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: Colonies of 8-32 (rarely 4, 64 or 128 cells) with large intercellular space; inner cells H-shaped, trapezoid to rectangular when matured; outer half of the marginal cells prolonged into two horn-like processes with a tuft of long bristles; daughter colonies formed in a vesicle released from mother cell; cell wall smooth (Photomicrographs of the fresh-water algae, 1994).

50 μm 100 μm 150 μm; x 400

Pediastrum duplex, x 400, x 640, Shinobazu-ike (pond), Ueno, Taito-ku, Tokyo, Japan, March 2001 by Y. Tsukii