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: Cell body ovoidal or ellipsoidal, 30-50 μm long, 15-20 μm wide, cauda nearly straight or slightly curved, keel reached at the base of the cauda, striae straight, paramylon bodies disc-shaped with a hole at its center, 1-3 in number (An Illustrated Guide to Freshwater Zooplankton in Japan, 2000).

Phacus caudatus Huebner var. caudatus: Cell body ovoid or pear-shaped, 15-20 μm diam., 30-50 μm long; lateral longitudinal ridge; anterior broad rounded; posterior straight long spine; a single discoid paramylon; (Illustrations of The Japanese Fresh-water Algae, 1977).