Genus: Colonies of a fixed number of cells, flat, circular in shape; cell body polygonal in shape, with horn-like projections; cell wall garanulated, wrinkled or notched; chloroplasts plate-like or reticular; asexual reproduction by zoospore; sexual reproduction by isogametes (Illustrations of The Japanese Fresh-water Algae, 1977). Species: Marginal cells with a single, long horn-like process (Photomicrographs of the Freshwater Algae, vol. 8, 1988). Var. simplex: Colonies with large perforations; marginal cells Y- to T-shaped, triangular to trapezoid at maturity, 12-52 μm long, 6-30 μm wide; inner cells Y-shaped, 7-35 μm long, 7-28 μm wide; cell wall granulated or smooth (Photomicrographs of the Freshwater Algae, vol. 11, 1993). Var. duodenarium: Colonies of 4, 8, 16, 32 cells, large intercellular space present, cells 15-25 μm long, 6-13 μm wide (Illustrations of The Japanese Fresh-water Algae, 1977).

31 μm 63 μm 94 μm; x 640

Pediastrum simplex Meyen var. duodenarium (Bailey) Rabenhorst, colony 64 μm in diam., x 640, Sakitama-kofun park (Sakitama tumuli park), Gyoda city, Saitama Pref., Japan, October 17, 2004 by Y. Tsukii
31 μm 63 μm 94 μm; x 640

Colony 55 μm in diam., x 640