The Baculum In Microtine Rodents Part 4

Microtus guentheri Danford and Alston

Fig. 23

Baculum: In the one specimen examined, stalk broad, greatest length (2.9 mm.) 1-1/2 times greatest breadth, 3-1/2 times greatest depth; three digitate processes ossified; median process slightly less than 1/2 length of stalk, broad, dorsally curved; curved lateral ossifications shorter and more slender than median ossification; basal tuberosities well developed, angular, confluent across posterior border of projecting shelf; in end-view tuberosities projecting ventrolaterally from central shelf; dorsal surface at medial constriction flat, ventral surface broadly and deeply concave; posterior profile in dorsal view trilobate, central lobe formed by posteriorly flattened shelf, surface of attachment visible only on lateral lobes; at mid-point stalk almost twice as wide as deep, depth of shaft greater than width proximal to inflated terminus.

_Specimen examined_: One from Palestine, 67104.

Microtus fortis Buchner

Fig. 25

Baculum: Stalk large, greatest length (3.8 mm.) 1-4/5 times greatest breadth, 4-1/2 times greatest depth; three digitate processes ossified; median ossification almost 1/3 length of stalk; lateral ossifications slender, smaller than median ossification; posterior profile of stalk in dorsal view trilobate, basal tuberosities well developed, confluent medially; in end-view dorsal concavity broader and deeper than ventral concavity; medial constriction p.r.o.nounced (less than 1/2 greatest depth); lateral profile at widest point of stalk convex, becoming abruptly concave as the f.l.a.n.g.e of the basal tuberosities grades into the shaft, then gradually converging to narrowest point 1/3 of length of stalk from the terminus; stalk wider than deep in proximal 2/3, circular in cross section in terminal 1/3, slight terminal inflation.

A specimen figured by Ognev (1950:297) has the same general proportions, slender lateral processes, and proximal placement of the point of greatest breadth.

_Specimens examined_: Two from Chipo-ri, Korea, 60443, 63841.

Microtus monta.n.u.s (Peale)

Figs. 19, 20 and 21

Baculum: Stalk broad, greatest length (varying with subspecies from 2.3 to 3.1 mm.) 1-1/2 to 1-3/4 times greatest breadth, 3-1/3 to 4-1/3 times greatest depth; three ossified processes, median one largest, more than twice as wide and as deep as shorter, slenderer, lateral processes; median process laterally compressed distally except in one specimen in which moderately inflated distally, proximally enlarged in some specimens (Fig. 21) and 1/3 to 2/5 length of stalk; base broad, posterior profile in dorsal view evenly convex throughout, at widest point of stalk abruptly incurved; basal tuberosities moderately to strongly developed, medially confluent; in end-view base wider ventrally than dorsally, dorsal concavity slightly to much deeper than the nearly flattened ventral concavity; medial constriction 2/3 to 4/5 of greatest depth; shaft relatively slender, at mid-point of stalk slightly wider than high and 1/4 as wide as base of stalk, terminally rounded or slightly inflated; lateral profile in dorsal view a gradual curve from point of greatest width anteriorly onto shaft.

The different subspecies figured show the essential characteristics of the species, differing primarily in size.

_Specimens examined_: Fourteen, of three subspecies; _Microtus monta.n.u.s amosus_, 1/2 mi. E Soldier Summit, Wasatch Co., Utah, 62241; _M. monta.n.u.s fusus_, La Manga Pa.s.s, Conejos Co., Colorado, 42164; 5 mi. N, 26 mi. W Saguache, 9500 ft., Saguache Co., Colorado, 42307, 42315; 5 mi. N, 27 mi.

W Saguache, 9350 ft., Saguache Co., Colorado, 42308; 5 mi. N, 28 mi. W Saguache, 9325 ft., Saguache Co., Colorado, 42309; 5 mi. S, 24 mi. W Antonito, 9600 ft., Conejos Co., Colorado, 42327, 42330; Prater Canyon, Mesa Verde National Park, Montezuma Co., Colorado, 69456, 69457, 69463; _Microtus monta.n.u.s na.n.u.s_, 2 mi. N, 2 mi. W Pocatello, Bannock Co., Idaho, 57470, 57472; 3/4 mi. N, 2 mi. W Allenspark, 8400 ft., Boulder Co., Colorado, 50330.

Microtus townsendii (Bachman)

Fig. 41

Baculum: Stalk broad, greatest length (3.0 mm.) 1-1/2 times greatest breadth, 4-1/2 times greatest depth; three ossified processes, median one largest, deeper and more than twice as wide as curved, shorter, compressed lateral processes and more than 2/5 as long as stalk; base broad, in dorsal view posterior profile trilobate, basal tuberosities visible; basal tuberosities well developed, medially confluent; in end-view base wider ventrally than dorsally, dorsal concavity deeper than ventral concavity; medial constriction 3/5 of greatest depth; shaft broad, at mid-point more than twice as wide as high and 1/3 as wide as base of stalk, terminally rounded.

