WeRead Powered by ReaderPub
The Baculum in Microtine Rodents cover

The Baculum in Microtine Rodents

Chapter 41: DISCUSSION
By Sydney Anderson
Open in WeRead

Explore more books like this:

Archaeology & AnthropologyScience - Biology

About This Book

A comparative anatomical study of the baculum in microtine rodents describing its orientation relative to the urethra and corpus cavernosum, typical structure (elongate stalk, expanded base, and terminal digitate processes), and variation in curvature, proportions, and secondary ossifications across species and developmental stages. The author documents methods for removing, clearing, and staining penis specimens (potassium hydroxide and Alizarin red‑S) to reveal bacular morphology, presents measurements and illustrations, and discusses how baculum characters vary with age and body size and how they can be used, alongside other traits, to assess species-level relationships.

Microtus townsendii (Bachman)

Fig. 41

Baculum: Stalk broad, greatest length (3.0 mm.) 1½ times greatest breadth, 4½ 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.) 12/3 to 2 times greatest width, 4 to 5½ 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 ¼ 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, México, 63101; Valle de Bravo, México, 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., Michoacán, 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.) 21/3 times greatest breadth, 4½ times greatest depth; median process ossified, ¼ 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, ½ 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.) 11/3 to 12/3 times greatest breadth, up to 3¾ 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 ½ 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 ½ 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, 2½mi. W Ft. Morgan, Morgan Co., Colorado, 75082.

Microtus agrestis (Linnaeus)

Fig. 18

Baculum: Greatest length of stalk (2.9 mm.) twice greatest breadth, 4½ times greatest depth; stalk well developed, shaft not flattened dorsoventrally; large median ossified process, minute lateral ossifications in single specimen examined; length of stalk 2½ 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 ¾ 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.) 12/3 to 2 times greatest breadth, 2½ 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 ¼ 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, 2½ 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; 1½ mi. N, ¼ 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.) 12/3 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 1½ 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) 1¾ times greatest breadth, 4 times greatest depth; median process ossified, size small, less than ¼ 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.) 11/3 to 12/3 times greatest breadth, 31/3 to 32/3 times greatest depth; median process ossified, with ventral depression, process ¼ 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 ½ 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 Tlapacoyán, 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 Cockrum (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 massive, greatest length (4.7 mm.) 1¾ 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 22/3 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 ½ 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, 2½ mi. E Robertson, Uinta Co., 26049; Farson, Sweetwater 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 mexicanus 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
2´. Size large (total length of baculum more than 5.5 mm.) 14
3´. Width at mid-point of stalk less than 1/3 greatest breadth of stalk, 8
4. Stalk, viewed from proximal end hour-glass shaped, and width of stalk less than ½ length of stalk.... Phenacomys intermedius, p. 197
4´. Stalk not both hour-glass shaped when viewed from proximal end, and with width less than ½ 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
6. Stalk more or less straight, base not deflected. Microtus oeconomus, p. 204
6´. Stalk spatulate, and base deflected from axis of shaft.... Microtus guatemalensis, p. 198
7. Base enlarged, depth nearly ½ of breadth.... Lemmus trimucronatus, p. 193
7´. Base moderately enlarged, depth near 1/3 of breadth.... Microtus pennsylvanicus, p. 206, or Microtus townsendii, p. 204
8. Base hour-glass shaped as viewed from proximal end.... Phenacomys intermedius, p. 197
8´. Not so 9
9. Lateral processes separated from tip of shaft by more than the thickness of the lateral process 10
9´. Lateral processes separated from tip of shaft by less than the thickness of the lateral process 11
10. Lateral processes more than ½ the width of median process.... Microtus longicaudus, p. 201
10´. Lateral processes slender, less than ½ the width of median process.... Microtus montanus, p. 204
11. Lateral ossifications equal in length to median ossification.... Clethrionomys, p. 194
11´. Lateral ossifications shorter than median ossification 12
12. Size small, less than 3.4 mm. in total length.... Microtus oregoni, p. 199
12´. Size medium, more than 3.4 mm. in total length 13
13. Greatest width of stalk at a point about 1/3 of length of stalk from base.... Microtus chrotorrhinus (Hamilton, 1946:382).  
13´. Greatest width of stalk at a point less than 1/3 of length of stalk from base.... Lagurus curtatus, p. 210
14. Size of baculum larger, base more than 3 mm. wide, processes all well developed.... Ondatra zibethicus, p. 198
14´. Size of baculum smaller, base less than 3 mm. wide, processes poorly developed in some animals.... Neofiber alleni, p. 209
15. At least one digital ossification present 16
15´. Digital ossifications not present.... Dicrostonyx groenlandicus, p. 193
16. Breadth of stalk at least ½ length of stalk 17
16´. Breadth of stalk less than ½ length of stalk 19
17. Length of stalk greater than 3.6 mm. and less than 1½ times its greatest breadth.... Microtus richardsoni, p. 199
17´. Length of stalk usually less than 3.6 mm., or if more than 3.6 mm. (up to 4.0 mm.) then length 1½ times or more its greatest breadth 18
18. Median process attenuate distally in dorsal view, and relatively long (more than twice its own breadth), 1/5 to 3/5 the length of stalk; breadth of stalk usually 2/3 or more length of stalk.... Microtus miurus, p. 200
18´. Median process relatively blunt distally in dorsal view, relatively short (usually less than ¼ length of stalk), breadth of stalk usually less than 2/3 length of stalk.... Pitymys, p. 208, Pedomys, p. 207, or Microtus mexicanus, p. 205
19. Distal processes small and firmly ankylosed to distal end of shaft.... Phenacomys longicaudus, p. 197
19´. Distal processes if present not firmly ankylosed to distal end of shaft 20
20. Dorsal concavity of base as viewed from proximal end usually deeper than ventral concavity.... Microtus mexicanus, p. 205
20´. Dorsal and ventral concavities of base equal in depth or ventral one the deeper 21
21. Total length of baculum more than 3.6 mm.... Microtus californicus, p. 205
21´. Total length of baculum less than 3.6 mm.... Synaptomys cooperi, p. 194

