Description of Pterocystis
with a periplast with an internal layer of tangential plate-scales and external one of radial spicules. The latter consist of a shaft and a thin membranous basal region which may extend along the shaft and may form a stalk-like protrusion at the base of the spicule. The membranous region is not clearly differentiated into lateral and basal regions as in Echinocystis
. Ten species all from freshwater, Type species: P. pteracantha (Siemensma, 1981) Siemensma et Roijackers, 1988.