A4. Cells divided into paired semi-cells that are approximate mirror images of each other: Desmids

B1. Cells mostly solitary unless dividing

C1. Cells are rectangular or lens-shaped (length greater than width): Genicularina, Gonatozygon, Penium, Roya, Spirotaenia

C2. Semi-cell with depressed or notched apex: Euastrum, Cosmarium (in part)

C3. Cells are crescent-shaped, with little resemblance to other desmids: Closterium

C4. Cells angular in cross-section (mostly tri-radiate); some species with arm-like lateral extensions; very large, diverse genera: Staurastrum and Staurodesmus

C5. Cells are spherical, oval, or elliptical in cross-section (view from apex) and lack long spines: Cosmarium