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
2´. Size large (total length of baculum more than 5.5 mm.)	14
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 ½ length of stalk.... Phenacomys intermedius,	[p. 197]
4´. Stalk not both hour-glass shaped when viewed from proximal end, and with width less than ½ length of stalk	5
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 ½ 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 ½ the width of median process.... Microtus longicaudus,	[p. 201]
10´. Lateral processes slender, less than ½ 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 ½ length of stalk	17
16´. Breadth of stalk less than ½ length of stalk	19
17. Length of stalk greater than 3.6 mm. and less than 1½ 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 1½ 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 ¼ 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]