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. intext.
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. intext.
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 (NaturalHistory), 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 84photomicrographies, 98 figures et 5 schemas dans le texte.
1955. Nouveaux documents sur les chromosomes des Muridae. Problèmes de cytologie comparée et de taxonomie chez lesMicrotinae. 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 intext.
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 platesin 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 platesin 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. intext.
Transmitted August 14, 1959.

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