Pinnularia subgibba has expanded valve margins.
Pinnularia procera can be difficult to separate, however it has narrower valves, (occasionally) undulate margins, and a broader fascia.
Pinnularia laxa has narrower valves, a higher stria density, and a broader fascia.
Pinnularia delicata has capitate apices and narrower valves.
Pinnularia rivularia has a lower stria density and larger valves.
BioData Reference
Krammer 2000 Pl 64 Fig 1-3 9