> Dichotomous key
North Carolina Atlantic Drainages

Disclaimers: (1) This key isn't literally dichotomous. (2) It is intended for use with the 36 species and subspecies of freshwater gastropods inhabiting Atlantic drainages of the state of North 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 (16)


2a) Operculum concentric . . . . . . . (3)
2b) Operculum not concentric . . . . . . . (8)


3a) Adults under 6 mm maximum shell dimension . . . . Valvata bicarinata
3b) Adults greater than 6 mm maximum shell dimension . . . . (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 . . . . Viviparus georgianus
5b) Aperture 50% of shell height or less . . . . (6)


6a) Umbilicus slightly perforate . . . Bellamya japonica
6b) Umbilicus imperforate . . . (7)

7a) Shell smooth . . . . Campeloma decisum
7b) Shell bearing fine spiral cords . . . . Lioplax subcarinata

8a) Adults over 6 mm shell length . . . . . Family Pleuroceridae (9)
8b) Adults under 6 mm shell length . . . . . Family Hydrobiidae (13)

9a) Aperture height 50% of shell height or greater . . . . Leptoxis carinata
9b) Aperture height much less than 50% of shell height . . . . (10)

10a) Shell costate . . . . . . (11)
10b) Shell uncostate . . . . (12)

11a) Shell with faint costae around apex only . . . Goniobasis catenaria dislocata
11b) Shell strongly costate throughout . . . Goniobasis catenaria catenaria

12a) Shell with one strong carination . . . . Goniobasis proxima
12b) Shell either entirely smooth, or with fine spiral cords . . . Goniobasis virginica

13a) Penis simple, with a single duct  . . . (14)
13b) Penis with a single duct and a glandular, terminal lobe . . . Floridobia sp.
13c) Penis with a single duct and fringed with numerous papillae . . . Littoridinops tenuipes
13d) Penis with two ducts . . . (15)

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 . . . Lyogyrus granum

16a) Shell dextral . . . . Family Lymnaeidae (17)
16b) Shell sinistral, not planispiral . . . . Family Physidae (19)
16c) Shell sinistral, planispiral . . . . Family Planorbidae (22)
16d) Shell patelliform . . . . Family Ancylidae (29)

17a) Aperture greater than 50% shell height . . . Lymnaea columella
17b) Aperture less than 50% shell height . . . (18)

18a) Lateral teeth of the radula tricuspid . . . . Lymnaea humilis
18b) Lateral teeth of the radula bicuspid . . . . Lymnaea cubensis

19a) One-part penial sheath . . . . . Physa acuta
19b) Two-part penial sheath . . . . (20)

20a) Shell slender, body generally dark. . . . Physa species A.
20b) Animal lightly pigmented, shell with shoulders more pronounced . . . . (21)

21a) Whorls convex . . . Physa gyrina
21b) Whorls not notably convex . . . Physa pomilia

22a) Adult greater than 10 mm shell diameter . . . . (23)
22b) Adult less than 10 mm shell diameter . . . . (25)

23a) Spire appears deeply indented when viewed from either aspect . . . Helisoma anceps
23b) Spire appears flattened when viewed from one aspect . . . . (24)

24a) Aperture strikingly flared . . . . Helisoma magnifica
24b) Aperture not flared . . . . Helisoma trivolvis

25a) Shell with rounded periphery . . . (26)
25b) Shell carinate . . . (28)

26a) Adults smaller than 4 mm maximum shell dimension . . . Gyraulus parvus
26b) Adults larger than 4 mm maximum shell dimension . . . . (27)

27a) Whorls tightly concentric . . . Planorbula armigera
27b) Whorls expanding markedly with growth . . . Helisoma eucosmium.

28a) Carina strong, approximately mid-whorl . . . Promenetus exacuous
28b) Carina weak, distinctly off mid-whorl . . . Menetus dilatatus.

29a) Apex distinctly to the right of midline . . . Hebetancylus excentricus
29b) Apex approximately in midline . . . . (30)

30a) Apex entirely smooth . . . . Laevapex fuscus
30b) Apex bearing fine striae . . . (31)

31a) Shell damaged easily with forceps . . . Ferrissia fragilis
31b) Shell thicker . . . . Ferrissia rivularis