Вероятно, мы нашли категорию простейших по вашему запросу:

Genus Heteroglypha

Shell is wedge-shaped, fundus is rounded, sides sub-parallel, anterior end only slighlty smaller than fundus. In lateral view, shell is tapered almost to point at apertural end. Aperture is terminal, slit-like or slightly arched, located at right angle to the broad diameter, with clear organic rim. Scales are elliptical, arranged as in Euglypha. Ecology: freshwater. Type species: Heteroglypha delicatula Thomas et Gauthier-Lièvre, 1959