Genus: Free-swimming; rigid oval; ventral surface convex; dorsal surface with a longitudinal groove; flagella anterior; one long, directed anteriorly; the other shorter and trailing; fresh water (Kudo, 1966). Species:

50 μm 100 μm 150 μm; x 400

N. apocamptus Stokes: Much compressed; dorsoventrally flattened; ventral face concave; 6-10 μm long, 5-6 μm wide, 3-4 μm thick; long flagellum straight during swimming, with minute waves at the tip (Illustrated Guide, 1985).