This key is a guide to orientate yourself. It is no guarantee for a definite identification. In fact it is often very difficult to identify a species based on one shell. You have to know the variation within a population at a specific location. And, to be honest, we don’t know how many species there are and what separates them.
It is very hard to identify a species based on morphology alone, because most original descriptions are inadequate. A number of species is not distinguishable.
| 1 | Shell strongly curved, like a saddle; aperture elliptical | A. excavata |
| – | Shell not strongly curved; aperture circular | 2 |
| 2 | Shell base with some conical projections | A. dentata |
| – | Shell base with numerous pointed projections | A. rota |
| – | Shell base without projections | 3 |
| 3 | Shell resembling a pie or sand cake | A. formosa |
| – | Shell not resembling a pie or sand cake | 4 |
| 4 | Shell balloon shaped, spherical | A. mitrata |
| – | Shell hemispherical or flattened | 5 |
| 5 | Shell height < 1/3 of shell diameter | 6 |
| – | Shell height > 1/3 of shell diameter | 7 |
| 6 | Shell diameter > 180 µm | A. megastoma |
| – | Shell diameter < 180 µm | A. discoides |
| 7 | Shell base usually not purely circular, but irregular or angular | 8 |
| – | Shell base usually circular | 9 |
| 8 | Shell diameter < 140 µm | A. catinus |
| – | Shell diameter > 150 µm | A. artocrea |
| 9 | Shell surface with folds, sculptures or depressions | 10 |
| – | Shell surface usually smooth | 13 |
| 10 | Dorsal surface with some dents or sculptures | A. arenaria |
| – | Shell surface angulated or with depressions | 11 |
| 11 | Dorsal surface with depressions | A. intermedia A. gibbosa |
| – | Sides with rib-like structures | 12 |
| 12 | Shell apex pointed, in side view resembling a tent | A. conica |
| – | Shell apex flat, in side view resembling a trapezoid | A. costata |
| 13 | Aperture irregular, crenulated | A. crenulata |
| – | Aperture smooth | 14 |
| 14 | Shell base flat like the edge of a hat, striated | |
| – | Shell base not flat, not striated | 15 |
| 15 | Shell diameter > 75 µm; shell with basal rim | A. vulgaris |
| – | Shell diameter < 75 µm; shell hemispherical without any rim | A. hemisphaerica |
