Disclaimers: (1) This key isn't literally dichotomous. (2) It is intended for use with the 44 freshwater gastropod species and subspecies confirmed for Missouri, plus 2 unconfirmed.  The management will not be responsible for any loss, damage, or injury resulting from its application elsewhere.

1a) Operculum present. . . Subclass Prosobranchia (2)
1b) Operculum absent . . . Subclass Pulmonata (25)

2a) Operculum concentric [photo] . . . (3)
2b) Operculum paucispiral [photo] . . . (8)

3a) Aperture rounded . . . (4)
3b) Aperture oblong, noticeably longer than wide . . . (6)

4a) Shell width approximately equal to, or sometimes greater than, shell height . . . Viviparus intertextus.
4b) Shell narrower than high . . . (5)

5a) Whorls rounded, with color bands . . . Viviparus georgianus.
5b) Whorls with flattened sides, unbanded . . . Viviparus subpurpureus.

6a) Shell bearing spiral cords . . . Lioplax subcarinata.
6b) Shell smooth . . . Campeloma (7)

7a) Adult shell robust, shouldered . . . Campeloma decisum crassulum.
7b) Adult shell unshouldered . . . Campeloma decisum decisum.

8a) Adults over 10 mm shell length . . . Family Pleuroceridae (9)
8b) Adults under 10 mm shell length . . . Hydrobioid taxa (15)

9a) Shell height 1.7 times body whorl height or more . . . (10)
9b) Shell height less than 1.7 times body whorl height . . . (13)

10a) Shell costate around early whorls . . . Pleurocera laqueata alveare (unconfirmed).
10b) No costae . . . (11)

11a) Body whorl angulate, aperture trapezoidal . . . (12)
11b) Body whorl rounded . . . Pleurocera canaliculata lawrencii.

12a) Shell robust, aperture usually demonstrating spiral cords . . . Pleurocera canaliculata canaliculata
12b) Shell gracile, no spiral cords . . . Pleurocera canaliculata acuta

13a) Shell pustulate . . . Lithasia verrucosa
13b) Shell knobby . . . Lithasia armigera armigera
13c) Shell entirely smooth . . . (14)

14a) Ratio of shell height / body whorl height approximately 1.2 . . . Leptoxis arkansensis.
14b) Ratio of shell height / body whorl height approximately 1.4. Body dark, often black.  Typical of springs and spring runs . . . Pleurocera simplex ozarkensis.
14c) Ratio of shell height / body whorl height approximately 1.5. Body brown, tan, or orangish. Widely distributed in waters of the interior highlands . . . Pleurocera potosiensis.

15a) Primarily terrestrial in life habit, found above the water level . . . Pomatiopsis lapidaria
15b) Entirely aquatic in life habit . . . (16)

16a) Penis with a single duct . . . (17)
16b) Penis with two ducts . . . (24)

17a) Penis simple, unlobed . . . (18)
17b) Penis with a glandular, bladelike lobe . . . (20)
17c) Penis with multiple lobes . . . (21)

18a) Inhabiting aboveground waters . . . (19)
18b) Endemic to Tumbling Creek Cave . . . Antrobia culveri.

19a) Adults bearing robust shells less than 6 mm standard length . . . Somatogyrus integra.
19b) Globose shells greater than 6 mm standard length . . . Birgella subglobosa (unconfirmed).

20a) Shell with rounded whorls, impressed sutures, uncarinate . . . (21)
20b) Shell fusiform, flat-sided, usually carinate . . . Marstonia scalariformis

21a) Shell acutely spired . . . Cincinnatia integra
21b) Shell apex blunt . . . Probythinella emarginata

22a) Penis with two accessory lobes, inhabits aboveground waters . . . Fontigens aldrichi.
22b) Penis with three accessory lobes, cave-dwelling . . . (23)

23a) Teloconch with spiral striae . . . Fontigens proserpina.
23b) Teloconch smooth . . . Fontigens antroecetes.

24a) Inhabits aboveground waters . . . Amnicola limosa.
24b) Obligate cave-dweller . . . Amnicola stygia.

25a) Shell dextral . . . Family Lymnaeidae (26)
25b) Shell sinistral, not planispiral . . . Family Physidae (30)
25c) Shell planispiral . . . Family Planorbidae (32)
25d) Shell patelliform . . . Family Ancylidae (38)

26a) Shell with fine spiral periostracal lines . . . Lymnaea caperata.
26b) Shell smooth . . . (27)

27a) Adult shell length less than 10 mm . . . Lymnaea humilis.
27b) Adult shells larger . . . (28)

28a) Aperture length less than 60% shell height . . . Lymnaea elodes
28b) Aperture greater than 60% shell height . . . (29)

29a) Whorls convex, sutures deeply impressed . . . Lymnaea cockerelli
29b) Whorls flat, sutures shallow . . . Lymnaea columella

30a) One-part, muscular penial sheath . . . Physa acuta.
30b) One-part, glandular penial sheath . . . Physa jennessi.
30c) Two-part penial sheath . . . (31)

31a) Adults greater than 7 mm shell length, whorls convex . . . Physa gyrina
31b) Adults less than 7 mm shell length, whorls not notably convex . . . Physa pomilia.

32a) Adult greater than 10 mm shell diameter . . . (33)
32b) Adult less than 10 mm shell diameter . . . (35)

33a) Spire appears deeply indented when viewed from either aspect . . . Helisoma anceps.
33b) Spire appears flattened when viewed from one aspect . . . (34)

34a) Juvenile shell bearing threadlike striae [figure] and acute keel . . . Helisoma trivolvis.
34b) Juvenile shell glossy, no keel . . . Helisoma scalare duryi (extralimital).

35a) Shell with periostracal hairs, at least on early whorls . . . Gyraulus deflectus
35b) Shell bare . . . (36).

36a) Periphery carinate . . . Menetus dilatatus.
36b) Periphery rounded . . . (37)

37a) Adult shell never greater than 5 mm diameter . . . Gyraulus parvus
37b) Adult shell usually greater than 5 mm diameter . . .Planorbula armigera

38a) Apex bearing fine radial striae, visible only under magnification . . . (39)
38b) Apex entirely smooth . . . Laevapex fuscus

39a) Apex distinctly to the right of the midline [photo] . . . Ferrissia fragilis
39b) Apex approximately in the midline [photo] . . . Ferrissia rivularis