Genus: Colonies composed of 16, 32, 64 or 128 (mostly 32) cells; 2-layered gelatinous envelope; mamillations anterior or posterior; cells spherical to ovoid with 2 equal-length apical flagella; eyespot present; anisogamy (Illustrated Guide, 1985).
Species: Colonies of 32 or 64 cells, spherical in shape, 60-120 μm in diam.; cells arranging separately; cell body spherical, 8-10 μm in diam.; a chloroplast cup-shaped, with a pyrenoid (Illustrations of the Japanese fresh-water algae, 1977).