> Dichotomous key
   Virginia Atlantic Drainages

Disclaimers: (1) This key isn't literally dichotomous. (2) It is intended for use with the 35 species and subspecies of freshwater gastropods inhabiting Atlantic drainages of the Commonwealth of Virginia.  (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 (10)

2a) Operculum calcareous . . . . Bithynia tentaculata
2b) Operculum corneous & concentric . . . . . . . (3)
2c) Operculum corneous & not concentric . . . . . . . (5)

3a) Shell with color bands, aperture greater than 50% 
      of shell height . . . . Viviparus georgianus
3b) Shell unbanded, aperture 50% of shell height or less . . . . (4)

4a) Umbilicus slightly perforate . . . Bellamya japonica
4b) Umbilicus imperforate  . . . . Campeloma decisum

5a) Adults over 6 mm shell length . . . . . Family Pleuroceridae 
    (13 species statewide, exit to a separate key)
5b) Adults under 6 mm shell length . . . . . Family Hydrobiidae (6)

6a) Penis simple, with a single duct  . . . Somatogyrus virginicus
6b) Penis with a single duct and fringed with numerous papillae . . .         Littoridinops tenuipes
6c) Penis with a single duct and two accessory lobes . . . (7)
6d) Penis with a single duct and three accessory lobes . . . 
    Fontigens bottimeri
6e) Penis with two ducts . . . (9)

7a) Proximal penial lobe containing tubular gland . . . 
    Fontigens nickliniana
7b) Proximal penial lobe containing bulbous gland . . . (8)

8a) Shell height less than 1.5 times width . . . Fontigens morrisoni
8b) Shell height greater than 1.5 times width . . . Fontigens orolibas

9a) Operculum entirely paucispiral . . . . Amnicola limosa
9b) Operculum initially multispiral, becoming paucispiral . . .
    Lyogyrus granum

10a) Shell dextral . . . . Family Lymnaeidae (11)
10b) Shell sinistral, not planispiral . . . . Family Physidae (12)
10c) Shell sinistral, planispiral . . . . Family Planorbidae (15)
10d) Shell patelliform . . . . Family Ancylidae (20)

11a) Aperture greater than 50% shell height . . . Lymnaea columella
11b) Aperture less than 50% shell height  . . . . Lymnaea humilis

12a) One-part penial sheath . . . . . Physa acuta
12b) Two-part penial sheath . . . . (13)

13a) Shell slender, body generally dark. . . . Physa species A.
13b) Animal lightly pigmented, shell with shoulders 
    more pronounced . . . . (14)

14a) Whorls convex . . . Physa gyrina
14b) Whorls not notably convex . . . Physa pomilia

15a) Adult greater than 10 mm shell diameter . . . . (16)
15b) Adult less than 10 mm shell diameter . . . . (17)

16a) Spire appears deeply indented when viewed from either aspect
    . . . Helisoma anceps
16b) Spire appears flattened when viewed from one aspect . .
    . . . Helisoma trivolvis

17a) Shell covered with periostracal hairs . . . Gyraulus deflectus
17b) Shell bare, with rounded periphery . . . (18)
17c) Shell bare, periphery carinate . . . (19)

18a) Adults smaller than 4 mm maximum shell dimension . . . 
    Gyraulus parvus
18b) Adults larger than 4 mm maximum shell dimension  . . .
    Planorbula armigera

19a) Carina strong, approximately mid-whorl . . .
    Promenetus exacuous
19b) Carina weak, distinctly off mid-whorl . . . Menetus dilatatus.

20a) Apex distinctly to the right of midline . . .
     Hebetancylus excentricus
20b) Apex approximately in midline . . . . (21)

21a) Apex entirely smooth . . . . Laevapex fuscus
21b) Apex bearing fine striae . . . (22)

22a) Shell damaged easily with forceps . . . Ferrissia fragilis
22b) Shell thicker . . . . Ferrissia rivularis