Disclaimers: (1) This key isn't literally dichotomous. (2) It is intended for use only with the 70 species and subspecies of freshwater gastropods confirmed for the drainages of the Tennessee and Cumberland Rivers, including southern Kentucky, southwestern Virginia, western North Carolina, and North Alabama. 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 (49)

2a) Operculum concentric [photo] . . . Family Viviparidae (3)
2c) Operculum paucispiral [photo] . . . (9)

3a) Aperture rounded . . . Viviparus (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) Shell rounded, with color bands . . . Viviparus georgianus.
5b) Shell with flattened sides, unbanded . . . Viviparus subpurpureus.

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

7a) Adult shell aperture less than 50% of shell height . . . Campeloma decisum decampi.
7b) Aperture greater than 50% of shell height . . . (8)

8a) Adult shell robust, shouldered . . . Campeloma decisum crassulum.
8b) Adult shell unshouldered . . . Campeloma decisum decisum.

9a) Adults over 10 mm shell length . . . Family Pleuroceridae (10)
9b) Adults under 10 mm shell length . . . Hydrobioid taxa (33)

10a) Adult shells reaching standard lengths greater than 35 mm . . . (11)
10b) Adult shells smaller . . . (12)

11a) Shell with prominent siphonal canal, often bearing spines . . . Io fluvialis.
11b) Shell acutely conical . . . Pleurocera canaliculata canaliculata.

12a) Shell pustulate, tuberculate, or spiney . . . (13)
12b) Shell bearing both striae and costae . . . (16)
12c) Shell bearing striae, no costae . . . Pleurocera troostiana troostiana.
12d) Shell bearing costae, no striae . . . (20)
12e) Shell carinate, at least on upper whorls, but otherwise unsculptured . . . (21)
12f) Shell entirely without sculpture . . . (27)

13a) Shell apex costate . . . Pleurocera laqueata alveare.
13b) No costae . . . (14)

14a) Shell bluntly knobbed, around shoulders only . . . Lithasia geniculata geniculata.
14b) Shell bearing blunt pustules over entire surface . . . Lithasia verrucosa.
14c) Shell bearing acute protuberances . . . (15)

15a) Single row of protuberances low, barely noticeable . . . Lithasia armigera duttoniana.
15b) Single row of strong proturbances or spines . . . Lithasia armigera armigera.
15c) Two rows of strong protuberances or spines . . . Lithasia armigera jayana.

16a) Shell slender, acute, aperture less than 0.37 adult shell length . . . (17)
16b) Shell more bluntly conical, aperture greater than 0.37 adult shell length . . . (19)

17a) Shells costate throughout . . . Pleurocera troostiana edgariana.
17b) Shells without costae on the body whorl . . . (18)

18a) Apical costae strong . . . Pleurocera troostiana perstriata.
18b) Apical costae weak, variable . . . Pleurocera troostiana lyonii.

19a) Hiwassee drainage of East Tennessee . . . Pleurocera catenaria catenaria.
19b) Middle Tennessee and North Alabama . . . Pleurocera nassula.

20a) Costae fine and weak, restricted to the earliest whorls . . . Pleurocera laqueata castanea.
20b) Costae stronger, always present on penultimate whorl, sometimes extending to the body whorl . . . Pleurocera laqueata laqueata.

21a) Shell ovoid or squarish, standard length approximately equal to standard width . . .
    Leptoxis carinata.
21b) Standard shell length much greater than width . . . (22)

22a) Inhabits softwater rivers and streams, shell usually at least slightly carinate throughout,
    apex typically eroded . . . Pleurocera proxima.
22b) Inhabits hardwater rivers and streams, apex not typically eroded . . . (23)

23a) Body of animal typically orange or brown, sometimes with black flecks . . . (24)
23b) Body primarily black. . . (26)

24a) Shell sub-peripherally angulate . . . Pleurocera canaliculata pyrenellum.
24b) No sub-peripheral angulation . . . (25)

25a) Carination usually extending several whorls beyond apex . . .
    Pleurocera clavaeformis clavaeformis.
25b) Carination at the apex only . . . Pleurocera semicarinata semicarinata.

26a) Shell broader, apex length better fitting the regression A = 0.157B + 2.46, where B is 
    body whorl length . . . Pleurocera simplex.
26b) Shell more slender, apex length better fitting the regression A = 0.556B - 0.09, where B
    is body whorl length . . . Pleurocera gabbiana.

27a) Shell ovoid or squarish, standard length approximately equal to standard width . . . (28)
27b) Standard shell length much greater than width . . . (30)

28a) Standard length of adult shell typically exceeding 15 mm . . . Leptoxis crassa.
28b) Standard length of adult shell less . . . (29)

29a) Shell umbilicate . . . Leptoxis praerosa umbilicata.
29b) No umbilicus . . . Leptoxis praerosa praerosa.

30a) Ratio of aperture length to standard length greater than 0.48 . . . (31)
30b) Ratio of aperture length to standard length ranging from 0.48 to 0.43 . . . (32)
30c) Ratio of aperture length to standard length less than 0.43 . . .
    Pleurocera clavaeformis unciale.

