Rossithidium Guide
Credit: Marina Potapova
  1. Frustules heterovalvar
  2. Raphe valve concave, rapheless valve convex
  3. Valves very small, linear-elliptic to lanceolate
  4. Striae uni- or biseriate
  5. Terminal raphe fissures absent

The valve outline is linear, elongate or lanceolate, with rounded apices. The striae are parallel throughout the valve, on both raphe and rapheless valves. The striae are variable and may be uni- or biseriate. The axial area is narrow. The raphe is straight with expanded proximal raphe ends. Terminal raphe fissures are absent. The central area on the raphe valve varies in both size and shape. Areolae are present on the valve mantle.

Rossithidium is similar to Achnanthidium. Rossithidium was established to include three linear species of Achnanthidium (Round and Bukhtiyarova 1996) including Achnanthes linearis W.Smith. However, Kingston (2003) questioned the separation of Rossithidium from Achnanthidium. Many ecological surveys do not discriminate the species in Achnanthidium and Rossithidium.