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: 120-170 μm long, 45-70 μm wide; usually slightly twisted; a long caudal prolongation; flagellum about 1/2 body length; stigma prominent; a discoidal paramylon (paramylum) body central; pellicle longitudinally striated (Kudo, 1966). var. longicauda: cell body 85-170 μm long, 45-70 μm wide; locomotive flagellum shorter than the cell body; a single paramylon body ring shaped (An Illustrated Guide to Freshwater Zooplankton in Japan, 2000).