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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.

1Shell strongly curved, like a saddle; aperture ellipticalA. excavata
 –Shell not strongly curved; aperture circular2
2Shell base with some conical projectionsA. dentata
 –Shell base with numerous pointed projectionsA. rota
 –Shell base without projections 3
3Shell resembling a pie or sand cake  A. formosa
 –Shell not resembling a pie or sand cake4
4Shell balloon shaped, spherical A. mitrata
 –Shell  hemispherical or flattened 5
5Shell height < 1/3 of shell diameter6
 –Shell height > 1/3 of shell diameter7
6Shell  diameter > 180 µmA. megastoma
 –Shell  diameter < 180 µmA. discoides
7Shell base usually not purely circular, but irregular or angular8
 –Shell base usually circular 9
8Shell diameter < 140 µmA. catinus
 –Shell diameter > 150 µmA. artocrea
9Shell 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
11Dorsal surface with depressionsA. intermedia
A. gibbosa
 –Sides with rib-like structures12
12Shell apex pointed, in side view resembling a tentA. conica
Shell apex flat, in side view resembling a trapezoidA. costata
13Aperture irregular, crenulatedA. crenulata
 –Aperture smooth14
14Shell base flat like the edge of a hat, striated

A. brasiliensis

 –Shell base not flat, not striated

15

15Shell diameter > 75 µm;  shell with basal rimA. vulgaris
 –Shell diameter < 75 µm; shell hemispherical without any rimA. hemisphaerica
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