| 1 | Shell distinctly compressed laterally | 2 |
| – | Shell not or slightly compressed laterally | 7 |
| 2 | Oval aperture | 3 |
| – | Circular aperture | C. compressa angustioris |
| 3 | Shell length 45–63 µm | C. angustastoma |
| – | Shell length 32–45 µm | C. paludosa |
| – | Shell is smaller than 25 µm | 4 |
| 4 | Shell width > length | C. compressa australis |
| – | Shell width < length | 5 |
| 5 | Shell lanceolate | C. lanceolata |
| – | Shell pyriform | C. brevicolla |
| – | Shell ovate | 6 |
| 6 | Thick organic lip around the aperture | C. compressa |
| – | No lip around the aperture | C. compressa ovata |
| 7 | Shell surface crenulate | 8 |
| – | Shell surface not crenulate | 10 |
| 8 | Shell shape ovoid | 9 |
| – | Shell shape globose or subglobose | C. crenulata globosa |
| 9 | Shell length < 20 µm | C. crenulata |
| – | Shell length > 23 µm | C. splendida |
| 10 | Shell pyriform with prominent neck; surface covered by rare sand grains | 11 |
| – | Shell ovoid or globular, without prominent neck | 13 |
| 11 | Shell surface covered by rare sand grains | 12 |
| – | Shell surface smooth | C. collum |
| 12 | Shell length 16–26 µm; aperture 4–5 µm; neck 1/3 of shell length | C. sacculus |
| – | Shell length 27–30 µm; aperture 10–13 µm; neck 1/4 of shell length | C. sacculus sakotschawi |
| 13 | Lateral sides conical (angular) | C. angulata |
| – | Lateral sides rounded | 14 |
| 14 | Aperture diameter equal to shell width | 15 |
| – | Aperture diameter considerably smaller than shell width | 16 |
| 15 | Shell width equal to shell length | C. pusilla |
| – | Shell width considerably smaller than shell length | C. pusilla conica |
| 16 | Shell length > 40 µm | 27 |
| – | Shell length < 40 µm | 17 |
| 17 | Shell length 8–12 µm | 18 |
| – | Shell length higher | 20 |
| 18 | Shell length higher than shell width | C. minuta |
| – | Shell length considerably smaller than shell width | 19 |
| 19 | Shell surface covered by scarce sand grains | C. patinata |
| – | Shell surface smooth | C. bassini |
| 20 | Shell surface covered by scarce sand grains | 25 |
| – | Shell surface smooth | 21 |
| 21 | Aperture large, its diameter about 1/3–1/4 of maximal shell width | 23 |
| – | Aperture small, its diameter about 1/6–1/8 of maximal shell width | 22 |
| 22 | Aperture diameter 1.5–2.5 µm | C. leachi |
| – | Aperture diameter 3.5–4.5 µm | C. vulgaris |
| 23 | Shell ovoid | 24 |
| – | Shell pyriform | C. crenulata glabra |
| 24 | Shell colorless | C. oviformis |
| – | Shell yellowish-brown, oval | C. oviformis fusca |
| 25 | Shell subsphaerical, width/length ratio > 0.8 | C. horrida |
| – | Shell elongated, width/length ratio < 0.7 | 26 |
| 26 | Shell length 24–27 µm | C. voigti |
| – | Shell length 20–24 µm | C. psammophila |
| – | Shell length 17–21 µm | C. angusta |
| 27 | Shell length 40 µm, broadest width at the middle of the shell | C. apiculata |
| – | Shell length 55 µm, broadest width at the upper third of the shell | C. valida |
| | |
