Pyxidicula cymbalum Penard, 1902
Diagnosis: Shell patelliform, transparent, usually light brown, and distinctly punctate. Aperture round, with a double rim: one curved inward and the other outward. Contractile vacuole single. Pseudopodia unknown.
Dimensions: According to Penard (1902): shell diameter 85–90 µm. My measurements over the years: 69–120 µm. Specimens from a Mongolian fen (Łuców et al., 2025): shell diameter 56–93 µm; building units 0.9–1.1 µm across; rim 7.4–8.8 µm wide (n = 21).
Ecology: Wet moss and Sphagnum, but also found in various water types, e.g. drinking‑water basins (Waterlaboratorium – Haarlem).
Geographical distribution: Probably cosmopolitan. Common, but easily overlooked.
Remarks: Although not mentioned by Penard (1902), two rims are present: an outer rim, which is always present, and an inner rim, which may be very small, absent, or difficult to detect. Both rims may form an angle of 180° or less, appearing as a V in cross‑section. The outer rim may show radiating striations or an alveolar structure (see below).
