Trigonopyxis arcula Penard, 1912
Diagnosis: Shell brownish, hemispherical; pseudostome central, invaginated, frequently triangular but more often irregular, surrounded by a distinct ring of organic cement; nucleus ovular.
Dimensions: Diameter of shell, my measurements: 87-173 µm. Within a population the size of the shells can vary considerably.
Ecology: Feeding on bacteria and fungi. Common in acid forest soil, litter, mosses (e.g. Sphagnum).