Вероятно, мы нашли категорию простейших по вашему запросу:

Genus Planocarina

Shell is elongated-pyriform, with a distinct neck, lateral margins tapering towards the aperture. A flat keel surrounds the entire posterior part of the shell. Shell is hyaline or slightly yellowish, composed of circular to elongated shell plates probably recycled from euglyphid testate amoeba prey. Type species: Planocarina carinata (Archer, 1867)