DISCUSSION

Owing to shortness of lower incisors and present geographic distribution of the species, Hinton (1926:35) considered the Tribe Lemmi (lemmings) to be more primitive than the Tribe Microti (voles). The surviving lemmings are specialized in many features and therefore are considered as advanced end-products of an evolutionary radiation of a primitive microtine stock, of which all earlier stages are extinct.

Hinton regarded Dicrostonyx as the most primitive of the genera of lemmings on account of its more complex molar teeth (complexity was considered to be primitive), and on account of the presence of three primitive longitudinal rows of tubercles in unworn molars. The other three genera were arranged in order of increasing specialization as follows: Synaptomys, Myopus, Lemmus.

If the baculum tended to retain its primitive character while specializations in the external anatomy developed, and if the above arrangement is correct the most primitive bacula would be found in Dicrostonyx and in Synaptomys. The baculum in these two genera in comparison to that in Myopus (as figured by Ognev, 1948:512) and Lemmus has a slenderer stalk and smaller digital ossifications or none at all. The baculum in the genera of lemmings increases in robustness and the development of processes from Dicrostonyx, to Synaptomys, to Myopus, to Lemmus—the same order outlined above for total of specialization. The two extremes in this series are near the extremes of variation in bacula to be found in all microtines. The baculum in lemmings as a group cannot then be considered more primitive than in voles as a group, although the voles are usually considered to be more advanced. The situation in the voles, as we shall see, casts a different light on the matter.

The voles, Tribe Microti, were considered by Hinton (1926:40) to be more advanced than the lemmings because the incisors of the voles are longer and the root of their last lower molar is lingual to the root of the incisor. Hinton thought also that the murine ancestors of microtines had shorter incisors and that the backward extension of the incisors in the voles is a more ancient feature than the hypsodonty of the molars. A trend in the molar teeth has been toward greater hypsodonty. The voles in which the molars are least hypsodont are thus considered primitive. These include the living genera Clethrionomys, Phenacomys, Ondatra, Dolomys, Ellobius, and Prometheomys. Therefore, the baculum, in these assumedly primitive genera, would be expected to resemble the baculum in the lemmings or at least the most primitive lemmings. This is not the case.

