Ancyromonas sigmoides Kent, 1880. The cell outline is oval. Cells are 3 to 7 microns long, 2 to 3.5 microns wide and dorso-ventrally flattened. This species has a shallow groove ventrally near an anterio-lateral margin of the cell. The cells have a thin stiff anterior flagellum emerging from an anterior depression. The anterior flagellum can be easily overlooked and beats slowly. The posterior flagellum is about 1.5 times the length of the cell and may not be acronematic. The cells move by gliding with the posterior flagellum trailing.