Achnanthidium Rivulare Guide
  1. Striae parallel at center valve
  2. Striae nearly parallel at apices
  3. Axial area narrow

Valves are linear-elliptic. Striae are nearly parallel throughout both valves, except striae may become slightly radiate at the apices of the rapheless valve. The axial area is narrow, but widened at the valve center. Areolae are usually not visible with LM.