Part 11

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

17'. Length of stalk usually less than 3.6 mm., or if more than 3.6 mm. (up to 4.0 mm.) then length 1-1/2 times or more its greatest breadth 18

18. Median process attenuate distally in dorsal view, and relatively long (more than twice its own breadth), 1/5 to 3/5 the length of stalk; breadth of stalk usually 2/3 or more length of stalk.... Microtus miurus, p. 200

18'. Median process relatively blunt distally in dorsal view, relatively short (usually less than 1/4 length of stalk), breadth of stalk usually less than 2/3 length of stalk.... Pitymys, p. 208, Pedomys, p. 207, or Microtus mexicanus, p. 205

19. Distal processes small and firmly ankylosed to distal end of shaft.... Phenacomys longicaudus, p. 197

19'. Distal processes if present not firmly ankylosed to distal end of shaft 20

20. Dorsal concavity of base as viewed from proximal end usually deeper than ventral concavity.... Microtus mexicanus, p. 205

20'. Dorsal and ventral concavities of base equal in depth or ventral one the deeper 21

21. Total length of baculum more than 3.6 mm.... Microtus californicus, p. 205

21'. Total length of baculum less than 3.6 mm.... Synaptomys cooperi, p. 194

DISCUSSION