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, 145 deg.09'50",
69 deg.24'28", 2900 ft., Romanzof Mts.); 51151, 51152, 51154, 51164, 51166,
51169 (last 6 from Wahoo Lake, 69 deg.08', 146 deg.58', 2350 ft.); 51210, 51213
(last 2 from Porcupine Lake, 68 deg.51'57", 146 deg.29'50", 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.
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 Buechner
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 pronounced (less than 1/2 greatest depth);
lateral profile at widest point of stalk convex, becoming abruptly
concave as the flange 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 montanus (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 montanus
amosus_, 1/2 mi. E Soldier Summit, Wasatch Co., Utah, 62241; _M. montanus
fusus_, La Manga Pass, 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 montanus nanus_, 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., Washington, 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 mexicanus (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 mexicanus
mexicanus_, Las Vigas, Veracruz, 30692; Nevada de Toluca, Mexico, 63101;
Valle de Bravo, Mexico, 63094; _Microtus mexicanus 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 mexicanus phaeus_,
Sierra Patamba, 9000 ft., Michoacan, 63085; _Microtus mexicanus
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 Butte, 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; pronounced
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.
Microtus (Pedomys) ochrogaster (Wagner)
Fig. 31
Baculum: Stalk broad, greatest length (3.2-4.0 mm.) 1-2/3 to 2 times
greatest breadth, 2-1/2 to 4 times greatest depth; median process
ossified, relatively small, less than 3/10 length of stalk; lateral
processes arising from subterminal part of stalk, cartilaginous or with
small ossifications; posterior profile in dorsal view broadly rounded or
slightly angular, widest point of stalk 1/6 to 1/4 the length of stalk
from base; basal tuberosities well developed and medially confluent, in
end-view dorsally convex, or at least less deeply concave than ventrally;
shaft straight, base bent ventrally or more commonly dorsally; at
mid-point of stalk wider than high, often twice as wide as high; viewed
from above, lateral profile from point of greatest breadth to middle of
shaft a gradual sigmoid curve; slight terminal inflation of shaft.
_Specimens examined_: Forty-one, of three subspecies; _Microtus
ochrogaster haydeni_, Muir Springs, 2 mi. N, 2-1/2 mi. W Ft. Morgan,
Morgan Co., Colorado, 74995, 74998, 74999, 75002; 1 mi. W Laird, Yuma
Co., Colorado, 57304, 76833; 2 mi. N, 2 mi. W Haigler, Dundy Co.,
Nebraska, 75016; 2 mi. S Franklin, Franklin Co., Nebraska, 75043, 75044;
Atwood, Rawlins Co., Kansas, 75020, 75023, 75025, 75027, 75028; 1 mi. N,
2 mi. E Oberlin, Decatur Co., Kansas, 75030, 75032, 75034, 75035, 75036;
1-1/2 mi. N, 1/4 mi. E Norton, Norton Co., Kansas, 68327; 1 mi. SW
Norton, Norton Co., Kansas, 75037; 2 mi. S, 1 mi. W Norton, Norton Co.,
Kansas, 75038; _M. ochrogaster ochrogaster_, Rydal, Republic Co., Kansas,
75047-75053, 75060, 75062, 75063, 75066, 75070, 75071, 75073; 1 mi. N, 1
mi. W Holton, Jackson Co., Kansas, 75077; 2 mi. W Court House, Lawrence,
Douglas Co., Kansas, 76832; Univ. Kansas Natural History Reservation,
Douglas Co., Kansas, 68536; _M. ochrogaster taylori_, Meade County State
Park, Kansas, 68539, 68542.
Microtus (Pitymys) pinetorum (LeConte)
Figs. 27 and 28