Disclaimers: (1) This key isn't literally dichotomous. (2) It is intended for use with the 72 freshwater gastropod species and subspecies confirmed for the The Ohio drainage above the mouth of the Tennessee/Cumberland at ORM 920.  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 (41)

2a) Operculum multispiral [photo] . . . Valvata tricarinata 
2b) Operculum concentric [photo] . . . (3)
2c) Operculum paucispiral [photo] . . . (10)

3a) Aperture rounded . . . (4)
3b) Aperture oblong, noticeably longer than wide . . . (8)

4a) Adults strikingly large, shells greater than 40 mm standard length . . . (5)
4b) Adults smaller . . . (6)

5a) Shell uncarinate, aperture lip typically purple . . . Cipangopaludina chinensis
5b) Shell carinate, especially noticeable in juveniles, aperture never
     purple . . . Cipangopaludina japonica  [photo]

6a) Shell width approximately equal to, or sometimes greater than, shell height . . . Viviparus intertextus.
6b) Shell narrower than high . . . (7)

7a) Shell rounded, with color bands . . . Viviparus georgianus.
7b) Shell with flattened sides, unbanded . . . Viviparus subpurpureus.

8a) Shell bearing spiral cords . . . Lioplax subcarinata.
8b) Shell smooth . . . Campeloma (9)

9a) Adult shell robust, shouldered . . . Campeloma decisum crassulum.
9b) Adult shell unshouldered . . . Campeloma decisum decisum.

10a) Adults over 10 mm shell length . . . Family Pleuroceridae (11)
10b) Adults under 10 mm shell length . . . Hydrobioid taxa (27)

11a) Shell height 1.5 times width or less . . . (12)
11b) Shell height greater than 1.5 times width . . . (13)

12a) Shell entirely smooth, not carinate or pustulate . . . Leptoxis praerosa
12b) Shell usually at least slightly carinate . . . Leptoxis carinata
12c) Shell pustulate . . . Lithasia verrucosa
12d Shell knobby . . . Lithasia geniculata geniculata

13a) Body whorl angulate or pustulate . . . (14)
13b) Body whorl rounded (but may bear weak carination) . . . (17)

14a) Aperture demonstrating spiral cords . . . Pleurocera canaliculata canaliculata
14b) No spiral cords . . . (15)

15a) Body whorl smooth . . . Pleurocera canaliculata acuta
15b) Body whorl pustulate or spiny . . . (16)

16a) Apex costate . . . Pleurocera laqueata alveare
16b) Apex uncostate . . . Lithasia armigera

17a) Shell bearing spiral cords, at least on upper whorls . . . (18)
17b) No spiral cords . . . (19)

18a) Body whorl less than 50% of shell length . . . Pleurocera  troostiana lyonii
18b)  Body whorl 50% of shell length or greater . . . Pleurocera virginica

19a) Upper whorls costate . . . Pleurocera laqueata laqueata
19b) No costae . . . (20)

20a) Adult shells (final two whorls) not carinate  . . . (21)
20b) Adult shells (final two whorls) may bear carination . . . (26)

21a) Inhabiting rivers of low to moderate richness . . . Pleurocera simplex ebenum
21b) Inhabiting richer streams and rivers . . . (22)

22a) Upper Kanawha/New drainages . . . Pleurocera shenandoa
22b) Elsewhere . . . (23)

23a) Body and shell dark, usually black.  Smaller streams and springs . . . (24)
23b) Body and shell more orange/brown.  All lotic environments . . . (25)

24a) Shell broader, apex length better fitting the regression A =  0.157B + 2.46,
    where B is body whorl length . . . Pleurocera simplex simplex
24b) Shell more slender, apex length better fitting the regression A = 0.556B - 0.09,
    where B is body whorl length . . . Pleurocera gabbiana

25a) Shell heavily shouldered, inhabits large rivers . . . Pleurocera semicarinata obovata
25b) Shell conical, height twice width or greater . . . Pleurocera semicarinata semicarinata
25c) Shell conical, height less than twice width . . . Pleurocera semicarinata livescens

26a) Inhabits softwaters, body black, shell usually decollate . . . Pleurocera proxima
26b) Hard waters, body tan or orange, shell never decollate . . . Pleurocera clavaeformis clavaeformis

27a) Primarily terrestrial in life habit, found above the water 
    level . . . Family Pomatiopsidae (28)
27b) Entirely aquatic in life habit . . . (29)

28a) Whorls rounded, shell sutures deeply impressed, apex
    acute . . . Pomatiopsis cincinnatiensis
28b) Whorls more ovoid in outline, sutures not deeply impressed, shell more
    fusiform . . . Pomatiopsis lapidaria 

