Centropyxis lapponica Grospietsch, 1954
Diagnosis: Shell nearly circular in circumference, without spines. Aperture clear square with rounded corners. Shell viewed from the side irregularly domed. Shell quite transparent, not or only sporadically covered with xenosomes, mostly small diatoms. Edge of the opening curved inward and connected to the dorsal side in several places. There is a large U-shaped opening in the posterior part of the rim, a smaller U-shaped opening on both sides and two small U-shaped openings in the anterior side. Where parts of the edge are attached to the dorsal side, they are visible as dark structures.
Dimensions: Grospietsch (1954): shell diameter about 125 µm, height 45-48 µm, apertural diam. 48-58 µm, H/D 0.36-0.39. Dutch population: shell diam. 140-164 µm, average 153 µm, height 60-63 µm, aperture: smallest diam. 54-65 µm, widest diam. 59-78 µm (n=10).
Ecology: Freshwater and mosses. The type species was described from mosses in Swedish Lapland, hence the name lapponica. In 2021, I found a population in sediment of a pond in the Netherlands.
Remarks: The type species was only found in one sample that originated from moss cushions in a small quarried bog near Abisko, Swedish Lapland. It was not uncommon there (Grospietsch, 1954). The original description is rather short and Grospietsch doesn’t mention the inner structure, which is only visible in his drawing. The shell resembles C. gasparella, but the aperture is much wider, without the typical key-hole shape.