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: Cell body flat, large, variable in shape, longer than wide; strong constriction at center; each semicell with several notches; cell wall with many short spines (Illustrations of The Japanese Fresh-water Algae, 1977).

M. pusillum 1 clip cells 5-8 μm diam. spines ~30 μm S/L=3.8 Japan, 199?