distally, curved; base of stalk well developed, basal tuberosities medially confluent or separated by medial emargination, posterolateral faces flattened or rough; emarginations in the four adults examined; posterior profile in dorsal view bluntly pointed or flattened except for emargination posterially, abruptly curved at point of greatest width; shaft arising broadly from distal side of base of stalk; in end-view hour-gla.s.s shaped, medial constriction p.r.o.nounced, both dorsal and ventral concavities deep; shaft having relatively straight but distally convergent sides; at mid-point of stalk, 1 to 1-1/2 times as wide as deep; tip bluntly rounded, or slightly inflated.

The specimens from Quebec differ from the one from Wyoming in smaller size, relatively smaller lateral digital processes, larger more medial basal emargination, and slender shafts. The baculum of _Phenacomys intermedius_ differs much from that of _Phenacomys longicaudus_, described by Hamilton (1946:381) and by Dearden (1958:547). Dearden states that the three bacula examined by him of _Phenacomys longicaudus_ differ markedly from the specimen described by Hamilton. It seems to me that in major features the resemblance is greater between the specimens of _Phenacomys longicaudus_ examined by these two authors than between their specimens and specimens of other microtines, including _Phenacomys intermedius_. Neither Hamilton nor Dearden record the exact localities of capture, the collections in which the specimens are deposited, or the catalogue numbers of specimens. Consequently verification of identifications and observations is difficult.

_Specimens examined_: Five, of two subspecies; _P. intermedius intermedius_, 5.4 mi. S Moran, Teton Co., Wyoming, 3-C-309, collection of W. B. Quay; _P. intermedius celatus_, four (including one immature specimen) from Authiernord, Abitibi-ouest Co., Quebec, specimens in collection of Bristol Foster designated by numbers 2041-2044 of S.

Anderson's field catalogue. Smith and Foster (1957:107) were of the view that _Phenacomys ungava_ (including the above specimens from Quebec) may be specifically distinct from _Phenacomys intermedius_.

Ondatra zibethicus (Linnaeus)

Not figured

Baculum: In the single specimen examined, less mature than that figured by Hamilton (1946:384), the digitate processes are cartilaginous, the basal tuberosities are less well developed, and the shaft is slenderer throughout. The cartilaginous processes are of the same proportions as ossified processes in the figure mentioned. The shaft is also convex ventrally in lateral profile. The view of the side here considered to be anatomically the ventral side (adjacent to the urethra) is labelled dorsal view in Hamilton's specimen.

_Specimen examined_: One, from Reserve, Brown Co., Kansas, 72405.

Microtus (Herpetomys) guatemalensis Merriam

Figs. 42 and 43

Baculum: Stalk moderately elongate, greatest length (3.5 mm.) 2-1/3 times greatest breadth, spatulate, flattened throughout, greatest thickness 1/3 millimeter; three ossified processes; median process having three cornered base, curved dorsally, wider than high, 1/4 to 1/5 greatest length of stalk; each lateral process bent at middle, as long as median process, compressed laterally; base of stalk curved dorsally, tuberosities marginal, hence narrow, lateral excavations of tuberous margin not confluent medially; in end-view ventral concavity broad, no dorsal concavity, medial constriction but slightly less than greatest thickness (not depth); shaft wider than high throughout, at mid-point more than 3 times as wide as high; tip of shaft slightly inflated both laterally and dorsoventrally; lateral profile gradually sloping anteriorly from widest point of stalk.

Specimen number 65921 (Fig. 43) differs from number 65895 (Fig. 42) described above. Terminus of shaft of number 65921 has lateral lobes from which arise lateral cartilaginous processes; median terminal ossification irregular in shape, smaller, imbedded in terminally bilobate cartilage.

In the spatulate flattened stalk these two specimens are much alike. An immature specimen, number 65908, is smaller (length of stalk 2.6 mm.) also flattened and spatulate, has the terminal processes cartilaginous, the lateral processes bent medially, and proportions as in the adult.

The baculum shows no noteworthy resemblance to that of any other species of North American _Microtus_; on the other hand the differences between _M. guatemalensis_ and some other species are no greater than the differences between certain species included in the subgenus _Microtus_.

The baculum neither strengthens nor weakens the case for subgeneric rank for _M. (Herpetomys) guatemalensis_.

_Specimens examined_: Three from Guatemala; 65895 (2 mi. S San Juan Ixcoy), 65908, (3-1/2 mi. SW San Juan Ixcoy), 65921 (10 mi. E, 4 mi. S Totonicapan).

Microtus (Arvicola) richardsoni (DeKay)

Figs. 38 and 39