Achnanthes Bory 1822

Frustules are monoraphid and heterovalvar, with one valve (the R-valve, or RV) bearing the raphe-sternum, and one valve (the P-valve, or PV) bearing a pseudoraphe, or raphe-less sternum. On the RV, the raphe-sternum is generally central, with a median fascia. On the PV, there is no fascia, and the narrower pseudoraphe is more or less excentric. Valves are slightly bent around the transapical axis, the RV being concave and the PV convex, giving the frustule a characteristic V-shape when seen in girdle view. Striae are uniseriate or multiseriate, varying with the species. Cells are either solitary or form short chains, in the former case with the RV appressed to a substrate, in the latter case most frequently attached to a substrate by a mucilage stalk produced from the RV.

Originally conceived as a very broad, inclusive group, intensive study over the past two decades has resulted in the separation of most species into newly created genera. Those remaining are almost exclusively marine, with only a few exceptions; e.g., Achnanthes coarctata (Brébisson) Grunow, a quite common aerophil. Unfortunately, some species originally described in Achnanthes are now taxonomically "stranded": they clearly do not belong to the genus sensu stricto, but have not been formally moved to other genera.