29a) Restricted to subterranean waters in Kentucky and Indiana . . . (30)
29b) Restricted to subterranean waters elsewhere . . . (31)
29c) Not restricted to subterranean waters . . . (33)

30a) Shell with fine periostracum, width approximaely equal to length . . . Antroselates spiralis
30b) No periostracum, shell height much greater than length . . . Fontigens cryptica

31a)  Shell height over twice shell width . . . Fontigens turritella
31b) Shell height less than twice length . . . (32)

32a) Adult shells ovoid, length 2.0 mm or less . . . Fontigens benfieldi
32b) Adult shells pupiform, length 2.0 mm or less . . . Fontigens tartarea
32c) Adult shells elongate-conic, greater than 2.0 mm . . . Fontigens davisi

33a) Penis with a single duct . . . (34)
33b) Penis with two ducts . . . (40)

34a) Penis simple, unlobed . . . (35)
34b) Penis with one, bladelike lobe . . . (36)
34c) Penis with two lobes . . . (39)
34d) Penis with three lobes . . . Fontigens bottimeri

35a) Adults bearing robust shells less than 6 mm standard length . . . Somatogyrus integra
35b) Globose shells greater than 6 mm standard length . . . Birgella subglobosa

36a) Shell acutely spired, suture line strongly impressed . . . Cincinnatia integra
36b) Shell apex blunt. . . Probythinella emarginata
36c) Shell fusiform . . . (37)

37a) Adult shell 4 mm or greater. . . Marstonia lustrica
37b) Adult shell less than 4 mm  . . . (38)

38a) Shell carinate . . . Marstonia scalariformis
38b) Shell smooth . . . Marstonia letsoni

39a) Animal dark, penis with two tubular lobes . . . Fontigens nickliniana
39b) Animal pale, penis with one tubular and one bulbous lobe . . . Fontigens orolibas

40a) Operculum entirely paucispiral . . . . Amnicola limosa
40b) Operculum initially multispiral, becoming paucispiral . . . Lyogyrus granum

41a) Shell dextral . . . Family Lymnaeidae (42)
41b) Shell sinistral, not planispiral . . . Family Physidae (46)
41c) Shell sinistral, planispiral . . . Family Planorbidae (48)
41d) Shell patelliform . . . Family Ancylidae (55)

42a) Shell with fine spiral periostracal lines . . . Lymnaea caperata
42b) Shell smooth . . . (43)

43a) Adult shell length less than 10 mm . . . Lymnaea humilis
43b) Adult shells typically greater than 30 mm in standard length . . . Lymnaea stagnalis
43c) Adult shell length intermediate . . . (44)

44a) Aperture length less than 60% shell height . . . Lymnaea elodes
44b) Aperture greater than 60% shell height . . . (45)

45a) Populations on solid substrates subsurface in lakes and rivers. . . Lymnaea catascopium
45b) Populations amphibious . . . Lymnaea columella

46a) Elongatedly fusiform shell, penis lacks a preputial gland . . . Aplexa elongata
46b) Penis bearing a preputial gland . . . (47)

47a) One-part, muscular penial sheath . . . Physa acuta
47b) One-part, glandular penial sheath . . . Physa vernalis
47c) Two-part penial sheath . . . Physa gyrina

48a) Adult greater than 10 mm shell diameter . . . (49)
48b) Adult less than 10 mm shell diameter . . . (51)

49a) Adult lip flared . . . Helisoma campanulatum
49b) Adult lip not flared . . . (50)

50a) Spire appears deeply indented when viewed from either aspect . . . Helisoma anceps
50b) Spire appears flattened when viewed from one aspect . . . Helisoma trivolvis

51a) Shell with periostracal hairs, at least on early whorls . . . Gyraulus deflectus
51b) Shell bare, with rounded periphery . . . (52)
51c) Shell bare, periphery carinate . . . (54)

52a) Top and bottom of shell nearly identical . . . Gyraulus circumstriatus
52b) Top and bottom of shell clearly different . . . (53)

53a) Adult shell never greater than 5 mm diameter . . . Gyraulus parvus
53b) Adult shell usually greater than 5 mm diameter . . .Planorbula armigera

54a) Carina strong, approximately mid-whorl . . . Promenetus exacuous
54b) Carina weak, distinctly off mid-whorl . . . Menetus dilatatus

55a) Apex slightly indented and striate to the unaided eye . . . Rhodacmea filosa
55b) Apex bearing fine radial striae, visible only under magnification . . . (56)
55c) Apex entirely smooth . . . Laevapex fuscus 

56a) Apex distinctly to the right of the midline [photo] . . . Ferrissia fragilis
56b) Apex approximately in the midline [photo] . . . Ferrissia rivularis