The Baculum In Microtine Rodents Part 5

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

Baculum: Stalk broad, greatest length (2.5 to 2.7 mm.) 1-2/3 times greatest breadth, 4 times greatest depth; median process ossified, size small, 1/5 length of stalk, higher than wide, having small anterodorsal prominence in both specimens examined; lateral processes cartilaginous, relatively small, displaced posteriorly, attenuate; posterior margin in dorsal view broadly rounded, or having blunt median apex, convex throughout; basal tuberosities moderately well developed, medially confluent, barely visible in dorsal view when mature; in end-view median constriction 4/5 greatest depth, ventral concavity deeper than dorsal concavity, both comparatively shallow; stalk at mid-point 1-1/2 times as wide as deep; shaft relatively slender, bowed dorsally at tip, relatively straight otherwise; lateral profile in dorsal view a gradual concave slope from point of greatest width anteriorly to distal part of shaft.

_Specimens examined_: Two, from Douglas Co., Kansas, 76834 (2 mi. N Baldwin), 68545 (1 mi. NE Pleasant Grove).

Microtus (Pitymys) parvulus (Howell)

Fig. 40

Baculum: Stalk broad, greatest length (2.4 mm. in specimen examined) 1-3/4 times greatest breadth, 4 times greatest depth; median process ossified, size small, less than 1/4 length of stalk, wider than high, terminally flattened; lateral processes cartilaginous, relatively small, attenuate; posterior margin in dorsal view flattened, irregularly curved with concavities medially and laterally; basal tuberosities well developed, medially confluent; visible in dorsal view; in end-view median constriction 2/3 greatest depth, ventral concavity well-formed, no dorsal concavity; stalk at mid-point twice as wide as deep; shaft relatively slender, bowed dorsally toward tip; in dorsal view lateral profile a gradual concave slope from point of greatest width anteriorly to distal part of shaft; tip of shaft enlarged.

The baculum of _M. parvulus_ resembles that of _M. pinetorum_ more than it resembles the baculum of any other microtine studied, differing primarily in smaller size.

_Specimen examined_: One, from 1 mi. W Micanopy, Alachua Co., Florida, Univ. Florida No. 1508.

Microtus (Pitymys) quasiater (Coues)

Figs. 29 and 30

Baculum: Stalk broad, greatest length (2.6-3.2 mm.) 1-1/3 to 1-2/3 times greatest breadth, 3-1/3 to 3-2/3 times greatest depth; median process ossified, with ventral depression, process 1/4 to 1/3 length of stalk, appressed to tip of shaft, wider than high proximally, relatively broad terminally; lateral processes cartilaginous, small, attenuate; posterior profile of stalk in dorsal view broadly rounded, bilobate, or trilobate, median lobe formed by posterior projection of dorsal shelf between enlarged lateral tuberosities that form outer lobes, posterolateral faces of these tuberosities visible in dorsal view of stalk; in end-view dorsal surface slightly concave, ventral concavity broad and deep, median constriction 1/2 greatest depth; shaft flattened except tip that is more terete, and bowed dorsally; at mid-point, stalk twice as wide as high; shaft relatively slender terminally, narrower than median ossification.

The baculum of _M. quasiater_ is the largest and has the best developed base and median process of the three American species of the subgenus _Pitymys_. The three species closely resemble each other in basic form.

_Specimens examined_: Five, all from Veracruz; Teocelo, 4500 ft., 30709, 30711; 4 km. N Tlapacoyan, 1700 ft., 24466; 5 km. N Jalapa, 4500 ft., 19869, 19878.

Microtus (Pitymys) fatioi (Mottaz)

Fig. 26

The baculum of a single specimen (KU 67103) of _M. fatioi_ from Zermatt, Valais, Switzerland, was examined. The baculum is immature, as evidenced by its small size, slender stalk and absence of ossified processes, therefore no characterization is included.

The baculum of another Old World species of the subgenus _Pitymys_, _M.

pyrenaicus_ from France, figured and described by Didier (1954:242-243), differs from all New World _Pitymys_ examined in processing ossified lateral processes.

The status of _Pitymys_, as a genus or as a subgenus, is uncertain. Hall and c.o.c.krum (1953:448) considered the North American _Pitymys_ and _Pedomys_ as subgenera of _Microtus_. They did not state specifically the basis for this point of view, but mention the fact that these two subgenera (_Pitymys_ and _Pedomys_) closely resemble each other cranially. These authors did not study nor comment upon the status of the Old World _Pitymys_. It may be asked whether the Old World and New World _Pitymys_ have developed as fossorial _Microtus_ independently, or from an ancestor common to both groups and not common to any other _Microtus_.

Matthey (1955:202) found 62 chromosomes (2N) in both the New World _Pitymys pinetorum_ and the Old World _Pitymys duodecimcostatus_. This suggests, but does not prove, common ancestry.

