Arcella gibbosa Penard, 1890
Diagnosis: shell colorless, yellow or brown, circular in dorsal view and domed or hemispherical in lateral view, with an enlargement at the basis (the keel or basal border). Apertural surface and basal collar are smooth, but the aboral surface has a series of regular depressions. Aperture invaginated, circular and with a distinct rim or lip. Variation appears to be restricted to the number or absence of depressions on the domed aboral region. The shell height/shell diameter ratio is in the range 0.49 -0.88.
Dimensions: literature: shell diameter 79-111 µm; height 48-76.5 µm; diameter of aperture 17-36.6 µm. My measurements: shell diameter 91-100 µm.
Remarks: Arcella gibbosa is a problematic species, because it is hard to distinguish from A. hemisphaerica and A. intermedia. These three species form together a morpho complex. More investigations and sufficient data are needed to help with any taxonomical solution.
The following subspecies are indistinguishable from A. gibbosa Penard, 1890:
Arcella gibbosa var. levis Deflandre, 1928
Arcella gibbosa var. mitriformis Deflandre, 1928
Arcella gibbosa var. aplanata Van Oye, 1956