The Baculum In Microtine Rodents Part 3

Baculum: Stalk broad, greatest length (3.7 to 4.3 mm.) 1-1/2 times greatest breadth, relatively flattened, greatest depth 1/3 greatest breadth; single median ossified process, in smaller of two specimens this ossification incomplete and of unusual shape (Fig. 39); length of stalk 4 times length of median process; concavities of basal tuberosities medially confluent, constriction less than 1/2 greatest depth; widest point of shaft less than 1/4 length of shaft from posteriormost point; shaft wider than high except at distal end that is inflated dorsally and sometimes laterally; both ventral and dorsal concavities of base of stalk broad and moderately deep; posterior profile in dorsal view evenly rounded or having marginal notch.

In the absence of ossified lateral processes my two specimens differ from bacula of _Microtus (Arvicola) terrestris_ figured by Didier (1943:79, 1954:245, 247, 248) and by Ognev (1950:591). The median process relative to the size of the shaft is smaller, and the shaft relative to its length is wider in _M. richardsoni_ than in _M. terrestris_. The stalk of _M.

(Arvicola) amphibius_ figured by Didier is like that of _M. richardsoni_ in its greater breadth and median notch on posterior border.

The relationship of the New World water rat, _M. richardsoni_, to the Old World water rats (genus _Arvicola_ of some European authors) is uncertain. Miller (1896:66) placed all of them in the subgenus _Arvicola_. Subsequent authors, stressing differences in the teeth, have placed _M. richardsoni_ in the subgenus _Aulacomys_ of Rhoads. Zimmerman (1955) has shown that teeth in some _Arvicola_ approach the more complex pattern of _M. richardsoni_. He argues also that _Arvicola_ is generically distinct from _Microtus_ on the grounds that the two groups have separate origins, _Arvicola_ having descended from the genus _Mimomys_ and _Microtus_ from some other group of microtines. This argument also was advanced by Hinton (1926:47-48). Pending further studies of the possible polyphyletic origin of other subgenera of the genus _Microtus_, I refer both _M. richardsoni_ and _M. terrestris_ to the subgenus _Arvicola_.

The evidence afforded by the bacula available is not conclusive as to relations of Old World and New World water rats. No general agreement on the number of species in this Palaearctic group has been reached, and bacula of only three or four of the numerous Old World subspecies have been figured. I have examined none.

_Specimens examined_: Two, from Wyoming; 42454 (31 mi. N Pinedale, 8025 ft., Sublette Co.), 37903 (23-1/2 mi. S, 5 mi. W Lander, 8600 ft., Fremont Co.).

Microtus (Chilotus) oregoni (Bachman)

Fig. 45

Baculum: Stalk broad, greatest length (2.2 mm.) 1-3/4 times greatest breadth, 3-1/2 times greatest depth; three well-developed ossified processes; median process 2/5 length of stalk, rounded or tapered terminally, proximal end opposed to tip of stalk and flattened obliquely; lateral processes 2/3 length of median process, deeper than wide, curved; tuberosities of stalk well developed, confluent medially, visible in dorsal view; in end-view dorsal concavity narrow, moderately deep, rounded, ventral concavity wide, deep, flattened; base wider ventrally than dorsally; shaft tapering more or less uniformly, terminally inflated.

In the relative sizes, to each other and to the stalk, of the three digitate ossifications _M. oregoni_ resembles closely the Old World representative of the same subgenus, _M. (Chilotus) socialis_, as figured by Argyropulo (1933b:181). In _M. oregoni_ the greatest width of the baculum is more proximal on the stalk than in the _M. socialis_ figured by Argyropulo but closely resembles the baculum of the _M. socialis_ figured by Didier (1954:242). In possessing a shallow emargination in the base of the stalk and in possessing a median process that is smaller than the lateral processes, _M. socialis_, as figured by Didier, differs from _M. oregoni_. The baculum figured by Argyropulo (_loc. cit._) of _Sumeriomys colchicus schidlovskii_ [ = _Microtus (Chilotus) socialis schidlovskii_ according to Ognev, 1950:392] differs from other _Chilotus_ that have been studied in having an unusually elongate median process and a more distal placement of the widest part of the stalk.

_Specimens examined_: Three, of the subspecies _M. oregoni oregoni_, from 5 mi. N Orick, Humboldt Co., California, 3-C-248, collection of W. B.

Quay; from Mary's Peak, Benton Co., Oregon, 66, collection of F. W.

Sturges; and from Sec. 3, T. 11S, R. 5W, Benton Co., Oregon, 79183.