The bacula that I have examined of Clethrionomys and Phenacomys have well-developed digital ossifications. In this they resemble the baculum of the genus Lemmus, the most advanced genus of lemmings according to Hinton. The baculum of Dolomys has not been studied. The baculum in Ondatra, and in Prometheomys as illustrated by Ognev (1948:552), also possesses well-developed processes. The baculum of Ellobius is small and lacks processes (as figured by Ognev, 1950:662). No ossification was found in a single specimen of Ellobius examined by me although the entire glans penis was removed and cleared without dissection. So far as known then, with the exception of Ellobius and Phenacomys longicaudus (Dearden, 1958:547), the primitive microtines having rooted molars possess bacula having three well-developed ossified processes.

Voles of the genus Microtus vary in the structure of the baculum almost as much as do the lemmings. Within the single subgenus Microtus some individuals of Microtus mexicanus, for example, have minute ossified lateral processes and other individuals lack these processes; Microtus pennsylvanicus and some other species have proportionately large lateral ossifications. If the well-developed condition of the baculum in the microtines having rooted molars is primitive, then within the genus Microtus those species having well-developed bacula may be considered primitive.

The genera Lagurus and Neofiber have moderately developed or well-developed lateral processes. Neofiber exhibits a tendency, not prominent elsewhere, to have a proportionately smaller median process rather than reduced lateral processes.

American species of Microtus (genus and subgenus) that have moderately- to well-developed ossified lateral processes are M. townsendii, M. oeconomus, M. pennsylvanicus, M. montanus, and M. chrotorrhinus. Microtus of other subgenera having this type of baculum include M. (Herpetomys) guatemalensis, M. (Chilotus) oregoni, and M. (Chionomys) longicaudus.

American species of Microtus (genus and subgenus) in which the lateral ossifications are weakly developed or absent (although cartilaginous lateral processes are present) include M. mexicanus and M. californicus. In other subgenera, species of Microtus having reduced lateral ossifications are M. (Pedomys) ochrogaster, M. (Pitymys) pinetorum, M. (Pitymys) parvulus, M. (Pitymys) quasiater, M. (Arvicola) richardsoni, and M. (Stenocranius) miurus.

The microtines are essentially holarctic in distribution. Both of the tribes, the lemmings and the voles, as well as primitive representatives of each tribe (not considering Ellobius) occur in both the Old World and New World. It is not certain on which continent (or continents) the Microtinae first differentiated. It is certain, however, that at various times, both early and late in the evolution of the subfamily, representatives have crossed from Eurasia to North America or vice versa. Each of 10 or more microtines in the New World is more closely related to some microtine in the Old World than to any other microtine in the New World.

The similarities or differences in the baculum in Old World and New World representatives placed in the same genus or subgenus, or thought to be "companion species" have been commented upon in accounts of Lemmus, Dicrostonyx, Clethrionomys, Lagurus, Arvicola, Stenocranius, Chilotus, Chionomys, Pitymys, and in accounts of Microtus agrestis as compared with M. pennsylvanicus, and Microtus oeconomus (both Old World and New World).

The baculum in the Microtinae more closely resembles the baculum in the Cricetinae of the Old World than in the Murinae, or than in any other rodents known to me. This resemblance suggests relationship between Microtinae and Cricetinae.