Neofiber alleni True

Fig. 49

Baculum: Stalk ma.s.sive, greatest length (4.7 mm.) 1-3/4 times greatest breadth, 4 times greatest depth; ossification in digitate processes variable; in one (KU 27123) of two specimens examined lateral processes ossified and median process unossified, as in two specimens examined by Hamilton (1946:379) from "southern Florida"; in my other specimen (KU 27268) that is possibly more mature, median process ossified although less deeply stained than lateral ossifications or stalk; posterior profile in probable dorsal view roughly rounded; in end-view probable dorsal concavity deep, ventral concavity broad but shallow, and with center convex; median constriction 3/5 greatest depth; shaft heavy, least depth 2/3 greatest depth of base; stalk, at mid-point, slightly wider than deep and more than 1/3 width of base; lateral profile in dorsal view sharply incurved distal to point of greatest breadth, shaft therefore relatively distinct from basal part of stalk; slight subterminal constriction; tip less reduced in the two specimens examined than in two figured by Hamilton. In preparation, the tissues that make it possible to distinguish with certainty the dorsal and ventral surfaces of the baculum were removed in both specimens.

_Specimens examined_: Two, of the subspecies _Neofiber alleni alleni_, 2 mi. S Gainesville, Alachua Co., Florida, 27268; 1 mi. E Courtenay, Merritt Island, Brevard Co., Florida, 27123.

Lagurus curtatus (Cope)

Fig. 46

Baculum: Stalk slender, greatest length (2.5 mm.) 2 to 2-2/3 times greatest breadth, 4 to 5 times greatest depth; three ossified processes; median one more than 1/3 length of stalk, curved dorsally toward tip, proximally flattened and having acute lateral angles in dorsal view, wider than deep except in distal half; lateral processes smaller than median one, slenderer, shorter, of approximately same depth, also curved dorsally; base of stalk well developed, basal tuberosities medially confluent, in part visible in dorsal view, in end-view wider ventrally than dorsally, dorsal and ventral concavities of equal depth and both wide, medial constriction 1/2 greatest depth; posterior profile in dorsal view broadly bilobate; lateral profile with abrupt transition from basal tuberosities to gradually converging, slightly curved sides of shaft; shaft terminally inflated.

Dearden (1958:543) described and figured the bacula of six subspecies of _Lagurus curtatus_ and two Asiatic species, _Lagurus lagurus_ and _Lagurus luteus_. He examined at least 34 specimens of _L. curtatus_ and found geographic variation in size, breadth of shaft distally, and proportions of digital ossifications to each other and to the stalk. The description that I have given above pertains to _L. c. levidensis_.

The baculum of the Asiatic _Lagurus (Lagurus) lagurus_ figured by Ognev (1950:554) agrees with that of _Lagurus (Lemmiscus) curtatus_, described here, in the relatively elongate shaft and slender stalk, the proportions of the processes, and the well-formed and moderately enlarged base of the stalk. The bacula of three _Lagurus lagurus_ examined by Dearden (1958:545) were of older individuals than the specimen that Ognev figures and differ from it and from bacula of _Lagurus curtatus_ (all subspecies) in the unusual, almost heart shaped, median process, and in larger size.

_Lagurus luteus_ examined by Dearden (1958:545) differs from both _Lagurus lagurus_ and _Lagurus curtatus_ in lacking lateral digital ossifications and in having shorter median digital ossifications and wider base of stalk.

_Specimens examined_: Seven _Lagurus curtatus levidensis_ from Wyoming; 9 mi. S Robertson, Uinta Co., 26045, 26053; 8 mi. S, 2-1/2 mi. E Robertson, Uinta Co., 26049; Farson, Sweet.w.a.ter Co., 37906; 16 mi. S, 11 mi. W Waltman, Natrona Co., 42457; 32 mi. S, 22 mi. E Rock Springs, 42465, 42466.

The following key to the bacula in some adult North American Microtinae is intended to help point out some of the most important differences. It should be noted that not all species can be keyed out on the basis of the baculum. The most difficult group in this respect includes the species of _Microtus_ that have small or no ossified lateral processes, especially species of the subgenera _Pedomys_ and _Pitymys_, and the two species _Microtus californicus_ and _Microtus mexica.n.u.s_ of the subgenus _Microtus_. Another complicating factor is the variability of bacula evident in some species even in the small samples available. It is to be expected that additional specimens will show variations not yet observed.

KEY TO THE BACULA OF SOME NORTH AMERICAN MICROTINES

1. Length of lateral digital ossifications more than 1/3 breadth of stalk 2

1'. Length of lateral digital ossifications less than 1/3 breadth of stalk or absent 15

2. Size small (total length of baculum less than 5.5 mm.) 3

2'. Size large (total length of baculum more than 5.5 mm.) 14

3. Width at mid-point of stalk more than 1/3 greatest breadth of stalk 4

3'. Width at mid-point of stalk less than 1/3 greatest breadth of stalk, 8

4. Stalk, viewed from proximal end hour-gla.s.s shaped, and width of stalk less than 1/2 length of stalk.... _Phenacomys intermedius_, p. 197

4'. Stalk not both hour-gla.s.s shaped when viewed from proximal end, and with width less than 1/2 length of stalk 5

5. Shaft thin basally, thickness less than 1/3 of greatest breadth 6

5'. Shaft thick basally, thickness 1/3 or more of greatest breadth 7