| 1a. | Nose leaf present. | 2 |
| 1b. | Nose leaf absent. | 3 |
| 2. | Ear length greater than 17 mm. | Macrotus californicus |
| 3a. | Tail extends beyond tail membrane full 1/3 its length or more. | 20 |
| 3b. | Tail extends beyond tail membrane less than half its length or not at all. | 4 |
| 4a. | Pink ears 44 mm or greater, dorsal fur jet black with 2 white spots on shoulders and one on rump. | Euderma maculatum |
| 4b. | Ears not pink, no distictive dorsal spots. | 5 |
| 5a. | Ear length greater than or equal to 25 mm. | 6 |
| 5b. | Ear length less than or equal to 22 mm. | 8 |
| 6a. | Forearm 50 mm or greater, fur blond and lighter at base than at tips, pig-like snout with forward facing nostrils. | Antrozous pallidus |
| 6b. | Forearm 49 mm or less, fur various but darker at base than at tips. | 7 |
| 7a. | Prominent lumps above nose, no lappets in brow region. | Corynorinhus townsendii |
| 7b. | Lumbs on nose absent, prominent brow lappets. | Idionycterus phyllotis |
| 8a. | At least anterior half of dorsal surface of tail membrane well furred. | 9 |
| 8b. | Dorsal surface of tail membrane naked, scantily haired, or at most anterior third lightly furred. | 11 |
| 9a. | Color of hair black with many hairs distinctly silver tipped | Lasionycteris noctivagans |
| 9b. | Color various but never uniformly black. | 10 |
| 10a. | Color brownish or grayish with heavily frosted fur, yellow ring of fur surrounding throat, ears edged with black. | Lasiurus cinereus |
| 10b. | Color reddish, might be lightly tipped in white. Posterior 1/3 of tail membrane bare or scantily haired. | Lasiurus blosservillii |
| 11a. | Forearm length greater than 33 mm. | 13 |
| 11b. | Forearm length less than 33 mm. | 12 |
| 12a. | Tragus short, less than 5 mm, blunt and club-shapped. | Pipistrellus hesperus |
| 12b. | Tragus long, greater than 5 mm, and pointed. | 13 |
| 13a. | Distinct fringe of hair on trailing edge of tail membrane. | Myotis thysanodes |
| 13b. | Fringe absent or inconspicuous on trailing edge of tail membrane. | 14 |
| 14a. | Distinct keel on calcar. | 15 |
| 14b. | Keel absent or rudimentary. | 18 |
| 15a. | Forearm length greater than 42 mm. | Eptesicus fuscus |
| 15b. | Forearm length less than 42 mm. | 16 |
| 16a. | Forearm length greater than 37 mm, underside of wing furred from side to elbow. | Myotis volans |
| 16b. | Forearm length less than 36 mm, underside of wing unfurred. | 17 |
| 17a. | Thumb length greater than 4.2 mm, long shiny hair appears to be tri-colored, when viewed from above, fur does not extend down from rostrum. | Myotis cililabrum |
| 17b. | Thumb length less than 4.2 mm, fur dull and bi-colored, when viewed from above, fur extends midway down on rostrum. | Myotis californicus |
| 18a. | Ear length is greater than or equal to 19 mm. Ears, wing and tail membrane blackish and opaque. | Myotis evotis |
| 18b. | Ear length is less than or equal to 18 mm. | 19 |
| 19a. | Forearm length less than or equal to 36 mm. | Myotis yumanensis |
| 19b. | Forearm length greater than 36 mm. Color glossy brown to reddish brown, no bare patch between shoulder blades, ears always dark, forearm length between 36 and 41 mm. | Myotis lucifugus occultus |
| 20a. | Forearm greater than or equal to 73 mm, ear length between 36 and 47 mm, no deep verticle grooves in upper lip. | Eumops perotis |
| 20b. | Forearm length less than or equal to 66 mm, lips with verticle grooves. | 21 |
| 21a. | Ears united at base, leading edge of ear closest to eye broad, rounded. | 22 |
| 21b. | Ears not united at base, leading edge of ear closest to eye fine, not rounded. | Tadarida brasiliensis |
| 22a. | Forearm length less than 48 mm. | Nyctinomops femmorosacca |
| 22b. | Forearm length greater than or equal to 58 mm. | Nyctinomops macrotus |
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