1. Operculum present. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Operculum absent. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2. Operculum oval and concentric (with growth rings like a tree); paucispiral, or multispiral. . . . . . . . . . . . . . . . . . . . . . 3
Operculum circular and spiral; plumose gill extends outside of shell. . . . . . . . . . . . . . . . . . . . . . . . . . Valvatidae, Valvata
3. Operculum concentric, corneous (hardened). Aperture narrow or subequal to length of spire; oral tentacles always absent. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Viviparidae
Operculum paucispiral, or multispiral; operculum calcareous or corneous. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Males, if present with a penis; adult often under 10 mm, but may reach 15 mm; shell rarely with any sculpturing. . . . . . . 5
Males, if present without a verge; adults typically 15 to 75 mm; shell typically with strong sculpturing. . . . . . . Pleuroceridae
5. Operculum multispiral or paucispiral. . . . . . . Hydrobiidae*
Operculum concentric. . .Bithyniidae, Bithynia tentaculata
6. Shell without any vestige of a spiral. . . . . . . . Ancylidae, 9
Shell with a spire, even if reduced or depressed (panospiral). 7
7. Tentacles narrow, filiform. . . . . . . . . . . . . . . . . . . . . . . 8
Tentacles, broad flat, triangular; haemogloben absent; coiled shell always dextral, pateliform shell with apex central or sinestral; never planospiral. . . . . . . . . . . . . Lymnaeidae, 17
8. Pseudobranch absent; shell obvious sinistral (coiling to the left), with an acute, posteriorly directed spire; mantle margin generally with digitate or serate lobes; blood cleara, lacking haemoglobin. . . . . . . . . . . . . . . . . . . . . . . . . Physidae, 10
Pseudobrach present as a cup-like lobe projecting from under the mantle posteriolaterally on the left side; shell patelliform (limpets), discoidal (ram's horn snails), or if with a projecting spire, then appearaing destral (spiraling to the right); blood red with haemoglobin. . . . . . . . . . . . . . . . . . . Planorbidae, 11
9. Shell low and somewhat broad, nearly ovate, apex near or just right of midline, not pronounced; tantacles with a black core; found in quiet, lentic waters. . . . . . . Laevapex fuscus
Shell low to moderately high, apex clearly right of midline, generally pronounced; tentacles white. . . . . . . . Ferrissia sp.*
10. Shell fusiform, with a high spire and narrow, compressed aperture. surface glossy; usually associated wiwth temporary waters. . . . . . . . . . . . . . . . . . . . . . . . . Aplexa elongata
Shell globose to conical, aperture wide, spire low to elevated, surface glossy or not, in most aquatic habitats. . . . Physella*
11. Shell with lamellae (teeth) a short distance within aperture. . . . . . . . . . . . . . . . . . . . . . . . . . . . Planorbula armigera
Shell without lamellae or teeth. . . . . . . . . . . . . . . . . . . . . 12
12. Maximum fronta height (includig aperture) more than one-half shell diameter, aperture hight usually (except a few (Menetus) greater than whorl heighgt; specimens typically greater than 6 mm in diameter. . Helisoma anceps anceps
Maximum frontal height (including aperture) less than one-half shell diameter, aperature height not more than whorl height, shell ususally 6 mm or less in diameter. . . . . . . . . . . . . . . .13
13. Shell with any or combinaiton of deflected aperture, raised costae, or greater than 4 whorls. . . . . . . . . . . . Gyraulus, 15
Shell lacking any of the above features; carinate or not. . . . 14
14. Aperture extending considerably beyond body whorl. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Menetus
Aperture only slightly deeper than body whorl; shell flat. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Promenetus, 16
15. Shell with umbilicus deep and narrow, body whorl on opposite side of shell from aperture slightly wider than umbilicus; shoulder flattened obliquely. . Gyraulus deflectus
Shell with umbilicus shallow and wide, body whorl on opposite side of shell from aperture slightly narrower than umbilicus; shoulder nto carinate. . . . . . . . . . . . . . . . . Gyraulus crista
16. Shell body whorl peripherally carinate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Promentus exacuous
Shell body whorl with shoulder rounded, no carina. . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . Promenetus umbilicatellus
17. Aperture much longer than spire. . . . . . . . . . . . . . . . . 18
Aperture length equal to or less than spire. . . . . . . . . . . . . 19
18. Aperture lacrimate, shell with a tapered spire, find spiral lines present on body whorl. . . . Pseudosuccinae columella
Shell not as above. . . . . . . . . . . . . . Stagnicola sp. (in part)
19. Shell thin and llight colored, fragile, and with a veery acute spire, length to 30 mm. . . . . . . . . . . . . . . . . . Lymnaea sp.
Shell otherwise. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
20. Shell with surface microsculpture, consisting of lines or crenulation, whorls rounded, length usually over 15 mm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Stagnicola sp. (in part)
Shell without microsculpture, whorls often shouldered length generally less than 15 mm. . . . . . . . . . . . . . . . Fossaria sp.