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