_Specimens examined_: Three, all _M. t. townsendii_; Fort Lewis, Pierce Co., Was.h.i.+ngton, 57998, subadult; Sec. 33, T. 11S, R. 5W, Benton Co., Oregon, 79186; Sec. 5, T. 12S, R. 4W, Benton Co., Oregon, 79188.

Microtus oeconomus (Pallas)

Fig. 44

Baculum: Stalk broad and flattened, greatest length (3.5 mm.) 1-2/3 to 2 times greatest width, 4 to 5-1/2 times greatest depth; three ossified processes, median one largest, lateral processes slender, relatively small; length of median process 3/8 length of stalk; median process decurved, dorsoventrally flattened in some specimens, widened at base; attachment of processes to shaft displaced ventrally; base of stalk widened, posterior profile in dorsal view usually trilobate, in a few cases rounded, median lobe forming posterior shelf, lateral lobes dorsally raised and forming margins of lateral tuberosities; in end-view thickness frequently more or less uniform throughout central part, broad depression dorsally, ventral concavity narrower and shallower (as figured); base, and occasionally shaft, flattened, width at mid-point of stalk 2 to 3 times depth, narrowest point posterior to terminal inflation of shaft in terminal 1/3 of shaft.

The baculum of _M. oeconomus_ (Old World) figured by Ognev (1950:257) resembles but exceeds that of _M. oeconomus_ (New World) in the relatively large median process and slender lateral processes, but differs noticeably in the presence of a deep median notch in the base of the stalk. A specimen from Hungary is intermediate between Ognev's specimen and those from the New World in both size of median process and size of lateral processes, and has an unnotched base resembling that in Figure 44.

_Specimens examined_: Ten, of three subspecies; _M. oeconomus gilmorei_, Umiat, Alaska, 51354, 51361, 51399, 51408; Lake Schrader, Brooks Range, Alaska, 51422; _M. o. macfarlani_, 5 mi. NNE Gulkana, Alaska, 43039, 43041; 20 mi. NE Anchorage, Alaska, 43044; Kelsall Lake, British Columbia, 43048; _M. o. mehelyi_, Kisbalatan, Hungary, 75159.

Microtus mexica.n.u.s (Saussure)

Figs. 35 and 36

Baculum: Stalk attenuate, greatest breadth relatively near proximal end; greatest length (3.1 to 3.4 mm.) more or less twice greatest breadth, 4 to 5 times greatest depth; usually a single process ossified; lateral processes relatively small, cartilaginous or (in three specimens, 63094, 69453, 68019) with small ossifications; median process relatively small, sometimes appressed to tip of shaft, in length less than 1/4 length of stalk; posterior profile in dorsal view rounded, flattened posteriorly, or in some specimens trilobate with angular edges; in end-view relative depths of dorsal and ventral concavities variable, dorsal usually deeper than ventral; distal end of stalk frequently bowed dorsally; shaft slender distally, sometimes slightly inflated terminally, or (in one specimen, 63085) near tip small lateral projections that are perhaps fused lateral ossifications; lateral profile in dorsal view a gradual slope anteriorly from point of greatest width to slender tip.

_Specimens examined_: Thirteen, of four subspecies; _Microtus mexica.n.u.s mexica.n.u.s_, Las Vigas, Veracruz, 30692; Nevada de Toluca, Mexico, 63101; Valle de Bravo, Mexico, 63094; _Microtus mexica.n.u.s mogollonensis_, Mt.

Taylor, Valencia Co., New Mexico, 63298, 76830; Park Well, Mesa Verde National Park, Montezuma Co., Colorado, 69448, 69453; Upper Nutria, McKinley Co., New Mexico, 69997, 70000; _Microtus mexica.n.u.s phaeus_, Sierra Patamba, 9000 ft., Michoacan, 63085; _Microtus mexica.n.u.s subsimus_, 2 mi. E Mesa de Tablas, Coahuila, 58916; 13 mi. E San Antonio de las Alazanas, Coahuila, 68019, 68021.

Microtus californicus (Peale)

Fig. 37

Baculum: Stalk elongate, greatest length (3.0 mm.) 2-1/3 times greatest breadth, 4-1/2 times greatest depth; median process ossified, 1/4 length of stalk, basally broadened, flattened and shallowly grooved ventrally to fit tip of shaft, to which the process is closely appressed; lateral processes cartilaginous; ends of stalk bowed upwardly; posterior profile of base of stalk rounded or slightly trilobate if posterolateral concavities form in tuberosities; moderate development of tuberosities, in end-view dorsal concavity slightly deeper and narrower than ventral concavity, both comparatively shallow, median constriction 4/5 greatest depth; shaft curved, more or less terete at mid-point of stalk, terminally inflated dorsally; lateral profile in dorsal view gradually curved from point of greatest width anteriorly onto shaft.

