FWGNA > Freshwater Gastropods of Virginia > Dichotomous Key
Dichotomous Key
Virginia Atlantic Drainages

Disclaimers: (1) This key isn't literally dichotomous. (2) It is intended for use with the 41 species and subspecies of freshwater gastropods confirmed for the Atlantic drainages of the Commonwealth of Virginia, plus 5 species unconfirmed (U). The pleurocerid key is valid statewide. 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 (19)

2a) Operculum calcareous . . . Bithynia tentaculata
2b) Operculum corneous . . . (3)

3a) Operculum multispiral [photo] . . . (4)
3b) Operculum concentric [photo] . . . (5)
3c) Operculum paucispiral [photo] . . . (9)

4a) Shells typically almost planispiral, bearing two carinae . . . Valvata bicarinata (U)
4b) Shells bearing 3 carinae . . . Valvata tricarinata 

5a) Shell with color bands, aperture greater than 50% of shell height . . . Viviparus georgianus
5b) Shell unbanded, aperture 50% of shell height or less . . . (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 . . . Family Pleuroceridae
    (13 pleuroceridae species statewide, separate key below)
9b) Adults under 10 mm shell length . . . (10)

10a) Primarily terrestrial in life habit, found above the
     water level . . . Pomatiopsis lapidaria 
10b) Entirely aquatic in life habit . . . Family Hydrobiidae (11)

11a) Penis with a single duct . . . (12)
11b) Penis with two ducts . . . (18)

12a) Penis simple, unlobed . . . (13)
12b) Penis fringed with numerous papillae . . . Littorinidops tenuipes
12c) Penis with glandular terminal lobe . . . Marstonia lustrica (U)
12d) Penis with two accessory lobes . . . (15)
12e) Penis with three accessory lobes . . . Fontigens bottimeri

13a) Habitat rivers & streams . . . (14)
13b) Tiny, white, obligate cave-dweller . . . Holsingeria unthanksensis

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

15a) Proximal penial lobe containing tubular gland . . . Fontigens nickliniana
15b) Proximal penial lobe containing bulbous gland . . . (16)

16a) Shell height less than 1.5 times width . . . Fontigens morrisoni
16b) Shell height greater than 1.5 times width . . .  (17)

17a) Shell pupoid, never greater than 2.3 mm maximum dimension, obligate cave-dweller . . . Fontigens tartarea
17b) Shell elongate-conic, up to 4.0 mm maximum dimension, caves and springs . . . Fontigens orolibas

18a) Operculum entirely paucispiral . . . . Amnicola limosa
18b) Operculum initially multispiral, becoming paucispiral . . . Lyogyrus granum

19a) Shell dextral . . . Family Lymnaeidae (20)
19b) Shell sinistral, not planispiral . . . Family Physidae (22)
19c) Shell sinistral, planispiral . . . Family Planorbidae (26)
19d) Shell patelliform . . . Family Ancylidae (31)

20a) Apex bearing periostracum – finely hairy, esp. in juveniles . . . Lymnaea caperata (U)
20b) No periostracum . . . (21)

21a) Aperture greater than 50% shell height . . . Lymnaea columella
21b) Aperture less than 50% shell height . . . Lymnaea humilis

22a) Penis lacks a preputial gland . . . Aplexa hypnorum(U)
22b) Penis bearing a preputial gland . . . (23)

23a) One-part penial sheath . . . Physa acuta
23b) Two-part penial sheath . . . (24)

24a) Shell slender, body generally dark . . . Physa carolinae.
24b) Animal lightly pigmented, shell with shoulders more pronounced . . . (25)

25a) Whorls convex . . . Physa gyrina
25b) Whorls not notably convex . . . Physa pomilia

26a) Adult greater than 10 mm shell diameter . . . (27)
26b) Adult less than 10 mm shell diameter . . . (28)

27a) Spire appears deeply indented when viewed from either aspect . . . Helisoma anceps
27b) Spire appears flattened when viewed from one aspect . . . Helisoma trivolvis

28a) Shell covered with periostracal hairs . . . Gyraulus deflectus
28b) Shell bare, with rounded periphery . . . (29)
28c) Shell bare, periphery carinate . . . (30)

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

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 the midline . . . Ferrissia fragilis
32b) Apex approximately in the midline . . . Ferrissia rivularis   [photo]

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

> Key to the Pleuroceridae of Virginia

Disclaimer: This key is intended for use with the 12 species and subspecies of pleurocerid gastropods inhabiting the Commonwealth of Virginia. The management will not be responsible for any loss, damage, or injury resulting from its application elsewhere.
     Drainages are A = Atlantic, N = New, S = Big Sandy,  T = Tennessee

1a) Shell with prominent siphonal canal, often bearing spines . . . Io fluvialis (T)
1b) No siphonal canal . . . (2)

2a) Shell height less than 1.7 times width . . . (3)
2b) Shell height greater than 1.7 times width . . . (4)

3a) Shell never carinate, tentacles constricted at eye,
    body bluish gray . . . Leptoxis praerosa (T, N)
3b) Shell often carinate, tentacles taper gradually,
    body dark with orange or yellow hue . . . Leptoxis carinata (N, A)

4a) Shells bearing costae, at least on early whorls . . . (5)
4b) Shells completely uncostate . . . (6)

5a) Spiral cords present over entire surface of shell . . . Pleurocera catenaria catenaria (A)
5b) If spiral cords are present, they are restricted to juvenile
    whorls only . . . Pleurocera catenaria dislocata (A)

6a) Shell usually bearing many fine spiral cords, at least on
    early whorls . . . Pleurocera virginica (A)
6b) Shell without fine spiral cords . . . (7)

7a) Body whorl greater than 55% of shell length . . . (8)
7b) Body whorl less than 55% of shell length . . . (10)

8a) Animal orange with dark flecks . . . Pleurocera simplex ebenum (S)
8b) Body of the animal primarily black . . . (9)

9a) Inhabits softwater rivers and streams, shell usually at least slightly
    carinate throughout, apex typically eroded . . . Pleurocera proxima (N, A)
9b) Inhabits hardwater streams, shell not carinate except in early
    whorls, apex not eroded . . . (12)

10a) Shell usually with flat, unshouldered whorls, animal orange with black flecks . . . (11)
10b) Shell with rounded whorls, animal not orange . . . Pleurocera shenandoa (N, A)

11a) Shell height greater than 3 times shell width . . . Pleurocera troostiana (T)
11b) Shell height less than 3 times shell width . . . Pleurocera clavaeformis (T)

12a) Shell broader, apex length better fitting the regression A = 0.157B + 2.46,
    where B is body whorl length . . . 
Pleurocera simplex simplex (N, T)
12b) Shell more slender, apex length better fitting the regression A = 0.556B - 0.09,
    where B is body whorl length . . . Pleurocera gabbiana (T)