Guide to Pinnularia undula
- Valves linear or slightly triundulate
- Central and distal undulation of about the same width
- Apices rostrate to subcapitate
- Central area large, round
- External depressions small, round depressions
Valves are linear, becoming slightly triundulate in larger specimens. The central and distal undulations are about the same width. Apices are rostrate to subcapitate. The central area is large and round. Small round depressions are present on the external valve surface in the axial and central areas. The striae are radiate at center valve, becoming convergent at the apices.