> Dichotomous key

Disclaimers:  (1) This key isn’t literally dichotomous.  (2) It is intended for use with the 32 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 (15)

2a) Operculum concentric . . . . . . . (3)
2b) Operculum not concentric . . . . . . . (10)

3a) Penis arising from the mantle edge, labial palps whiplike, eggs
   calcareous . . .  Family Ampullariidae (4)
3b)  Penis modified from a right tentacle, labial palps not whiplike,
   females ovoviviparous . . . Family Viviparidae (5)

4a) Shell whorls gently sloping, sutures not indented or incised . . .
    Pomacea paludosa.  
4b) Sutures strongly incised . . . Pomacea canaliculata 

5a) Shell width approximately equal to, or sometimes greater than
   shell height . . .  Viviparus intertextus
5b) Shell narrower than high . . . . (6)

6a) Aperture greater than 50% of shell height . . . . (7)
6b) Aperture 50% of shell height or less . . . . (8)

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

8a) Umbilicus slightly perforate . . . Bellamya japonica
8b) Umbilicus imperforate . . .  (9)

9a) Shell smooth . . . . Campeloma decisum
9b) Shell bearing fine spiral cords . . . . Lioplax subcarinata

10a) Adults over 6 mm shell length . . . . . Family Pleuroceridae (11)
10b) Adults under 6 mm shell length . . . . . Family Hydrobiidae (12)

11a) Shell uncostate . . . . Goniobasis proxima
11b) Shell with faint costae around apex only . . .
      Goniobasis catenaria dislocata

11c) Shell costate throughout . . . Goniobasis catenaria catenaria

 

12a) Penis simple . . . (13)
12b) Penis bifurcated . . . (14)

12c) Penis fringed with numerous papillae . . . Littoridinops tenuipes

13a) Adults never greater than 6 mm . . . Somatogyrus virginicus
13b) Adults generally greater than 6 mm . . . Gillia altilis

14a) Operculum entirely paucispiral . . . . Amnicola limosa
14b) Operculum initially multispiral, becoming paucispiral . . .
       Lyogyrus granum

 

15a) Shell dextral . . . . Family Lymnaeidae (16)

15b) Shell sinistral, not planispiral . . . . Family Physidae (18)

15c) Shell sinistral, planispiral . . . . Family Planorbidae (20)

15d) Shell patelliform . . . . Family Ancylidae (25)

 

16a) Aperture greater than 50% shell height . . .

    Lymnaea columella

16b) Aperture less than 50% shell height . . .  (17)

17a)  Lateral teeth of radula bicuspid . . . Lymnaea cubensis

17b)  Lateral teeth of radula tricuspid . . . Lymnaea humilis

 

18a) One part penial sheath . . . . . Physa acuta

18b) Penial sheath constricted into two parts . . . (19)

19a) Shell slender, body generally dark . . . . Physa species A.
19b) Animal lightly pigmented, Shell with shoulders
       more pronounced .  . Physa pomilia

20a) Adult greater than 5 mm shell diameter . . . . (21)

20b) Adult less than 5 mm shell diameter . . . . (23)

 

21a) Shell easily crushed with finger pressure . . .
       Biomphalaria obstructa

21b) Shell stronger . . . (22)

 

22a) Spire appears deeply indented when viewed from either

    aspect ... Helisoma anceps

22b) Spire appears flattened when viewed from one aspect . . . .
       Helisoma trivolvis

 

23a) Shell with rounded periphery . . . . Gyraulus parvus

23b) Shell carinate . . .  (24) 

24a) Carina strong, approximately mid-whorl . . .
     Promenetus exacuous

24b) Carina weak, distinctly off mid-whorl . . .  Menetus dilatatus.

25a) Apex distinctly to the right of midline . . .
        Hebetancylus excentricus
25b) Apex approximately in midline . . . . (26)

26a) Apex entirely smooth . . . . Laevapex fuscus

26b) Apex bearing fine striae . . . (27)

 

27a) Shell damaged easily with forceps . . . Ferrissia fragilis

27b) Shell thicker . . . . Ferrissia rivularis