Genus: Unicellular; variable in shape; a constriction at the center of the cell body; mostly longer than wide; flattened; each semicell hemispherical, spherical, ellipsoidal, rectangular, pyramidal or kidney-shaped; no apical indentation (Illustrations of The Japanese Fresh-water Algae, 1977). Species: Cell body small, 16-(17-26.5) μm long, (10.5-14)-18 μm wide, isthmus 10-(10.5-14)-17 μm, L/W=1.25-1.5; semicells nearly spherical or inverted broad ovoidal; both ends broader than the center; apex slightly flat; cell wall smooth; var. perminutum: 9-13.5 μm long, 5.6-8.5 μm wide, isthmus 6-7 μm, L/W=2; semicells inverted ovoidal; apex rounded (Illustrations of The Japanese Fresh-water Algae, 1977).

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

Cosmarium (pseudarctoum ?), x 640, Kaga city, Ishikawa Pref., 2000 by Y. Tsukii
31 μm 63 μm 94 μm; x 640