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.