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 usally smooth

  13
10

Dorsal surface with some dents or sculptures 

A. arenaria
 -

Shell surface angulated or woth 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

A. brasiliensis

 -

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