Disclaimers:  (1) This key isn’t literally dichotomous.  (2) It is intended for use with the 39 species and subspecies of freshwater gastropods inhabiting the state of South Carolina.  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 (18)

2a) Operculum multispiral [photo] . . . (3)
2b) Operculum concentric [photo] . . . (4)
2c) Operculum paucispiral [photo] . . . (11)

3a) Adult shell bearing two carina . . . Valvata bicarinata.
3b) Adult shell bearing three carina . . . Valvata tricarinata.

4a) Penis arising from the mantle edge, labial palps whiplike, eggs calcareous . . .  Family Ampullariidae (5)
4b)  Penis modified from a right tentacle, labial palps not whiplike, females ovoviviparous . . . Family Viviparidae (6)

5a) Shell whorls gently sloping, sutures not indented or incised . . . Pomacea paludosa. 
5b) Sutures strongly incised . . . Pomacea maculata.

6a) Aperture rounded . . . (7)
6b) Aperture oblong, noticably longer than wide . . . (10)

7a) Adults strikingly large, shells in excess of 40 mm standard length . . . Cipangopaludina japonica.
7b) Adults smaller . . . Viviparus (8)

8a) Shell width approximately equal to, or sometimes greater than, shell height . . . Viviparus intertextus.
8b) Shell narrower than high . . . (9)

9a) Shell rounded, with color bands . . . Viviparus georgianus.
9b) Shell with flattened sides, unbanded . . . Viviparus subpurpureus.

10a) Shell bearing spiral cords . . . Lioplax subcarinata.
10b) Shell smooth . . . Campeloma decisum decisum.

11a) Adults over 10 mm shell length, all female, ovoviviparous brooders. . . Melanoides tuberculata.
11b) Adults over 10 mm shell length, both sexes present, oviparous . . . . . Family Pleuroceridae (12)
11c) Adults under 10 mm shell length . . . . . Hydrobioid families (13)

12a) Shell uncostate . . . . Pleurocera proxima.
12b) Shell costate (sometimes faint, around apex only).  Spiral cords, if present, restricted to juvenile whorls only . . . Pleurocera catenaria dislocata.
12c) Shell costate, bearing spiral cords throughout . . . Pleurocera catenaria catenaria.

13a) Penis simple . . . (14)
13b) Penis bifurcated . . . (15)
13c) Penis fringed with numerous papillae . . . (17)

14a) Adults never greater than 6 mm . . . Somatogyrus virginicus.
14b) Adults generally greater than 6 mm . . . Gillia altilis.

15a) Operculum entirely paucispiral . . . . Amnicola limosa.
15b) Operculum initially multispiral, becoming paucispiral . . . (16)

16a) Shell length greater than shell width . . . Lyogyrus granum.
16b) Shell length equal to or greater than shell width . . . Lyogyrus latus.

17a) Shells polymorphic, including morphs with carinae and spines . . . Pyrgophorus parvulus.
17b) Shells never carinate or spiny . . . Littoridinops tenuipes.

18a) Shell dextral . . . . Family Lymnaeidae (19)
18b) Shell sinistral, not planispiral . . . . Family Physidae (21)
18c) Shell sinistral, planispiral . . . . Family Planorbidae (23)
18d) Shell patelliform . . . . Family Ancylidae (30)

19a) Aperture greater than 50% shell height . . . Lymnaea columella.
19b) Aperture less than 50% shell height . . .  (20)

20a)  First lateral tooth of radula bicuspid . . . Lymnaea cubensis/viator.
20b)  First lateral tooth of radula tricuspid . . . Lymnaea humilis.   [photos]

21a) One part penial sheath . . . . . Physa acuta.
21b) Penial sheath constricted into two parts . . . (22)

22a) Shell slender, body generally dark . . . . Physa carolinae.
22b) Animal lightly pigmented, Shell with shoulders more pronounced . . . Physa pomilia.

23a) Adult greater than 5 mm shell diameter . . . . (24)
23b) Adult less than 5 mm shell diameter . . . . (28)

24a) Shell easily crushed with finger pressure . . . Biomphalaria havanensis.
24b) Shell stronger . . . (25)

25a) Spire appears deeply indented when viewed from either aspect . . . Helisoma anceps.
25b) Spire appears flattened when viewed from one aspect . . . . (26)

26a) Juvenile shell bearing threadlike striae & acute keel . . . Helisoma trivolvis.
26b) Juvenile shell glossy, no keel [figure] . . . (27)

27a) Shell higher than wide in aperture view . . . Helisoma scalare scalare.
27b) Shell wider than high in aperture view . . . Helisoma scalare duryi.

28a) Shell with rounded periphery . . . . Gyraulus parvus.
28b) Shell carinate . . .  (29) 

29a) Carina strong, approximately mid-whorl . . . Promenetus exacuous.
29b) Carina weak, distinctly off mid-whorl . . . Menetus dilatatus.

30a) Apex bearing fine radial striae . . . (31).
30b) Apex entirely smooth . . . . (32)

31a) Apex distinctly to the right of midline . . . Ferrissia fragilis.
31b) Apex approximately in the midline . . . Ferrissia rivularis.  [photos]

32a) Apex approximately in the midline . . . . Laevapex fuscus.
32b) Apex distinctly to the right of midline . . . Hebetancylus excentricus.