Pinnularia subgibba var lanceolata guide
  1. Valves linear-lanceolate
  2. Apices rostrate
  3. Fascia narrow
  4. Axial area broad, rhombic

Valves are linear-lanceolate, with rostrate apices. The axial area is broad, with a central transverse fascia. In smaller specimens, the fascia may be absent.