31a) Inhabits small softwater rivers and streams, adult shell length never exceeds 20 mm, 
    apex typically eroded . . . Pleurocera modesta.
31b) Inhabits larger, richer rivers, heavily shelled, adults may exceed 20 mm . . .
    Lithasia geniculata fuliginosa.

32a) There is a population of Lithasia geniculata immediately downstream . . . 
    Lithasia geniculata pinguis.
32b) No population of Lithasia geniculata immediately downstream . . .
    Pleurocera simplex ebenum.

33a) Primarily terrestrial in life habit, found above the water level . . . Family 
    Pomatiopsidae (34)
33b) Entirely aquatic in life habit . . . (35)

34a) Whorls rounded, shell sutures deeply impressed, apex acute . . . 
    Pomatiopsis cincinnatiensis.
34b) Whorls more ovoid in outline, sutures not deeply impressed, shell more fusiform . . .
    Pomatiopsis lapidaria.

35a) Penis with a single duct . . . (36)
35b) Penis with two ducts . . . Family Amnicolidae (48)

36a) Penis simple, unlobed . . . Family Lithoglyphidae, primarily (37)
36b) Penis with glandular terminal lobe . . . Family Hydrobiidae (ss) primarily (41)
36c) Penis with two accessory lobes . . . Family Fontigentidae (46)

37a) Obligate cave-dweller bearing a light, slender shell . . . Holsingeria unthanksensis.
37b) Not cave-dwelling . . . (38)

38a) Adults bearing globose shells greater than 6 mm standard length . . . Birgella subglobosa.
38b) Adults bearing robust shells less than 6 mm standard length . . . (39)

39a) Shell umbilicate . . . Somatogyrus currierianus umbilicata.
39b) Shell not umbilicate . . .  (40)

40a) Inhabits the Hiwassee R . . . Somatogyrus virginicus.
40b) Inhabits rivers and streams elsewhere in the Tennessee/Cumberland . . . Somatogyrus
    currierianus currierianus.

41a) Shell acutely spired, suture line strongly impressed . . . Cincinnatia integra.
41b) Shell apex blunt. . . Probythinella emarginata.
41c) Shell slender, fusiform . . . (42)

42a) Umbilicus open . . . Marstonia olivacea.
42b) Umbilicus closed . . . (43)

43a) Shell carinate, penis slender with small glandular lobe . . . Marstonia scalariformis.
43b) Shell variable, penis broad and spatulate . . . (44)

44a) Shell sutures not strikingly impressed, penial lobe with a single large gland, inhabits
    impoundments, large rivers, and more lentic environments . . . Marstonia arga.
44b) Shell sutures more impressed, Penial lobe bearing multiple glands, inhabits rivers and
    streams . . . (45)

45a) Whorls angulate . . . Marstonia pachyta angulobasis.
45b) Whorls not angulate . . . Marstonia pachyta pachyta.

46a) Animal unpigmented, penis with one tubular and one bulbous lobe . . . Fontigens orolibas.
46b) Both accessory lobes on the penis tubular . . . (47)

47a) Animal pigmented, shell elongate, widespread in East Tennessee and SW Virginia . . .
    Fontigens nickliniana.
47b) Animal unpigmented, shell ovate-conic, endemic to Scott County, VA . . . 
    Fontigens hershleri.

48a) Operculum entirely paucispiral . . . . Amnicola limosa.
48b) Operculum initially multispiral, becoming paucispiral . . . Lyogyrus granum.

49a) Shell dextral . . . Family Lymnaeidae (50)
49b) Shell sinistral, not planispiral . . . Family Physidae (52)
49c) Shell sinistral, planispiral . . . Family Planorbidae (54)
49d) Shell patelliform . . . Family Ancylidae (57)

50a) Aperture greater than 50% shell height . . . Lymnaea columella.
50b) Aperture less than 50% shell height . . .  (51)

52a) One-part penial sheath . . . Physa acuta.
52b) Two-part penial sheath . . . (53)

53a) Whorls convex . . . Physa gyrina.
53b) Whorls not notably convex . . . Physa pomilia.

54a) Adult greater than 10 mm shell diameter . . . (55)
54b) Adult less than 4 mm shell diameter . . . (56)
54c) Adult shell 4 - 10 mm diameter . . . Planorbula armigera.

55a) Spire appears deeply indented when viewed from either aspect . . . Helisoma anceps.
55b) Spire appears flattened when viewed from one aspect . . . Helisoma trivolvis.

56a) Shell bearing rounded periphery . . . Gyraulus parvus.
56b) Shell with weak carination, off mid-whorl . . . Menetus dilatatus.

57a) Apex entirely smooth . . . Laevapex fuscus.
57b) Apex bearing fine radial striae . . . (58)

58a) Apex distinctly to the right of the midline . . . Ferrissia fragilis.
58b) Apex approximately in the midline . . . Ferrissia rivularis.   [photo]