Guide to Pinnularia decrescens
- Valve margin slightly triundulate
- Central valve expansion slightly wider than distal expansion
- Apices rostrate
- Central area large
- Striae radiate in the middle, convergent toward the apices
Valves have a slightly triundulate margin, with the central expansion slightly wider than the distal expansion. Apices are rostrate. The axial area is linear-lanceolate, 1/4-1/3 of the valve width. The central area is large and elliptic to rhombic. Striae are radiate in the middle, becoming strongly convergent toward the apices.