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:

31 μm 63 μm 94 μm; x 640

Images of collecting locality: No.1, Ko-numa (pond/marsh), Minakami town (formerly Tsukiyono town), Gunma Pref., Japan, October 12, 2008 by Y. Tsukii