Guide to Neidium

Neidium valves are linear to linear elliptical. Ends are variable in shape across species and may be rounded, capitate, rostrate or protracted. Longitudinal lines are present, formed by internal canals positioned along the valve margins. The distal raphe end  is straight and terminates in a thickened, raised helictoglossa. In many species, this raphe end appears to bifurcate. However,  this apparent bifurcation is actually the edges of a lacinia (termed "apical flap" in some works). Interruptions in the striae, or Voigt discontinuities, are frequently present in Neidium. Voigt discontinuities are positioned on the secondary side of the axial area along each raphe branch. The striae are uniseriate and composed of rounded, distinct areolae. Two plastids are present in living cells, positioned against the girdle.

Neidium is rarely abundant in collections, but the genus is broadly distributed, often growing in slightly acidic waters.