Nitzschia Sigma Guide
Credit: Pat Kociolek
  1. Valve apices curved in opposite directions
  2. Apices small, capitate
  3. Striae distinctly punctate

In valve view, the valve apices curve in opposite directions. Valves lack an expanded central gap in the fibulae (called a central nodule by some). The valve apices are small, rounded, and capitate. The striae are distinctly punctate. In girdle view, the frustule has a distinct sigmoid shape.