Pinnularia nobilis, P. nobilis var. regularis, P. roland-schmidtii, and P. pulchella var. canadensis have larger internal opening of alveoli, whose edges collectively appear in LM as a longitudinal line transecting the striae close to the central area.
Pinnularia roland-schmidtii, a species so far only known from South America, has a higher valve length/width ratio.
Pinnularia nobilis and P. nobilis var. regularis differ by more linear valves and a complex raphe.
Pinnularia pulchella var. canadensis is smaller and has large drop-shaped proximal raphe ends.
Pinnularia alabamae is larger and has capitate-cuneate apices and spines on the valve margin.