_Specimens examined_: Two, of two subspecies; _Microtus californicus californicus_, 1 mi. NE Berkeley, in Contra Costa Co., California, 76828; _Microtus californicus mohavensis_, 1/2 mi. SE Victorville, San Bernardino Co., California, 63745.

Microtus pennsylvanicus (Ord)

Figs. 14, 15, 16 and 17

Baculum: Stalk heavy, broad, greatest length (2.2 to 3.0 mm.) 1-1/3 to 1-2/3 times greatest breadth, up to 3-3/4 times greatest depth; three ossified processes, median one largest, usually not twice so deep as lateral ossifications; median process usually distinctly widened basally, in length approximately 1/2 length of stalk; base broad, frequently angular laterally and basally, sometimes bilobate; basal tuberosities well developed, medially confluent; in end-view more or less uniformly biconvex or ventral surface more flattened than dorsal surface, medial constriction 1/2 to 2/3 greatest depth; shaft relatively heavy, at mid-point stalk almost twice as wide as deep and 1/3 as wide as base of stalk; shaft terminally rounded and sometimes slightly inflated; lateral profile in dorsal view abruptly or gradually curved anterior to point of greatest width and then gradually curved anteriorly.

Specimens examined averaged slightly smaller and were more variable than those described by Hamilton (1946:382). The greater variation may be in part geographic, as five subspecies are represented. Lateral processes are the last to ossify. One specimen (75082) with well-ossified median process lacks any lateral ossification. Four bacula of _M.

pennsylvanicus_ (locality not specified) studied by Dearden (1958:547) agree in general with the description above.

One specimen shows a break, perhaps resulting from injury, in the shaft (Fig. 14). One specimen has a posteromedian spine on the median digital ossification (Fig. 16). Comparison with _M. agrestis_ is included with the description of _M. agrestis_.

_Specimens examined_: Thirteen, of six subspecies; _Microtus pennsylvanicus alcorni_, 20 mi. NE Anchorage, Alaska, 43043; _Microtus pennsylvanicus finitus_, Laird, Yuma Co., Colorado, 68544; _Microtus pennsylvanicus modestus_, 5 mi. N, 26 mi. W Saguache, 9500 ft., Saguache Co., Colorado, 42306; 3 mi. N, 16 mi. W Saguache, 8500 ft., Saguache Co., Colorado, 42416, 42417, 42418; 1 mi. S, 2 mi. E Eagle Nest, 8100 ft., Colfax Co., New Mexico, 42430, 42439; _Microtus pennsylvanicus pennsylvanicus_, 2 mi. S, 3 mi. E Ft. Thompson, 1370 ft., Buffalo Co., South Dakota, 42379; Vermillion, Clay Co., South Dakota, 37070; _Microtus pennsylvanicus pullatus_, 12 mi. S, 5 mi. E b.u.t.te, Silver Bow Co., Montana, 57501, 57503; _Microtus pennsylvanicus uligocola_, Muir Springs, 2 mi. N, 2-1/2 mi. W Ft. Morgan, Morgan Co., Colorado, 75082.

Microtus agrestis (Linnaeus)

Fig. 18

Baculum: Greatest length of stalk (2.9 mm.) twice greatest breadth, 4-1/2 times greatest depth; stalk well developed, shaft not flattened dorsoventrally; large median ossified process, minute lateral ossifications in single specimen examined; length of stalk 2-1/2 times length of median ossification which is higher than wide, slightly decurved, sagittate in dorsal view, with three-cornered base; basal tuberosities of stalk moderately well developed, medially joined; posterior profile in dorsal view evenly rounded; ventral concavity broader than, but of comparable depth to, dorsal concavity in end-view, base of stalk wider ventrally, constriction 3/4 greatest depth; at mid-point of stalk shaft is but slightly wider than high; p.r.o.nounced terminal inflation of shaft; lateral profile in dorsal view sloping abruptly from widest point of stalk anteriorly onto stalk which then tapers more gradually to terminal inflation.

From the baculum of its New World counterpart, namely _Microtus pennsylvanicus_, my specimen of _Microtus agrestis_ and the specimen figured by Didier (1954:239) differ in their minute lateral processes, relatively larger median processes, and more elongate, less dorsoventrally flattened shafts.

The specimen of _M. agrestis_ figured by Ognev (1950:320), in dorsal view has lateral concavities producing a somewhat trilobate outline in the base of the stalk, and the lateral processes are well developed; the median process is larger and bulbous, wider distally than proximally.

Without larger numbers of bacula of _M. agrestis_ I am unable to reconcile these differences. The differences between _M. agrestis_ and _M. pennsylvanicus_ seem comparable to the differences between some other species of _Microtus_.

_Specimen examined_: One, from Gryon, Switzerland, 67102.