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 | Aperture oval | 2 |
| – | Aperture circular | 5 |
| 2 | Shell strongly curved, like a saddle | A. excavata |
| – | Shell otherwise | 3 |
| 3 | Aperture without pores | A. lichenophila |
| – | Aperture with pores | 4 |
| 4 | Dorsal surface without ribs | A. ovaliformis |
| – | Dorsal surface with more or less clear ribs | A. catinus |
| 5 | Shell with spines or projections | 6 |
| – | Shell otherwise | 8 |
| 6 | Shell base with numerous sharply pointed spines | A. rota |
| – | Shell with fewer and less sharply pointed projections | 7 |
| 7 | Shell base with some conical projections | A. dentata |
| – | Shell crowned with some conical projections | A. corona |
| 8 | Shell resembling a pie or sand cake | A. formosa |
| – | Shell not resembling a pie or sand cake | 9 |
| 9 | Shell balloon shaped, more or less spherical | 10 |
| – | Shell otherwise | 14 |
| 10 | Pseudostome smooth | 11 |
| – | Pseudostome with undulations or lobes | 12 |
| 11 | Shell with distinct neck | A. nordestina |
| – | Shell without distinct neck | A. jeanelli |
| 12 | Pseudostome cross-shaped, with four lobes | A. peruviana |
| – | Pseudostome with many lobes | 13 |
| 13 | Shell smooth, sometimes with small undulations, 80-200 µm in diameter | A. mitrata |
| – | Shell with distinct ribs, 80-90 µm in diameter | A. spectabilis |
| 14 | Shell diameter >= height | 26 |
| – | Shell diameter < height | 15 |
| 15 | Shell height < 1/3 of shell diameter | 16 |
| – | Shell height > 1/3 of shell diameter | 17 |
| 16 | Shell diameter > 180 µm | A. megastoma |
| – | Shell diameter < 180 µm | A. discoides |
| 17 | Shell base usually not purely circular, but irregular or angular | 18 |
| – | Shell base usually circular | 19 |
| 18 | Shell diameter < 140 µm | A. catinus |
| – | Shell diameter > 150 µm | A. artocrea |
| 19 | Shell surface with folds, sculptures or depressions | 20 |
| – | Shell surface usually smooth | 23 |
| 20 | Dorsal surface with some dents or sculptures | A. arenaria |
| – | Shell surface angular or with depressions | 21 |
| 21 | Dorsal surface with depressions | A. intermedia A. gibbosa |
| – | Sides with rib-like structures | 22 |
| 22 | Shell apex pointed, in side view resembling a tent | A. conica |
| – | Shell apex flat, in side view resembling a trapezoid | A. costata |
| 23 | Aperture irregular, crenulated | A. crenulata |
| – | Aperture smooth | 24 |
| 24 | Shell base flat like the edge of a hat, striated | |
| – | Shell base not flat, not striated |
25 |
| 25 | Shell diameter > 75 µm; shell with basal rim | A. vulgaris |
| – | Shell diameter < 75 µm; shell hemispherical without any rim | A. hemisphaerica |
| 26 | Shell hat-shaped, with broad flat edge | A. rukiensis |
| – | Shell otherwise | 27 |
| 27 | Shell resembling a a Greek amphora | A. apicata |
| – | Shell resembling Gandalf’s hat | A. gandalfi |