FWGNA > Freshwater Gastropods of North Carolina > Dichotomous Key
Dichotomous Key
North Carolina Atlantic Drainages

Disclaimers: (1) This key isn't literally dichotomous. (2) It is intended for use with the 37 species and subspecies of freshwater gastropods inhabiting Atlantic drainages of the state of North Carolina, plus two species unconfirmed (U). 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 paucispiral [photo] . . . (9)
2b) Operculum not paucispiral . . . (3)

3a) Operculum multispiral [photo] . . . Valvata bicarinata.
3b) Operculum concentric [photo] . . . (4)

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

5a) Shell with color bands . . . Viviparus georgianus.
5b) Shell unbanded . . . . (6)

6a) Umbilicus slightly perforate . . . (7)
6b) Umbilicus imperforate . . . (8)

7a) Shell uncarinate, aperture lip typically purple . . . Bellamya chinensis (U)
7b) Shell carinate, especially noticeable in juveniles, aperture never purple . . . Bellamya japonica.  [photo]

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

9a) Adults over 10 mm shell length, all female, ovoviviparous brooders . . . Melanoides tuberculata.
9b) Adults over 10 mm shell length, both sexes present, oviparous . . . . . Family Pleuroceridae (10).
9c) Adults under 10 mm shell length . . . . . (14)

10a) Aperture height 50% of shell height or greater . . . . Leptoxis carinata.
10b) Aperture height much less than 50% of shell height . . . . (11)

11a) Shell costate . . . . . . (12)
11b) Shell uncostate . . . . (13)

12a) Shell bearing spiral cords throughout . . . Pleurocera catenaria catenaria.
12b) If spiral cords are present, they are restricted to juvenile whorls only . . . Pleurocera catenaria dislocata.

13a) Shell with one strong carination . . . . Pleurocera proxima.
13b) Shell either entirely smooth, or with fine spiral cords . . . Pleurocera virginica.

14a) Primarily terrestrial in life habit, found above water level . . . Pomatiopsis lapidaria (U)
14b) Entirely aquatic in life habit . . . Family Hydrobiidae (15)

15a) Penis simple, with a single duct  . . . (16)
15b) Penis with a single duct and a glandular, terminal lobe . . . Floridobia sp.
15c) Penis with a single duct and fringed with numerous papillae . . . Littoridinops tenuipes
15d) Penis with two ducts . . . (17)

16a) Adults never greater than 6 mm . . . Somatogyrus virginicus.
16b) Adults generally greater than 6 mm . . . Gillia altilis.

17a) Operculum entirely paucispiral . . . . Amnicola limosa.
17b) Operculum initially multispiral, becoming paucispiral . . . Lyogyrus granum.

18a) Shell dextral . . . . Family Lymnaeidae (19)
18b) Shell sinistral, not planispiral . . . . Family Physidae (21)
18c) Shell sinistral, planispiral . . . . Family Planorbidae (24)
18d) Shell patelliform . . . . Family Ancylidae (31)

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

20a) Lateral teeth of the radula tricuspid . . . . Lymnaea humilis.
20b) Lateral teeth of the radula bicuspid . . . . Lymnaea cubensis.  [photo]

21a) One-part penial sheath . . .  Physa acuta.
21b) Two-part penial sheath . . . . (22)

22a) Shell slender, body generally dark. . . . Physa carolinae.
22b) Animal lightly pigmented, shell with shoulders more pronounced . . . (23)

23a) Whorls convex . . . Physa gyrina.
23b) Whorls not notably convex . . . Physa pomilia.

24a) Adult greater than 10 mm shell diameter . . . . (25)
24b) Adult less than 10 mm shell diameter . . . . (27)

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

26a) Aperture strikingly flared . . . . Helisoma magnifica.
26b) Aperture not flared . . . . Helisoma trivolvis.

27a) Shell with rounded periphery . . . (28)
27b) Shell carinate . . . (30)

28a) Adults smaller than 4 mm maximum shell dimension . . . Gyraulus parvus.
28b) Adults larger than 4 mm maximum shell dimension . . . . (29)

29a) Whorls tightly concentric . . . Planorbula armigera.
29b) Whorls expanding markedly with growth . . . Helisoma eucosmium.

30a) Carina strong, approximately mid-whorl . . . Promenetus exacuous.
30b) Carina weak, distinctly off mid-whorl . . . Menetus dilatatus.

31a) Apex bearing fine radial striae . . . (32)
31b) Apex entirely smooth . . . . (33)

32a) Apex distinctly to the right of midline . . .  Ferrissia fragilis.
32b) Apex approximately in the midline . . . Ferrissia rivularis.
 
33a) Apex approximately in the midline . . . . Laevapex fuscus.
33b) Apex distinctly to the right of the midline . . . Hebetancylus excentricus.