Вероятно, мы нашли категорию простейших по вашему запросу:

Genus Paracentropyxis

Shell is circular or slightly elliptical in frontal view; at the dorsal face, a groove separates the reminder of the shell from a small visor in from of tapered crescent. Ventral surface is concave; at median level the visor is rounded, forming a buccal cavity, ventral face and the side walls of this vestibulum are connected to the dorsal part of the shell; with small internal opening as in Centropyxis sylvatica. The slit-like aperture is hidden by the dorsal lip and an extension of the ventral shell wall (cryptostomia). Shell transparent and composed of exogenous mineral particles, which are embedded in a hyaline cement. Ecology: tropical soils. Monospecific. Type species: Paracentropyxis mimetica Bonnet, 1960. Note: Dekhtiar (2009) placed the genus Paracentropyxis in the family Centropyxidae with species Paracentropyxis sylvatica (Deflandre, 1929) Dekhtiar, 2009 (basionym Centropyxis sylvatica Deflandre, 1929) and Paracentropyxis matthesi (Rauenbusch, 1987) Dekhtiar, 2009 (basionym Centropyxis matthesi Rauenbusch, 1987).