Genus: Highly flattened; pellicle firm; body form constant (Kudo, 1966). Chloroplasts small, discoid, pyrenoids usually absent; most species flat and leaf-shaped; often with ridges or fins running helically or longitudinally (Illustrated Guide, 1985).
Species: About 30-50 μm long, 10-20 μm wide; circular in cross section; with caudal prolongation; pelicle obliquely ridged; flagellum as long as the body (Kudo, 19660). Cell body 28-55 μm long, 10-21 μm wide two paramylons thick disc-shaped, parietal (An Illustrated Guide to Freshwater Zooplankton in Japan, 2000).