Pinnularia macilenta guide
  1. Valve linear, with expanded center
  2. Axial area 1/4-1/2 valve width
  3. Fascia small, asymmetric, transverse
  4. Central area with four irregular markings

Valves are linear, with an expanded central area and slightly subcapitate apices. The axial area is 1/4-1/2 of the valve width. The central area is irregular and rhombic, with a small asymmetric transverse fascia. The small asymmetric fascia may be absent in smaller specimens. The central area may have four irregular markings.