Genus: Colonial; cell body with 2 or more spiny hairs
(Illustrations of The Japanese Fresh-water Algae, 1977).
Species: Tetrahedral colonies of 4 or 8 cells; cell body spherical 3-7 (-12) μm in diam., with 1-5 spiny projections 20-35 (-60) μm long; a sngle chloroplast parietal, plate-like, with a pyrenoid (Photomicrographs of the Freshwater Algae, vol. 2, 1984).
Micractinium pussillum Fresenius var. elegans G. M. Smith: a colony of 4-32 spherical cells, cell body 3-5 μm diam., with 2-3 needle-like projections 20-30 μm long, a single cup-shaped chloroplast with a pyrenoid (Illustrations of The Japanese Fresh-water Algae, 1977).