LITERATURE CITED

Argyropulo, A. I.
1933a. Die Gattungen und Arten der Hamster (Cricetinae Murray, 1866) der Paläarktik. Zeitschr. f. Säugetierkunde, 8:129-149, 27 figs. in text.
1933b. über zwei neue paläarktische Wühlmäuse. Zeitschr. f. Säugetierkunde, 8:180-183, 3 figs. in text.
Callery, R.
1951. Development of the os genitale in the golden hamster, Mesocricetus (Cricetus) auratus. Jour. Mamm., 32:204-207, 1 fig. in text.
Chamberlain, J. L.
1954. The Block Island meadow mouse, Microtus provectus. Jour. Mamm., 35:587-589, 2 tables in text.
Dearden, L. C.
1958. The baculum in Lagurus and related microtines. Jour. Mamm., 39:541-553, 1 fig. in text.
Didier, R.
1943. L'os pénien des Campagnols de France du Genre Arvicola. Mammalia, 7:74-79, 10 figs. in text.
1954. Etude systématique de l'os pénien des Mammifères (suite), Rongeurs: Muridés. Mammalia, 18:237-256, 14 figs. in text.
Ellerman, J. R.
1941. The families and genera of living rodents. Vol. II. Family Muridae. The British Museum (Natural History), London, pp. xii + 690, 50 figs.
Friley, Charles E.
1947. Preparation and preservation of the baculum of mammals. Jour. Mamm., 28:395-397, 1 fig. in text.
Hall, E. R., and E. L. Cockrum.
1953. A synopsis of the North American microtine rodents. Univ. Kansas Publ., Mus. Nat. Hist., 5:373-498, 149 figs. in text.
Hamilton, W. J., Jr.
1946. A study of the baculum in some North American Microtinae. Jour. Mamm., 27:378-387, 3 figs. in text.
Hibbard, C. W., and G. C. Rinker.
1942. A new bog-lemming (Synaptomys) from Meade County, Kansas. Univ. Kansas Sci. Bull., 28:25-35, 3 figs. in text.
1943. A new meadow mouse (Microtus ochrogaster taylori) from Meade County, Kansas. Univ. Kansas Sci. Bull., 29:255-268, 5 figs. in text.
Hinton, M. A. C.
1926. Monograph of the voles and lemmings (Microtinae), living and extinct, Vol. I. British Museum (Natural History), London, pp. xvi + 488, plus 15 plates, 110 figs. in text.
Matthey, R.
1953. Les Chromosomes des Muridae. Revue Suisse de Zoologie, 60:225-283, avec les planches 1 à 4 groupant 84 photomicrographies, 98 figures et 5 schemas dans le texte.
1955. Nouveaux documents sur les chromosomes des Muridae. Problèmes de cytologie comparée et de taxonomie chez les Microtinae. Revue Suisse de Zoologie, 62:163-206, avec 114 figures.
Miller, G. S.
1896. Genera and subgenera of voles and lemmings. North American Fauna No. 12, pp. 1-85, 40 figs. and 3 plates in text.
Ognev, S. I.
1948. The mammals of Russia (USSR) and adjacent countries (The mammals of Eastern Europe and Northern Asia), Vol. 6. Publ. Acad. Sci. USSR, pp. 1-587, 260 figs., 12 maps, and 11 color plates in text (in Russian).
1950. The mammals of Russia (USSR) and adjacent countries (The mammals of Eastern Europe and Northern Asia), Vol. 7. Publ. Acad. Sci. USSR, pp. 1-736, 347 figs., 15 maps, and 10 color plates in text (in Russian).
Ruth, E. B.
1934. The os priapi: A study in bone development. Anat. Rec., 60:231-249, 16 figs. in 3 plates.
Smith, D. A., and J. B. Foster.
1957. Notes on the small mammals of Churchill, Manitoba. Jour. Mamm., 38:98-115, 3 figs. and 3 tables in text.
Wheeler, B.
1956. Comparison of the Block Island "species" of Microtus with M. pennsylvanicus. Evolution, 10:176-186, 4 figs. and 2 tables in text.
White, J. A.
1951. A practical method for mounting the bacula of small mammals. Jour. Mamm., 32:125.
Zimmerman, K.
1955. Die Gattung Arvicola Lac. im System der Microtinae. Säugetierkundliche Mitteilungen, 3:110-112, 2 figs. in text.
Transmitted August 14, 1959.

28-774

UNIVERSITY OF KANSAS PUBLICATIONS
MUSEUM OF NATURAL HISTORY

Institutional libraries interested in publications exchange may obtain this series by addressing the Exchange Librarian, University of Kansas Library, Lawrence, Kansas. Copies for individuals, persons working in a particular field of study, may be obtained by addressing instead the Museum of Natural History, University of Kansas, Lawrence, Kansas. There is no provision for sale of this series by the University Library which meets institutional requests, or by the Museum of Natural History which meets the requests of individuals. However, when individuals request copies from the Museum, 25 cents should be included, for each separate number that is 100 pages or more in length, for the purpose of defraying the costs of wrapping and mailing.

