Arcella conica (Playfair, 1918)
Syn.: Arcella pyramidalis Van Oye, 1926
Diagnosis: shell hemispherical; dorsal surface with angular facets which are bordered by six or more prominent folds; aperture slightly invaginated, circular and bordered by a collar.
Dimensions: in literature diameter 68-100 µm; aperture 20-33 µm wide; my measurements: 49-98 µm.
Remarks: Variation in this species appears to be restricted to the arrangement of the angular folds. Specimens can be confused with A. costata, which has a flat apex. Playfair compares the shell in side view with a marquee-tent.