Arcella rotundata  Playfair, 1918

Diagnosis: shell with a depressed dome in side-view, expanded at the sides above the baseline; no basal angles, the margin of the dome merging into the base in a single, broadly rounded curve.

Dimensions: Diameter 47-62 µm, height 25-30 µm. Playfair: 47-54 µm, height 25-30 µm, pseudostome 17 µm.
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Remarks: In a recent study Lahr and Lopes (2009) showed that Arcella hemisphaerica and A. rotundata are indistinguishable. They consider both species as a single entity. See A. hemisphaerica Perty, 1852.
The main feature that could separate individuals of A. rotundata from those of A. hemisphaerica is the larger semi-circle contour in lateral view, but this could also fall within the natural variation. Further observations and measurements are needed.