Microtus (Stenocranius) gregalis (Pallas)

Fig. 34

Baculum: Length of stalk (2.4 mm.) 1-3/4, times greatest breadth, 4-1/3 times greatest depth; median ossified process well developed, more than 1/3 length of stalk, higher than wide, slightly bowed, closely appressed to terminus of shaft; basal tuberosities of stalk moderately developed, confluent medially, posterior profile of medial apex rounded in dorsal view, lateral indentations present, hence trilobate outline; in proximal end-view base wider ventrally, ventral concavity broader than dorsal concavity but of equal depth, medial constriction 2/3 greatest depth; shaft slender in distal part, inflated terminally, and wider than high at mid-point of stalk; lateral profile a smooth slope of gradually decreasing curvature from point of greatest width to near distal end.

The baculum of this species figured by Ognev (1950:461) differs in having lateral ossified processes, and a more rounded base of the stalk.

Resemblance to the New World _Stenocranius_ is discussed below.

_Specimen examined_: One from "Eastern Europe," 8059.

Microtus (Stenocranius) miurus Osgood

Figs. 32 and 33

Baculum: Length of stalk (2.8 mm.) 1-1/2 times greatest breadth, 3-1/2 times greatest depth; median process ossified, 2/5 to 3/5 length of stalk, laterally compressed, sometimes arched in dorsoventral plane; lateral processes cartilaginous, slender; basal tuberosities well developed, averaging less enlarged than shown in Figure 32, but more angular in lateral outline than shown in Figure 33; tuberosities confluent posteriorly; posterior profile smoothly rounded to trilobate, curvature at point of greatest breadth usually acute; in proximal end-view base wider dorsally, deep dorsal concavity, shallow ventral concavity, medial constriction 3/5 of greatest depth; shaft slender anteriorly, at mid-point of stalk twice as wide as high, at tip higher than wide, laterally inflated; lateral profile in most specimens abruptly curved anterior to point of greatest breadth.

The single specimen of the Old World _M. (Stenocranius) gregalis_ examined resembles the New World _M. (Stenocranius) miurus_ in the angular lateral profile at the point of greatest breadth of the stalk, slender shaft in comparison to broad base of stalk, and presence of a single well-developed laterally compressed median process. The base of the stalk in the baculum of _M. gregalis_ is less well developed and smaller than in the baculum of _M. miurus_.

_Specimens examined_: Nine, all of the subspecies _Microtus miurus muriei_, from the Brooks Range, Alaska; 51077 (Lake Schrader, 14509'50", 6924'28", 2900 ft., Romanzof Mts.); 51151, 51152, 51154, 51164, 51166, 51169 (last 6 from Wahoo Lake, 6908', 14658', 2350 ft.); 51210, 51213 (last 2 from Porcupine Lake, 6851'57", 14629'50", 3140 ft.).

Microtus (Chionomys) nivalis Martins

Fig. 47

Baculum: Greatest length of stalk (2.7 mm.) 2-1/4 times greatest breadth, 4-1/2 times greatest depth; three digitate processes, lateral processes mostly cartilaginous in single adult examined; median process well ossified, approximately 1/3 length of stalk, basally notched, not arched, laterally compressed distally; base of stalk broad and flat, basal tuberosities well developed, separate; posterior profile in dorsal view rounded, convex except for medial notch separating tuberosities; dorsal and ventral concavities deep, broad, equal; medial constriction less than 1/2 greatest depth; in dorsal view shaft tapering gradually from widest point, terminally rounded; at mid-point of stalk almost twice as wide as high.

In the elongate, largely cartilaginous lateral processes of the baculum, the specimen described above resembles _M. longicaudus_. The size of the median process in comparison to the size of the stalk is also the same.

The lateral processes have larger ossifications and the base of the stalk is more robust in _M. longicaudus_ than in _M. nivalis_.

The well ossified lateral processes and enlarged base of Didier's (1954:240) specimen suggest that it is of a more mature individual than the one described above. These specimens of _M. nivalis_, as well as the specimens of _M. longicaudus_, exhibit dorso-ventral flattening of the mid-part of the base of the stalk.

The baculum of a specimen from Switzerland is weakly developed, of small size (shaft 2.0 mm. in length), slender, thin, spatulate, and terminally inflated. Digital processes were not observed, perhaps owing to excessive maceration in preparation. The general appearance of the baculum is that of an immature individual, although the animal was not small (165 mm.

total length in preservative).

_Specimens examined_: Two _Microtus nivalis nivalis_; Zermatt, Valais, Switzerland, 67105; Wetterstein, Germany, 65127.

