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. echinulatum: Colonies without perforations; cell wall densely covered with smallspines (Photomicrographs of the Freshwater Algae, vol. 11, 1988). Var. simplex: Colonies of 4, 8, 16, 32 cells, intercellular space absent, cells 20-30 μm long, 6-15 μm wide (Illustrations of The Japanese Fresh-water Algae, 1977).

31 μm 63 μm 94 μm; x 640

Images of collecting locality: Yasuragi-no-ike (pond), Oura central park, Oura town, Gunma Pref., Japan, February 2006 by Y. Tsukii

Pediastrum simplex (Meyen 1829) Lemmermann var. simplex: Colonies of 4, 8, 16, 32 cells with no intercellular space, cells 20-30 μm long, 6-15 μm wide (Illustrations of The Japanese Fresh-water Algae, 1977).

* Pediastrum simplex var. duodenarium (Bailey) Rabenhorst: Colonies of 4, 8, 16, 32 cells with large intercellular space, cells 15-25 μm long, 6-13 μm wide (Illustrations of The Japanese Fresh-water Algae, 1977).