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Aperture usually surrounded by pores. Aperture diameter 0.45 of shell diameter. Shell diameter 70-160 µm.

Aperture surrounded by pores. Aperture diameter ≥ 0.4 of shell diameter. Shell very flat; diameter 140-402 µm.

Aperture surrounded by pores. Distinct basal rim. Height ca 1/3 of shell diameter.

Shell elliptical, quadrate or circular. Shell often with distinct dorsal folds or ribs. Aperture small and with pores.       

Shell with six or more ribs; dorsal surface with ribs and depressions; aperture smooth, without crenulations.

Shell with flattened top and aperture with crenulated border; usually with six ribs.

Shell in side view trapezoid, with six or more ribs; aperture smooth without crenulations.

 

Shell in lateral view trapezoid, with 5-7 ribs. Aperture with pores.

Shell with basal border. Dorsal surface with depressions. Diameter c. 80-110 µm.

Arcella

Shell with basal border. Dorsal surface with depressions. Diameter c. 80 µm. 

Arcella

Shell in lateral view hemispherical, with acute or somewhat rounded corners,  sometimes with a basal border. Diameter c. 35-55 µm. 

Shell hemispherical, with smooth and broad rounded corners in lateral view. 

Arcella
Arcella

Shell with distinct basal border. Ratio diameter aperture/shell 0.2.

Arcella crenulata
Arcella crenulata

Shell dome shaped; aperture with irregular crenulated border.       

Arcella excavata

Shell formed like a saddle. Aperture elliptical.       

Shell balloon shaped.       

Test broader than high; height c. 80-90 µm.  

Shell with undulations, more or less spherical. 

Shell spherical, wrinkled.       

 

Shell shaped like an old hat.

Arcella formosa

Shell resembling a pie; prominent folds or ribs often with darker color.       

Arcella brasiliensis
Arcella

Shell with striated border

Arcella

Shell with a number of projections.

Arcella

Shell border with numerous small spines.

Arcella multilobata

 

Shell flattened, with crenulated aperture.

 

Shell crowned with some spines.

 

Shell with high dome and distinct neck.

Arcella gandalfi

 

Shell like Gandalf’s hat.

 

Shell sub-hemispherical, round smooth aperture, thickened edges

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