Disclaimers: (1) This key isn't literally dichotomous. (2) It is intended for use with the 33 freshwater gastropod species and subspecies confirmed for Kansas, Nebraska, South Dakota and North Dakota.  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 (9)

2a) Operculum multispiral [photo] . . . Valvata tricarinata 
2b) Operculum concentric [photo] . . . (3)
2c) Operculum paucispiral [photo] . . . (4)

3a) Aperture rounded, adult shell length typically greater than 40 mm . . . Cipangopaludina chinensis
3b) Aperture oblong, adult shell length generally less than 40 mm . . . Campeloma decisum decisum.

4a) Adults over 10 mm shell length . . . (5)
4b) Adults under 10 mm shell length . . . (7)

5a) All female, ovoviviparous brooders . . . Melanoides tuberculata
5b) Oviparous, both sexes present . . . (6)

6a) Ratio of shell length to width generally 2.1 or less . . . Pleurocera potosiensis
6b) Ratio of shell length to width much greater than 2.1 . . . Pleurocera canaliculata acuta

7a) Penis with a single duct . . . (8)
7b) Penis with two ducts . . . Amnicola limosa

8a) Shell acutely spired, suture line strongly impressed . . . Cincinnatia integra
8b) Shell apex blunt. . . Probythinella emarginata

9a) Shell dextral . . . Family Lymnaeidae (10)
9b) Shell sinistral, not planispiral . . . Family Physidae (15)
9c) Shell sinistral, planispiral . . . Family Planorbidae (18)
9d) Shell patelliform . . . Family Ancylidae (25)

10a) Shell with fine spiral periostracal lines . . . Lymnaea caperata
10b) Shell smooth . . . (11)

11a) Adult shells typically greater than 30 mm in standard length . . . Lymnaea stagnalis
11b) Adult shell length less than 10 mm . . . (12)
11c) Adult shell length intermediate . . . (13)

12a) First lateral tooth of radula bicuspid [photo] . . . Lymnaea cubensis/viator
12b) First lateral tooth of radula tricuspid [photo] . . . Lymnaea humilis

13a) Aperture length less than 60% shell height . . . Lymnaea elodes
13b) Aperture greater than 60% shell height . . . (14)

14a) Whorls convex, sutures deeply impressed . . . Lymnaea cockerelli
14b) Whorls flat, sutures shallow . . . Lymnaea columella

15a) Elongatedly fusiform shell, penis lacks a preputial gland . . . Aplexa elongata
15b) Penis bearing a preputial gland . . . (16)

16a) One-part, muscular penial sheath . . . Physa acuta
16b) One-part, glandular penial sheath . . . Physa jennessi
16c) Two-part penial sheath . . . (17)

17a) Adults greater than 7 mm shell length, whorls convex . . . Physa gyrina
17b) Adults less than 7 mm shell length, whorls not notably convex . . . Physa pomilia

18a) Adult greater than 10 mm shell diameter . . . (19)
18b) Adult less than 10 mm shell diameter . . . (20)

19a) Spire appears deeply indented when viewed from either aspect . . . Helisoma anceps
19b) Spire appears flattened when viewed from one aspect . . . Helisoma trivolvis

20a) Shell costate . . . Gyraulus crista
20b) Shell with periostracal hairs, at least on early whorls . . . Gyraulus deflectus
20c) Shell smooth and bare with rounded periphery . . . (21)
20d) Shell smooth and bare, periphery carinate . . . (24)

21a) Top and bottom of shell nearly identical . . . Gyraulus circumstriatus
21b) Top and bottom of shell clearly different . . . (22)

22a) Spire pit shallow and wide . . . Gyraulus parvus
22b) Spire pit deep and narrow . . . (23)

23a) Diameter of body whorl notably larger than earlier whorls . . . Promenetus umbilacatellus
23b) Diameter of body whorl similar to earlier whorls . . . Planorbula armigera

24a) Carina strong, approximately mid-whorl . . . Promenetus exacuous
24b) Carina weak, distinctly off mid-whorl . . . Menetus dilatatus

25a) Apex distinctly to the right of the midline [photo] . . . Ferrissia fragilis
25b) Apex approximately in the midline [photo] . . . Ferrissia rivularis