Pinnularia dactylus has smaller valves less than 38 µm in width and greater than 6 striae in 10 µm. Valves of P. mesogongyla are also less than 38 µm in width, a smaller central area and 5-6 striae in 10 µm. Valves of Pinnularia viridis are less than 30 µm wide and and 6-7 striae in 10 µm. Valves of P. flexuosa have broadly rounded apices.