topotypes of modestus (Davis, 1939:315). I have examined a number of mice from the Bitterroot Valley in western Montana and the color value for 12 adults is 2.7. They are slightly but not significantly paler than topotypes of modestus. This is a result of the cline mentioned above and does not indicate relationship with modestus. Some average measurements of 10 skulls from this series are as follows: condylobasilar length, 24.9; zygomatic breadth, 14.4; interorbital breadth, 3.4; lambdoidal breadth, 11.5; prelambdoidal breadth, 9.1; alveolar length of upper molar teeth, 6.5; and depth of braincase, 7.6. Average external measurements of 9 specimens are as follows: total length, 157; length of tail, 36; length of hind feet, 19.4. Mice from the Bitterroot Valley were compared with topotypes of modestus by the "method of pairs," and modestus had a larger foramen magnum in 6 of 6 pairs (Confidence Limit .97) and larger first upper molar teeth in 5 of 6 pairs (C. L. .75).

A comparison of topotypes of pullatus with modestus shows a similar difference in the teeth, modestus being larger, but in the size of the foramen magnum there is no difference. A comparison of the measurements of pullatus (Bitterroot Valley), pullatus (near topotypes), and modestus (topotypes) shows that the two series of topotypes differ significantly in condylobasilar length (of borderline significance), zygomatic breadth and lambdoidal breadth (both of which vary greatly with age), and length of molar series; the specimens from the Bitterroot Valley agree with pullatus rather than modestus in all of these characters. The specimens from the Bitterroot Valley are smaller than pullatus (topotypes) in total length; they more closely resemble modestus in length of tail, and the hind foot is shorter than in either modestus or pullatus (which do not differ significantly). Specimens from western Montana resemble modestus in certain respects but in most respects resemble topotypes of pullatus and are referred to pullatus. Specimens from Coeur d'Alene, Idaho, and from Blackfoot, Montana (marginal records of modestus, Hall and Cockrum, 1953:410) may be referred to drummondi. Marginal records of pullatus in Montana to my knowledge are: Florence, Ravalli Co.; Highwood Mtns., Chouteau Co.; 7 mi. NE Hilger, Fergus Co.; 10 mi. NW Park City, Stillwater Co. (all represented by specimens in the USBS). The line separating pullatus and drummondi is tentatively drawn as shown in Figure 1.

Microtus pennsylvanicus insperatus (Allen)

Table 1. Average Measurements (in Millimeters) of Adult
Microtus pennsylvanicus