Part 10

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.) 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, 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

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 1/2 length of stalk.... Phenacomys intermedius, p. 197

4'. Stalk not both hour-glass 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

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 1/2 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