Part 6 (1/2)

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-gla.s.s 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 monta.n.u.s_, 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