Вероятно, мы нашли категорию простейших по вашему запросу:



Genus Cyclopyxis

Shell is circular in frontal view, hemispherical in lateral view. Dorsal face is often rough, covered by large sand grains, ventral face is smooth. Aperture is invaginated, in most species circular, few species with irregular or lobed aperture; margin never with thick organic rim but often with small mineral particles. Ecology: freshwater, mosses, soils. Type species: Cyclopyxis arcelloides (Penard, 1902)