Pinnularia socialis guide
  1. Cells joined at valve margin into colonies
  2. Valves broadly linear
  3. Axial area broad
  4. Striae are parallel or very weakly radiate

Cells are joined by their girdle sides in small colonies of 2 - 4. The axial area is broad, 1/3 to 1/2 of the valve width. Striae are parallel, or very weakly radiate.