Pinnularia undula major guide
  1. Valves tri- or bidundulate
  2. Central undulation slight
  3. Apices rostrate to subcapitate
  4. Central area large, round
  5. Axial and central areas with small round depressions

Valves are linear and triundulate with central undulation more narrow than the two distal and often barely noticeable, so that valves may appear biundulate. Apices are rostrate to subcapitate, about half of the width of the distal undulations. The central area is large and round. Small round depressions are found on axial and central areas. The striae are radiate at center valve, becoming convergent at the apices.