1. Nose leaf present; tail vertebrae not extending to edge of interfemoral membrane; no diastema in upper incisor series, bony palate complete in front: Family Phyllostomatidae, 2
1'. No nose leaf; tail extending at least to edge of interfemoral membrane; diastema in upper incisor row present; if absent, then tail extending well posterior to edge of interfemoral membrane: 3
2. Tail extremely reduced; zygomatic arch complete; two lower incisors on each side: Leptonycteris sanborni.
2'. Tail more than 10 mm: zygomatic arch incomplete; no lower incisors: Choeronycteris mexicana.
3. Tail extending conspicuously beyond free edge of interfemoral membrane: Family Molossidae, 4
3'. Tail extending at most a few mm beyond free edge of interfemoral membrane: Family Vespertilionidae, 7
4. Forearm more than 65 mm; upper lips without deep vertical grooves; palate without anterior emargination: Eumops perotis.
4'. Forearm less than 65 mm; upper lips with deep vertical grooves; palate with anterior emargination: 5
5. Forearm less than 45 mm; ears not united at base; second phalanx of fourth finger more than 5 mm; breadth of rostrum markedly greater than interorbital breadth: Tadarida brasiliensis.
5'. Forearm greater than 45 mm; ears united at base; second phalanx of fourth finger less than 5 mm; breadth of rostrum little more than interorbital breadth:6
6. Skull longer than 21 mm: Nyctinomops macrotis.
6'. Skull less than 21 mm: Nyctinomops femorosacca.
7. One upper incisor on each side: 8
7'. Two upper incisors on each side: 11
8. Total teeth, 28; ears long and naked; upper surface of interfemoral membrane largely naked: Antrozous pallidus.
8'. Total teeth, 30 to 32; ears short, rounded, and partly furred; upper surface of interfemoral membrane wholly or at least one-half furred: 9
9. One pair of upper premolars: Lasiurus ega.
9'. Two pairs of upper premolars (first of which is minute, medially displaced, and not visible in lateral view of tooth-row): 10
10. Skull longer than 15 mm: Lasiurus cinereus.
10'. Skull shorter than 15 mm: Lasiurus borealis.
11.Interfemoral membrane densely furred dorsally for more than one-half its length; fur black, tipped with white: Lasionycteris noctivagans.
11'. Interfemoral membrane furred for, at most, its basal one-third; fur not black tipped with white: 12
12. Cheek teeth (postcanine teeth) 6/6: (Myotis), 17
12'. Cheek teeth fewer than 6/6: 13
13. Length of ear more than 25 mm: 14
13'. Length of ear less than 25 mm: 16
14. Dorsal color black with three large white spots; postcanine teeth 5-5: Euderma maculatum.
14'. Dorsal color not as above; postcanine teeth 5/6: 15
15. A pair of anterior accessory lappets between base of ears: Idionycteris phyllotis.
15'. No anterior accessory lappets between bases of ears: Corynorhinus townsendii.
16. One pair of upper premolars: Eptesicus fuscus.
16'. Two pairs of upper premolars: Pipistrellus hesperus.
17. Calcar with well-marked keel: 18
17'. Calcar without well-marked keel: 20
18.Ventral surface of plagiopatagium furred to elbow: Myotis volans.
18'. Ventral surface of plagiopatagium not furred to elbow: 19
19. Rostral breadth less than 5.1 mm; skull length less than 13.5 mm: Myotis californicus.
19'. Rostral breadth more than 5.1 mm; skull length more than 13.5 mm: Myotis ciliolabrum.
20. Free edge of uropatagium with distinct fringeof macroscopically visible short hairs: Myotis thysanodes.
20'. Free edge of uropatagium without a distinct fringe of macroscopically visible hairs: 21
21. Well-developed sagittal crest present: Myotis velifer.
21'. No well-developed sagittal crest: 22
22. Ears more than 16 mm: 23
22'. Ears less than 16 mm: 24
23. Ears black: Myotis evotis.
23'. Ears brownish (may be dark, but not black): Myotis auriculus.
24.Dorsal fur usually with a slight sheen; mastoid breadth usually 7.5 mm or more: Myotis lucifugus.
24'. Dorsal fur usually lacking a sheen; mastoid breadth generally 7.4 mm or less: Myotis yumanensis.
Key modified from Findley et al. (1975).
Last Update: 4 Jan 2008