Pyxidicula cymbalum, diameter 90 µm
Pyxidicula cymbalum Penard, 1902
Diagnosis: Shell patelliform, colorless, but usually brown in color, distinctly punctuated. Aperture round, bordered by a more or less narrow rim. Contractile vacuole single. Pseudopodia unknown.
Dimensions: diameter 85-90 µm (Penard), my measurements 69-120 µm.
Ecology: wet moss and Sphagnum, but also different water types, e.g. in drink water basins (Waterlaboratorium – Haarlem). Common, but easily overlooked.
Remarks: In fact there are two rims, an outer rim which is always present, and an inner rim which can be very small or absent. Both rims may form an angle of 180º or smaller, in cross section visible as a V. The outer rim may show radiating striations or an alveolar structure (see below).
Pyxidicula cymbalum, detail of the outer rim.
Pyxidicula cymbalum, structure of the dorsal part of the shell.
Original drawing of Penard (1902) in Faune Rhizopodique du Bassin du Léman.
The structure of the double margin. There is an inner margin and an outer rim. The inner margin is not always present or distinct. It can also be hidden by the outer rim (f2), if viewed from the dorsal side. (Drawing: Ferry Siemensma)
Stacked images, test from Austria, 89 µm.
Pyxidicula cymbalum, edge with distinct inner and outer rim, 92 µm
Pyxidicula cymbalum, arrows point to the outer rim (left) and the inner rim.
Old test, with relatively broad rim, 89 µm – Heelsumse beek, the Netherlands
Detail of a 95 µm large shell
Pyxidicula cymbalum,c. 75 µm
Pyxidicula cymbalum c. 69 µm
Pyxidicula cymbalum c. 82 µm