Tweet this page Like this page Email this page Share this page Home» Resources » Identification » Identification of algae » Freshwater algae » How to identify algae » Identification guide » Unicellular » Cell wall has the form of a rigid clear box » Semi-rectangular » Rotational symmetry of cell face outline 180 degrees, with the ends being opposite reflections of each other Rotational symmetry of cell face outline 180 degrees, with the ends being opposite reflections of each other Boat-shaped, sometimes with elongated ends, in valve view; striae faint. Nitzschia Weakly S-shaped in valve view. Gyrosigma