Genus: Cell body long cylindroid; a slight constriction at center; cross section of the semicells spherical, slightly swell at the center but without a folded structure as Docidium; slightly tapered toward both ends, which are truncated and smooth or surrounded by granules or various small structures; laterally straight or wavy, with spiny projections in some species; 2-10 ribbon-like chloroplasts in each semicell; with many pyrenoids; a vacuole containing active granules at both ends same as in Closterium; cell wall mostly dotted, but with spines or other patterns in some species (Illustrations of The Japanese Fresh-water Algae, 1977).

P. minutum 1 clip Hiroshima, 1998	P. minutum 1 clip Hiroshima, 1998	P. minutum 1 clip Hiroshima, 1998
P. minutum 2 clips Hiroshima, 1998	P. minutum 2 clips Hiroshima, 1998	P. minutum 2 clips Hiroshima, 1998
P. trabecula 2 clips Japan, 1998	P. kayei 3 clips Hiroshima, 1998