Genus: Variable in shape and in size; mostly longer than wide; strong constriction at center; semicell pyramid-shaped, apically flattened with indentation (or notch, some species lack this notch); laterally straight or wavy or indented; one chloroplast in each semicell; one pyrenoid for each chloroplast (Illustrations of The Japanese Fresh-water Algae, 1977). Subgenus: Atomum (Apical area slightly depressed) Section: Simplicia (Cell body small; cell surface smooth) Species: 25 μm long, 17.4 μm wide, isthmus 7 μm (Illustrations of The Japanese Fresh-water Algae, 1977).

31 μm 63 μm 94 μm; x 640

Images of collecting locality: Block 4, Kashibaru marsh, Karatsu city, Saga Pref., Japan, September 9, 2008 by Y. Tsukii