Family: Unicells or cells loosely attached forming 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 cylindroid, rounded at both ends; cell wall smooth or granulated or longitudinally lined; constriction or bands present in some species; outer cell wall yellowish or brownish; chloroplast has radially-arranged laminae which attached cell wall in full length; one or several pyrenoids in each semicell, aligned in chloroplasts (Illustrations of The Japanese Fresh-water Algae, 1977).

P. margaritaceum 3 clips 144 x 22 μm L/W= 6.5-7.0 Hiroshima, 1998