Arcella costata  Ehrenberg, 1847

Diagnosis: Shell in side view trapezoid, with a flat polygonal top; slopes with six or more angular facets, bordered by some prominent folds; aperture slightly invaginated, circular and bordered by a collar.

Dimensions: In literature diameters 52—100 µm. My measurements about 90 µm.

Ecology: Freshwater; on en between aquatic plants.

Remarks: Variation in this species appears to be restricted to the arrangement of the angular folds. Specimens can be confused with Arcella conica, which has a domed top.