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: Retusa (Cell body large; granulated dome(s) at the center of each semicell)