* An asterisk designates those numbers of which the Museum's supply (not the Library's supply) is exhausted. Numbers published to date, in this series, are as follows:

Vol. 1.
Nos. 1-26 and index. Pp. 1-638, 1946-1950.
*Vol. 2.
(Complete) Mammals of Washington. By Walter W. Dalquest. Pp. 1-444, 140 figures in text. April 9, 1948.
Vol. 3.
  1. *The avifauna of Micronesia, its origin, evolution, and distribution. By Rollin H. Baker. Pp. 1-359, 16 figures in text. June 12, 1951.
  2. *A quantitative study of the nocturnal migration of birds. By George H. Lowery, Jr. Pp. 361-472, 47 figures in text. June 29, 1951.
  3. Phylogeny of the waxwings and allied birds. By M. Dale Arvey. Pp. 473-530, 49 figures in text, 13 tables. October 10, 1951.
  4. Birds from the state of Veracruz, Mexico. By George H. Lowery, Jr., and Walter W. Dalquest. Pp. 531-649, 7 figures in text, 2 tables. October 10, 1951.
Index. Pp. 651-681.
*Vol. 4.
(Complete) American weasels. By E. Raymond Hall. Pp. 1-466, 41 plates, 31 figures in text. December 27, 1951.
Vol. 5.
Nos. 1-37 and index. Pp. 1-676, 1951-1953.
*Vol. 6.
(Complete) Mammals of Utah, taxonomy and distribution. By Stephen D. Durrant. Pp. 1-549, 91 figures in text, 30 tables. August 10, 1952.
Vol. 7.
  1. *Mammals of Kansas. By E. Lendell Cockrum. Pp. 1-303, 73 figures in text, 37 tables. August 25, 1952.
  2. Ecology of the opossum on a natural area in northeastern Kansas. By Henry S. Fitch and Lewis L. Sandidge. Pp. 305-338, 5 figures in text. August 24, 1953.
  3. The silky pocket mice (Perognathus flavus) of Mexico. By Rollin H. Baker. Pp. 339-347, 1 figure in text. February 15, 1954.
  4. North American jumping mice (Genus Zapus). By Phillip H. Krutzsch. Pp. 349-472, 47 figures in text, 4 tables. April 21, 1954.
  5. Mammals from Southeastern Alaska. By Rollin H. Baker and James S. Findley. Pp. 473-477. April 21, 1954.
  6. Distribution of Some Nebraskan Mammals. By J. Knox Jones, Jr. Pp. 479-487. April 21, 1954.
  7. Subspeciation in the montane meadow mouse, Microtus montanus, in Wyoming and Colorado. By Sydney Anderson. Pp. 489-506, 2 figures in text. July 23, 1954.
  8. A new subspecies of bat (Myotis velifer) from southeastern California and Arizona. By Terry A. Vaughan. Pp. 507-512. July 23, 1954.
  9. Mammals of the San Gabriel mountains of California. By Terry A. Vaughan. Pp. 513-582, 1 figure in text, 12 tables. November 15, 1954.
  10. A new bat (Genus Pipistrellus) from northeastern Mexico. By Rollin H. Baker. Pp. 583-586. November 15, 1954.
  11. A new subspecies of pocket mouse from Kansas. By E. Raymond Hall. Pp. 587-590. November 15, 1954.
  12. Geographic variation in the pocket gopher, Cratogeomys castanops, in Coahuila, Mexico. By Robert J. Russell and Rollin H. Baker. Pp. 591-608. March 15, 1955.
  13. A new cottontail (Sylvilagus floridanus) from northeastern Mexico. By Rollin H. Baker. Pp. 609-612. April 8, 1955.
  14. Taxonomy and distribution of some American shrews. By James S. Findley. Pp. 613-618. June 10, 1955.
  15. The pigmy woodrat, Neotoma goldmani, its distribution and systematic position. By Dennis G. Rainey and Rollin H. Baker. Pp. 619-624, 2 figures in text. June 10, 1955.
Index. Pp. 625-651.
Vol. 8.
  1. Life history and ecology of the five-lined skink, Eumeces fasciatus. By Henry S. Fitch. Pp. 1-156, 26 figures in text. September 1, 1954.
  2. Myology and serology of the Avian Family Fringillidae, a taxonomic study. By William B. Stallcup. Pp. 157-211, 23 figures in text, 4 tables. November 15, 1954.
