> Dichotomous key
Georgia Atlantic Drainages

Disclaimers: (1) This key isn't literally dichotomous. (2) It is intended for use with the 38 species and subspecies of freshwater gastropods inhabiting Atlantic drainages of Georgia. 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 concentric . . . . . . . (3)
2b) Operculum not concentric . . . . . . . (7)

3a) Adults under 6 mm maximum shell dimension . . . . Valvata bicarinata   (Family Valvatidae).
3b) Adults greater than 6 mm maximum shell dimension . . . . (4)

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

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 . . . . Viviparus georgianus
6b) Aperture 50% of shell height or less . . . . . Campeloma decisum

7a) Adults over 6 mm shell length . . . . . Family Pleuroceridae (8)
7b) Adults under 6 mm shell length . . . . . Family Hydrobiidae (11)


8a) Shell entirely uncostate . . . . Goniobasis proxima
8b) Shell at least slightly costate . . . . (9)


9a) Shell strongly costate throughout . . . Goniobasis catenaria catenaria
9b) Shell with faint costae around apex only . . . . . (10)

10a) Shell slightly carinate . . . . Goniobasis catenaria dislocata
10b) Shell entirely without carination (vicinity of Hawkinsville) . . . Goniobasis floridensis timidus

11a) Penis simple, with a single duct . . . Somatogyrus virginicus 
11b) Penis with a single duct and a glandular terminal lobe . . . (12)
11c) Penis with a single duct and fringed with numerous papillae . . . Littoridinops tenuipes
11d) Penis with two ducts . . . (16)

12a) Umbillicus imperforate, adults can be greater than 4.5 mm . . . Notogillia sathon
12b) Shell smaller, umbillicus at least slightly open . . . (13)


13a) Penis with a small terminal lobe, more than two additional glandular areas, and large penial filament . . . Spilochlamys turgida
13b) Penis with a small terminal lobe and not more than two additional glandular areas . . . (14)
13c) Penis with a large terminal lobe and a short filament . . . (15) 

14a) Ogeechee River . . . Marstonia halcyon
14b) Springs and spring-fed tributaries of the Ocmulgee River . . . Marstonia agarhecta
14c) endemic to Rock Springs, a tributary of the Oconee River . . . Marstonia gaddisorum

15a) Ogeechee River . . . . Floridobia species A.
15b) Southern coastal . . . Floridobia floridana

16a) Operculum entirely paucispiral . . . . Amnicola limosa
16b) Operculum initially multispiral, becoming paucispiral . . . (17)

17a) Shell substantially higher than wide . . . . Lyogyrus granum
17b) Shell roughly as wide as high, sometimes wider . . .
 . . Lyogyrus latus

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 (29)

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

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

22a) Shell slender, body generally dark. . . . Physa species A.
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 5 mm shell diameter . . . . (25)
24b) Adult less than 5 mm shell diameter . . . . (27)

25a) Shell easily crushed with fingers . . . Biomphalaria obstructa
25b) Shell stronger . . . . (26) 

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

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


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