Aperture usually surrounded by pores. Aperture diameter 0.45 of shell diameter. Shell diameter 180 µm.


Aperture surrounded by pores. Aperture diameter ≥ 0.4 of shell diameter. Shell very flat; diameter 180 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.


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


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. 


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

Shell dome shaped; aperture with irregular crenulated border.       

Shell formed like a saddle. Aperture elliptical.       

Shell balloon shaped.       

Test broader than high; heigth 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.       


Shell with striated border


Shell with a number of projections.


Shell border with numerous small spines.


Shell flattened, with crenulated aperture.


Shell crowned with some spines.


Shell with high dome and distinct neck.


Shell like Gandalf’s hat.


Shell sub-hemispherical, round smooth aperture, thickened edges

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