Genus: Colonies of a fixed number of cells, flat, circular in shape; cell body polygonal in shape, with horn-like projections; cell wall garanulated, wrinkled or notched; chloroplasts plate-like or reticular; asexual reproduction by zoospore; sexual reproduction by isogametes (Illustrations of The Japanese Fresh-water Algae, 1977).
Species: Colonies of 8-32 (rarely 4, 64 or 128 cells) with large intercellular space; inner cells H or X-shaped, trapezoid to rectangular when matured; outer half of the marginal cells prolonged into two horn-like processes with a tuft of long bristles; daughter colonies formed in a vesicle released from mother cell; cell wall smooth (Photomicrographs of the fresh-water algae, 1994).
Google Image Search
Similar species -->> P. angertiniense
Horn-like processes swelled 
var. rotundatum
16-cells
16 = 10+5+1
Japan, 1998
var. rotundatum
16-cells
16 = 10+5+1
Tokyo, 2001
var. rotundatum
32-, 16-cells
16 = 10+5+1
Japan, 1998
var. rotundatum
15-cells ?
Japan, 1998
var. rotundatum
Shakujii Park
Tokyo, 2001
