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.
Ecology: 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 (Deflandre, 1928). Netherlands (Siemensma, unpubl.).
Remarks: Arcella mitriformis exhibits considerable variation in its overall shape.
A formal nomenclatural act is not required.
(1) Under Article 45.6.3, names published before 1961 using the abbreviation “var.” are deemed subspecific rather than infrasubspecific, and therefore fall under the rules for species‑group nominal taxa (Chapter 10).
(2) Under Article 46.1, names established at either species rank (species or subspecies) are simultaneously established at the other rank, with the same author and type. The authorship therefore remains unchanged.