Pinnularia macilenta is wider than all varieties of P. subgibba.
Pinnularia subgibba var. undulata has a similar valve outline and distinct transverse fascia, although larger specimens are difficult to distinguish.
Pinnularia gibba has more capitate apices.
Pinnularia kittelii has wider valves and the central area is indistinct.
Pinnularia tabellaria is less broad, with a higher stria density and more capitate apices.
BioData Reference
Krammer 2000 Pl 68 Fig 1-8 Pl 69 Fig 1-5