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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.

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

A. brasiliensis

 – 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
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