Family: Test rigid, chitinoid, often areolar, smooth; pseudostome ventral, round
(Illustrated Guide, 1985).
Genus: Test variable, chitinoid; binucleate (Illustrated Guide, 1985).
Arcella discoides Ehrenberg, 1871: Diam. 90-150 μm, height 25-30 μm; pseudostome, round, 30-40 μm; with or without ring of pores (Illustrated Guide, 1985).