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 8´. Not so 9 9. Lateral processes separated from tip of shaft by more than the thickness of the lateral process 10 9´. Lateral processes separated from tip of shaft by less than the thickness of the lateral process 11 10. Lateral processes more than 1/2 the width of median process.... _Microtus longicaudus_, p. 201 10´. Lateral processes slender, less than 1/2 the width of median process.... _Microtus montanus_, p. 204 11. Lateral ossifications equal in length to median ossification.... _Clethrionomys_, p. 194 11´. Lateral ossifications shorter than median ossification 12 12. Size small, less than 3.4 mm. in total length.... _Microtus oregoni_, p. 199 12´. Size medium, more than 3.4 mm. in total length 13 13. Greatest width of stalk at a point about 1/3 of length of stalk from base.... _Microtus chrotorrhinus_ (Hamilton, 1946:382). 13´. Greatest width of stalk at a point less than 1/3 of length of stalk from base.... _Lagurus curtatus_, p. 210 14. Size of baculum larger, base more than 3 mm. wide, processes all well developed.... _Ondatra zibethicus_, p. 198 14´. Size of baculum smaller, base less than 3 mm. wide, processes poorly developed in some animals.... _Neofiber alleni_, p. 209 15. At least one digital ossification present 16 15´. Digital ossifications not present.... _Dicrostonyx groenlandicus_, p. 193 16. Breadth of stalk at least 1/2 length of stalk 17 16´. Breadth of stalk less than 1/2 length of stalk 19 17. Length of stalk greater than 3.6 mm. and less than 1-1/2 times its greatest breadth.... _Microtus richardsoni_, p. 199