Arcella mitriformis (Deflandre, 1928) *)
Basionym: Arcella gibbosa var. mitriformis Deflandre, 1928

Diagnosis: Differs from A. gibbosa by its even more convex dorsal side, which does not widen towards the lower part. Marginal undulation is often absent; when present, it is generally located closer to the oral side than in the type. The maximum diameter is often found at mid-height of the shell, and consequently, the oral side has a smaller diameter. In lateral view, the dorsal side is undulated, ranging from a semicircle to three-quarters of a circle. The apertural side is rarely without a buccal tube.

Dimensions: According to Deflandre (1928): Shell diameter 60—88 µm; height 52—72 µm; diameter of aperture 17—28 µm; apertural invagination 10—18 µm. H/D ratio 0.63—0.93.

Habitat: Freshwater. In variable biological conditions, such as siliceous ponds and peat bogs on limestone.

Geographical distribution: North America, (?) Australia. In France: around Paris, Somme, Jura (Deflande, 1928). Netherlands (Siemensma, unpubl.).

Remarks: Arcella mitriformis exhibits considerable variation in its overall shape.