Вероятно, мы нашли категорию простейших по вашему запросу:

Genus Argynnia

Shell is ovoid, slightly compressed in cross section; covered with various euglyphid plates mixed with diatom fragments and mineral particles in porous cement, grayish. Aperture is terminal, circular, surrounded by siliceous plates giving a rough outline. Ecology: freshwater, moss, soil. Type species: Argynnia schwabei Jung, 1942