Neidium fossum guide
  1. Valves large
  2. Valves elliptic-lanceolate
  3. One broad longitudinal canal
  4. Multiple, narrow longitudinal canals

Valves are large, with a elliptic-lanceolate outline. Striae are nearly parallel across the valve face, becoming slightly convergent at the valve apices. Proximal raphe ends are relatively small. Axial area is hyaline. Innermost longitudinal canal is broad. Distal longitudinal canals on the mantle are narrow.