Family: Unicells or cells loosely attached to form hyphae; cell body varied in shape, a constriction at the center in most species; cell walls consisting of two or more parts, with small pores and various ornaments (Illustrations of The Japanese Fresh-water Algae, 1977).
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).

C. moniliforme var. moniliforme 1 clip 20 x 12 μm Hiroshima, 1998	Cosmarium sp. 2 clips Torinome kasen p. Kuroiso Tochigi, 1998	C. binum 4 clips Torinome kasen p. Kuroiso Tochigi, 1998
C. sportella var. subnudum 1 clip 48 x 38 μm Hiroshima, 1998	C. granatum 2 clips 37 x 24 μm Hiroshima, 1998	C. notabile ? var. arcticum 1 clip 38 x 24 μm single pyrenoid Hiroshima, 1998
Cosmarium sp. 1 clip μm Hiroshima, 1998	C. maculatum 3 clips 136 x 64 μm Hiroshima, 1998	Cosmarium sp. 3 clips 111 x 66 μm L/W=1.7 USA, 1999
C. obsoletum var. sitvense 3 clips 64 x 64 μm Hiroshima, 1998	C. obsoletum var. sitvense 2 clips 64 x 64 μm Hiroshima, 1998	Cosmarium sp. 2 clips μm Hiroshima, 1998
C. margispinatum var. cornutum 2 clips 60 x 48 μm Hiroshima, 1998	C. margispinatum var. cornutum 1 clip 60 x 48 μm Hiroshima, 1998	Cosmarium sp. 1 clip Japan, 1998