  3. An ecological study of the collared lizard (Crotaphytus collaris). By Henry S. Fitch. Pp. 213-274, 10 figures in text. February 10, 1956.
  4. A field study of the Kansas ant-eating frog, Gastrophryne olivacea. By Henry S. Fitch. Pp. 275-306, 9 figures in text. February 10, 1956.
  5. Check-list of the birds of Kansas. By Harrison B. Tordoff. Pp. 307-359, 1 figure in text. March 10, 1956.
  6. A population study of the prairie vole (Microtus ochrogaster) in northeastern Kansas. By Edwin P. Martin. Pp. 361-416, 19 figures in text. April 2, 1956.
  7. Temperature responses in free-living amphibians and reptiles of northeastern Kansas. By Henry S. Fitch. Pp. 417-476, 10 figures in text, 6 tables. June 1, 1956.
  8. Food of the crow, Corvus brachyrhynchos Brehm, in south-central Kansas. By Dwight Platt. Pp. 477-498, 4 tables. June 8, 1956.
  9. Ecological observations on the woodrat Neotoma floridana. By Henry S. Fitch and Dennis G. Rainey. Pp. 499-533, 3 figures in text. June 12, 1956.
  10. Eastern woodrat, Neotoma floridana; Life history and ecology. By Dennis G. Rainey. Pp. 535-646, 12 plates, 13 figures in text. August 15, 1956.
Index. Pp. 647-675.
Vol. 9.
  1. Speciation of the wandering shrew. By James S. Findley. Pp. 1-68, 18 figures in text. December 10, 1955.
  2. Additional records and extension of ranges of mammals from Utah. By Stephen D. Durrant, M. Raymond Lee, and Richard M. Hansen. Pp. 69-80. December 10, 1955.
  3. A new long-eared myotis (Myotis evotis) from northeastern Mexico. By Rollin H. Baker and Howard J. Stains. Pp. 81-84. December 10, 1955.
  4. Subspeciation in the meadow mouse, Microtus pennsylvanicus, in Wyoming. By Sydney Anderson. Pp. 85-104, 2 figures in text. May 10, 1956.
  5. The condylarth genus Ellipsodon. By Robert W. Wilson. Pp. 105-116, 6 figures in text. May 19, 1956.
  6. Additional remains of the multituberculate genus Eucosmodon. By Robert W. Wilson. Pp. 117-123, 10 figures in text. May 19, 1956.
  7. Mammals of Coahuila, Mexico. By Rollin H. Baker. Pp. 125-335, 75 figures in text. June 15, 1956.
  8. Comments on the taxonomic status of Apodemus peninsulae, with description of a new subspecies from North China. By J. Knox Jones, Jr. Pp. 337-346, 1 figure in text, 1 table. August 15, 1956.
  9. Extensions of known ranges of Mexican bats. By Sydney Anderson. Pp. 347-351. August 15, 1956.
  10. A new bat (Genus Leptonycteris) from Coahuila. By Howard J. Stains. Pp. 353-356. January 21, 1957.
  11. A new species of pocket gopher (Genus Pappogeomys) from Jalisco, Mexico. By Robert J. Russell. Pp. 357-361. January 21, 1957.
  12. Geographic variation in the pocket gopher, Thomomys bottae, in Colorado. By Phillip M. Youngman. Pp. 363-387, 7 figures in text. February 21, 1958.
  13. New bog lemming (genus Synaptomys) from Nebraska. By J. Knox Jones, Jr. Pp. 385-388. May 12, 1958.
  14. Pleistocene bats from San Josecito Cave, Nuevo León, México. By J. Knox Jones, Jr. Pp. 389-396. December 19, 1958.
  15. New subspecies of the rodent Baiomys from Central America. By Robert L. Packard. Pp. 397-404. December 19, 1958.
  16. Mammals of the Grand Mesa, Colorado. By Sydney Anderson. Pp. 405-414, 1 figure in text, May 20, 1959.
  17. Distribution, variation, and relationships of the montane vole, Microtus montanus. By Sydney Anderson. Pp. 415-511, 12 figures in text, 2 tables. August 1, 1959.
  18. Conspecificity of two pocket mice, Perognathus goldmani and P. artus. By E. Raymond Hall and Marilyn Bailey Ogilvie. Pp. 513-518, 1 map in text. January 14, 1960.