Microtus (Chionomys) longicaudus (Merriam)

Fig. 48

Baculum: Base of stalk well developed, greatest length (3 mm.) 1-3/4 times greatest breadth, 3-2/3 times greatest depth; three ossified processes; base of median process rounded; median process slightly curved in dorsoventral plane, in length almost 1/3 greatest length of stalk; ossifications in lateral processes variable in size, frequently widely separated from shaft by cartilage, rarely as large as median ossification; basal tuberosities usually well-developed, medially confluent; profile of base in dorsal view trilobate or irregularly convex throughout; constriction 1/2 greatest depth; shaft relatively straight or slightly bowed ventrally or dorsally, shaft at mid-point of stalk wider than high; tip of shaft laterally inflated; widest point of stalk approximately 1/4 length of stalk from proximal end; lateral profile in dorsal view tapers gradually onto shaft anteriorly from point of greatest width of stalk; shaft variable, from slender terminally and nearly parallel sided (Fig. 48), to broad distally and tapered.

In many of the features that distinguish _M. longicaudus_ (and the closely related insular species _M. coronarius_) from other North American _Microtus_, _longicaudus_ resembles the Old World species of the subgenus _Chionomys_ (that is to say, _M. nivalis_, _M. gud_, and _M.

roberti_). These features are medium size, long tail, grayish color, montane habitat, relatively short molar tooth-row, moderate sized and unconstricted incisive foramen, relatively decurved upper incisors, elongate nasals, relatively broad interorbital region without well-developed median ridge, and similar chromosomes (Matthey, 1955:178).

For these reasons I am here referring _Microtus longicaudus_ to the subgenus _Chionomys_; previously it has not been referred to that subgenus.

_Specimens examined_: Six, of three subspecies; _Microtus longicaudus littoralis_, Sullivan Island, Alaska, 42972, 42969; _M. l. mordax_, 3/4 mi. N, 2 mi. W Allenspark, 8400 ft., Boulder Co., Colorado, 50335, 76829; _M. l. sierrae_, Crane Flat, Mariposa Co., California, 50252, 50253.

Microtus arvalis (Pallas)

Fig. 22

Baculum: In the single specimen examined, stalk small, greatest length (2.3 mm.) 2-1/3 times greatest width, almost 6 times greatest depth, flattened proximally; three well-developed digitate processes, the median one ossified, the lateral processes cartilaginous; median ossification laterally compressed and decurved at tip, bilobate at base; basal tuberosities of stalk weakly developed, medially confluent; posterior profile in dorsal view evenly rounded; ventral concavity deeper and narrower than dorsal concavity, but both comparatively shallow; medial constriction 2/3 greatest depth; shaft straight, at mid-point twice as wide as deep; lateral profile tapering from greatest width gradually to parallel sides of distal third of stalk.

From the baculum of _Microtus arvalis_ figured by Ognev (1950:173), and from the baculum figured by Didier (1954:238) my specimen differs in the absence of lateral ossifications in the digitate processes, smaller and slenderer median ossification, and weaker base. These differences in part may be owing to a difference in age, my specimen being the less mature.

_Specimen examined_: One from Vidy, Switzerland, 67101.

Microtus orcadensis Millais

Fig. 24

Baculum: In the one specimen examined, stalk broad, greatest length (2.6 mm.) 1-1/2 times greatest breadth, 3-1/2 times greatest depth; three digitate processes ossified; median process relatively broad, in length more than 1/2 length of stalk, triangular in dorsal view, with small spurs posterolaterally, middorsal ridge posteriorly; lateral ossifications slightly curved, slenderer, less than 1/2 depth and less than 1/2 transverse thickness of median process; basal tuberosities well-developed, confluent medially; in end-view base wider dorsally than ventrally, dorsal concavity broader and more abruptly curved at mid-point than ventral concavity; constriction 1/2 greatest depth; posterior profile in dorsal view notched, setting off a posterior shelf; stalk including shaft wider than deep throughout, at mid-point width twice depth; lateral profile abruptly curved anterior to point of greatest width, sides of shaft tapering gradually anteriorly to rounded uninflated tip.

The baculum of this insular species, placed in the "_arvalis_" group by Ellerman (1941:595), resembles the baculum of both _Microtus agrestis_ and _Microtus guentheri_ more than it resembles the baculum of _Microtus arvalis_. Similarities in the chromosomes of _M. arvalis_ and _M.

orcadensis_ were noted by Matthey (1953:254, 279), who was of the opinion that _M. orcadensis_ is an insular derivative of the _arvalis_-group.

_Specimen examined_: One from the Orkney Islands, 67106.