Cells are free-swimming, solitary, more or less spherical in shape, with a single, parietal chloroplast. They are covered with siliceous scales of a single type. Two flagella are of unequal length. Except for the presence of scales, cells closely resemble those of the genus Ochromonas. Scales are more or less elliptical and perforated by irregularly arranged linear apertures. The genus comprises single species, P. vacuolata, occasionally found in freshwater and brackish plankton in Europe.