Achnanthes Guide
Credit: Mark Edlund
  1. Frustules heterovalvar - one valve with a raphe, the other lacking a raphe
  2. Raphe valve usually possesses a fascia
  3. Valves shallow and arched in girdle view
  4. Rapheless valve with a narrow, excentric sternum

The frustules of Achnanthes are heterovalvar. The raphe valve usually possesses a central area of thickened silica, called a fascia. The rapheless valve has no such central area and the sternum may be positioned near the valve margin. In girdle view, the mantle of the rapheless valve appears to be more ornamented than that of the raphe valve. The striae are uni-, bi-, or triseriate and composed of areolae covered by complex cribra, or sieve plates.

Cells may grow alone, or may form short chains. Living cells are usually attached to substrata by a mucilage stalk extending through one end of the raphe valve. Most species are found in marine habitats, but a few taxa occur inland including Achnanthes coarctata. Inland species are commonly associated with mosses and lichens in aerophillic habitats. Many common species previously classified in this genus are now recognized as species of Achnanthidium, Psammothidium, Planothidium, and other genera.