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:

50 μm 100 μm 150 μm; x 400 : 31 μm 63 μm 94 μm; x 640

Phacus curvicauda Swirenko: Cell body 24-26 μm diam., (25)-38-30 μm long; 2 paramylons (Illustrations of The Japanese Fresh-water Algae, 1977).