Pyxidicula patens Claparède et Lachman, 1858
Diagnosis: shell hemispherical, aperture nearly as large as the diameter of the shell; aperture bordered by a small inner ring, which is in fact a recurved part of the shell, and a broader tubular outer ring which is hyaline, thinner than the rest of the shell and more or less undulated and fragmented. The inner ring has a vacuolated structure. Nucleus single, with some dispersed small nucleoli.
Dimensions: Penard: diameter 35-50 µm, mean 40 µm, nucleus 10 µm; my measurements: 38-41 µm, no living specimens observed.
Ecology: freshwater, rare; in Sphagnum, but also in my garden pond.
Remarks: This species was described by Penard in 1902. I found it several times in the Netherlands. It was also recorded for Austria in 2012 by Angie Opitz (pers. comm.).
The description of Penard fits very well to our specimens.The most characteristic feature is the thin hyaline zoom. Penard describes this as follows: "Cette collerette ou frange circulaire est plus claire et plus mince que le reste de la coque, et ses bords tres amincis se voient la plupart du temps "ondulés et déchiquetés." (translated: This collar or circular fringe is clear and much thinner than the rest of the shell, and its very thinned edges are most of the time curled and shredded.)
Penard describes the nucleus as spherical, granular, with no clear distinction between nucleolus and nuclear juice. In a note he admits that he has isolated only one nucleus.