Arcella mitriformis – Microworld

A. mitriformis, dorsal view and lateral view

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. mitriformis, ventral view and dorsal view

Arcella mitriformis, 102 and 95 µm

*) Note: No new name needs to be established in this case under the International Code of Zoological Nomenclature. Because the name was published before 1961 using the abbreviation var., it is deemed subspecific under Article 45.6.4, unless explicitly proposed as infrasubspecific. As a species-group name, it is simultaneously established at both species and subspecies rank under the Principle of Coordination (Art. 46.1), with the same author, date, and name-bearing type. Accordingly, when treated at species rank, the correct name is Arcella mitriformis ,Deflandre, 1928.

Ferry Siemensma, created March 20, 2025; last modified April 29, 2026