Genus: Body small; ellipsoid or ovoid; somewhat compressed; pellicle with conspicuous spiral furrows; cytostome a narrow slit at anterior extremity; with strong cilia on right margin of slit; macronucleus ellipsoid to sausage-form; contractile vacuole posterior (Kudo, 1966). Brosse dikinetids extend forward only to oral dikinetid (Illustrated Guide, 1985). Species:

50 μm 100 μm 150 μm; x 400

P. socialis (Fabre-Domergue): 40-50 μm long; 28-32 μm wide; about 22 μm thick, salt water (Kudo, 1966).