  19. Records of harvest mice, Reithrodontomys, from Central America, with description of a new subspecies from Nicaragua. By Sydney Anderson and J. Knox Jones, Jr. Pp. 519-529. January 14, 1960.
  20. Small carnivores from San Josecito Cave (Pleistocene), Nuevo León, México. By E. Raymond Hall. Pp. 531-538, 1 figure in text. January 14, 1960.
  21. Pleistocene pocket gophers from San Josecito Cave, Nuevo León, México. By Robert J. Russell. Pp. 539-548, 1 figure in text, January 14, 1960.
More numbers will appear in volume 9.
Vol. 10.
  1. Studies of birds killed in nocturnal migration. By Harrison B. Tordoff and Robert M. Mengel. Pp. 1-44, 6 figures in text, 2 tables. September 12, 1956.
  2. Comparative breeding behavior of Ammospiza caudacuta and A. maritima. By Glen E. Woolfenden. Pp. 45-75, 6 plates, 1 figure. December 20, 1956.
  3. The forest habitat of the University of Kansas Natural History Reservation. By Henry S. Fitch and Ronald R. McGregor. Pp. 77-127, 2 plates, 7 figures in text, 4 tables. December 31, 1956.
  4. Aspects of reproduction and development in the prairie vole (Microtus ochrogaster). By Henry S. Fitch. Pp. 129-161, 8 figures in text, 4 tables. December 19, 1957.
  5. Birds found on the Arctic slope of northern Alaska. By James W. Bee. Pp. 163-211, plates 9-10, 1 figure in text. March 12, 1958.
  6. The wood rats of Colorado: distribution and ecology. By Robert B. Finley, Jr. Pp. 213-552, 34 plates, 8 figures in text, 35 tables. November 7, 1958.
  7. Home ranges and movements of the eastern cottontail in Kansas. By Donald W. Janes. Pp. 553-572, 4 plates, 3 figures in text. May 4, 1959.
  8. Natural history of the salamander Aneides hardyi. By Richard F. Johnston and Gerhard A. Schad. Pp. 573-585. October 8, 1959.
More numbers will appear in volume 10.
Vol. 11.
  1. The systematic status of the colubrid snake, Leptodeira discolor Günther. By William E. Duellman. Pp. 1-9, 4 figures. July 14, 1958.
  2. Natural history of the six-lined racerunner, Cnemidophorus sexlineatus. By Henry S. Fitch. Pp. 11-62, 9 figures, 9 tables. September 19, 1958.
  3. Home ranges, territories, and seasonal movements of vertebrates of the Natural History Reservation. By Henry S. Fitch. Pp. 63-326, 6 plates, 24 figures in text, 3 tables. December 12, 1958.
  4. A new snake of the genus Geophis from Chihuahua, Mexico. By John M. Legler. Pp. 327-334, 2 figures in text. January 28, 1959.
  5. A new tortoise, genus Gopherus, from north-central Mexico. By John M. Legler. Pp. 335-343. April 24, 1959.
  6. Fishes of Chautauqua, Cowley and Elk counties, Kansas. By Artie L. Metcalf. Pp. 345-400, 2 plates, 2 figures in text, 10 tables. May 6, 1959.
  7. Fishes of the Big Blue River Basin, Kansas. By W. L. Minckley. Pp. 401-442, 2 plates, 4 figures in text, 5 tables. May 8, 1959.
  8. Birds from Coahuila, México. By Emil K. Urban. Pp. 443-516. August 1, 1959.
  9. Description of a new softshell turtle from the southeastern United States. By Robert G. Webb. Pp. 517-525, 2 plates, 1 figure in text. August 14, 1959.
Another number will appear in volume 11.
Vol. 12.
  1. Functional morphology of three bats: Eumops, Myotis, Macrotus. By Terry A. Vaughan. Pp. 1-153, 4 plates, 24 figures in text. July 8, 1959.
  2. The ancestry of modern Amphibia: a review of the evidence. By Theodore H. Eaton, Jr. Pp. 155-180, 10 figures in text. July 10, 1959.
  3. The baculum in microtine rodents. By Sydney Anderson. Pp. 181-216, 49 figures in text. February 19, 1960.
More numbers will appear in volume 12.