Вероятно, мы нашли категорию простейших по вашему запросу:

Genus Trinema

In ventral view, shell is ovoid or broad oval, sometimes with almost parallel sides. In lateral view, the dorsal side is arched or flattened towards the aperture, ventral side is either flat or concave. Shell is covered by body-scales of two types: large circular plates and smaller circular or broad elliptical scales in between, overlapping. Aperture is sub-terminal or ventral, circular or oval, invaginated, surrounded by rows of toothed apertural plates. Ecology: freshwaters, mosses, soils. Type species: Trinema enchelys (Ehrenberg, 1838) Leidy, 1878