Genus: Filamentous algae without branching, no rhizoids, covered with gelatinous sheath;
cell body cylindrical or ellipsoidal, cells attached side by side or separated;
a single chloroplast band-form, with a pyrenoid
(Guide book to the "Photomicrographs of the Freshwater Algae", 1998).
Geminella mutabilis (Brébisson) Wille: Colonies linear filaments consisting of 10-20 cells; cell body short cylindroid, both ends rounded, 15-20 μm long, 8-10 μm in diam.; cells separated each other; a single chloroplast plate-like, with a pyrenoid (Illustrations of the Japanease Freshwater Algae, 1977). The thickness of the gelatinous sheath variable; planktonic; cell body ellipsoidal or cylindrical with rounded ends, 25-35 μm long, 9-20 μm in diam.; cells scarcely adherent to each other or equally separated one another; chloroplasts a single, plate-like forming a girdle, with a pyrenoid (Photomicrographs of the Fresh-water Algae, vol. 3 (1985).
Geminella ellipsoidea (Prescott) Smith var. ellipsoidea: Linear short filaments, free-floating, covered with a thick gelatinous sheath; cell body elliptic to ovate, 4-6 μm long, 7-9 μm in diam., transversely arranging in groups of 2 or 4 cells; a single chloroplast parietal, band-form, with a pyrenoid (Photomicrographs of the Fresh-water Algae, vol. 4 (1985).
Geminella planctonica Iyengar & Philipose: Thalli unbranched, short filaments clearly constricted at the cross wall, without mucilaginous sheath; cell body elontate cylindrical, 3-5 μm broad at the cross wall, 4-7 μm broad at the mid region of the cell; 8-24 μm long; chloroplasts a single, parietal, usually occupying the entire length of the cell; with 1-2 pyrenoids; (Photomicrog. Freshw. Alg. 13: 21, 1994)
Geminella minor (Nägeli) Hansgirg: Colonies a short lnear filament covered with a gelatinous sheath; cell body short cylindroid, 8-18 μm long, 4-8 μm wide; adjacent cells attached with each other (Illustrations of the Japanese Freshwater Algae, 1977).