> Dichotomous key

Disclaimers:  (1) This key isn’t literally dichotomous.  (2) It is intended for use with the 31 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 (14)

2a) Operculum concentric . . . . . . . (3)
2b) Operculum not concentric . . . . . . . (9)

3a) Penis arising from the mantle edge, labial palps whiplike, eggs
   calcareous . . .  Pomacea canaliculata   (Family Ampullariidae).
3b)  Penis modified from a right tentacle, labial palps not whiplike,
   females ovoviviparous . . . Family Viviparidae (4)

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

5a) Aperture greater than 50% of shell height . . . . (6)
5b) Aperture 50% of shell height or less . . . . (7)

 
6a) Shell rounded, with color bands . . . . Viviparus georgianus
6b) Shell with flattened shoulders, unbanded . . .
     Viviparus subpurpureus

7a) Umbilicus slightly perforate . . . Bellamya japonica
7b) Umbilicus imperforate . . .  (8)

8a) Shell smooth . . . . Campeloma decisum
8b) Shell bearing fine spiral cords . . . . Lioplax subcarinata

9a) Adults over 6 mm shell length . . . . . Family Pleuroceridae (10)
9b) Adults under 6 mm shell length . . . . . Family Hydrobiidae (11)

10a) Shell uncostate . . . . Goniobasis proxima
10b) Shell with faint costae around apex only . . .
      Goniobasis catenaria dislocata

10c) Shell costate throughout . . . Goniobasis catenaria catenaria

 

11a) Penis simple . . . (12)
11b) Penis bifurcated . . . (13)

11c) Penis fringed with numerous papillae . . . Littoridinops tenuipes

12a) Adults never greater than 6 mm . . . Somatogyrus virginicus
12b) Adults generally greater than 6 mm . . . Gillia altilis

13a) Operculum entirely paucispiral . . . . Amnicola limosa
13b) Operculum initially multispiral, becoming paucispiral . . .
       Lyogyrus granum

 

14a) Shell dextral . . . . Family Lymnaeidae (15)

14b) Shell sinistral, not planispiral . . . . Family Physidae (17)

14c) Shell sinistral, planispiral . . . . Family Planorbidae (19)

14d) Shell patelliform . . . . Family Ancylidae (24)

 

15a) Aperture greater than 50% shell height . . .

    Lymnaea columella

15b) Aperture less than 50% shell height . . .  (16)

16a)  Lateral teeth of radula bicuspid . . . Lymnaea cubensis

16b)  Lateral teeth of radula tricuspid . . . Lymnaea humilis

 

17a) One-part penial sheath . . . Physa acuta
17b) Penial sheath constricted into two parts . . . . (18)

18a) Shell slender, body generally dark . . . . Physa species A.
18b) Animal lightly pigmented, Shell with shoulders
       more pronounced . . . . Physa pomilia

19a) Adult greater than 5 mm shell diameter . . . . (20)

19b) Adult less than 5 mm shell diameter . . . . (22)

 

20a) Shell easily crushed with finger pressure . . .
       Biomphalaria obstructa

20b) Shell stronger . . . (21)

 

21a) Spire appears deeply indented when viewed from either

    aspect ... Helisoma anceps

21b) Spire appears flattened when viewed from one aspect . . . .
       Helisoma trivolvis

 

22a) Shell with rounded periphery . . . . Gyraulus parvus

22b) Shell carinate . . .  (23) 

23a) Carina strong, approximately mid-whorl . . .
     Promenetus exacuous

23b) Carina weak, distinctly off mid-whorl . . .  Menetus dilatatus.

24a) Apex distinctly to the right of midline . . .
        Hebetancylus excentricus
24b) Apex approximately in midline . . . . (25)

25a) Apex entirely smooth . . . . Laevapex fuscus

25b) Apex bearing fine striae . . . (26)

 

26a) Shell damaged easily with forceps . . . Ferrissia fragilis

26b) Shell thicker . . . . Ferrissia rivularis