Psammothidium Guide
Credit: Marina Potopova
  1. Frustules heterovalvar
  2. Raphe valve convex, rapheless valve concave
  3. Valves linear, elliptic or lanceolate
  4. Striae usually uniseriate
  5. Terminal raphe fissures may curve to opposite sides, or absent

Frustules are small, elliptic, lanceolate-elliptic, or linear-elliptic in shape. Frustules are bent in girdle view so that the raphe valve is convex and the rapheless is concave, an arrangement that differs from other achnanthoid genera. The length to width ratio ranges between 1.5 and 3.0. The valve apices are broadly rounded. The arrangement of areolae is similar on the raphe and rapheless valves. Striae are radiate and closely spaced, with nearly 30 striae in 10 ┬Ám. A distinct, rectangular central area is present on the raphe valve. The central area is more expanded on the rapheless valve than the raphe valve, at least in several species. The terminal raphe fissures vary in shape and orientation from very short and almost straight, to long and strongly deflected in opposite directions. In some species, terminal raphe fissures are absent.

Psammothidium contains a number of species (over 40, according to Index Nominum Algarum) including species formerly treated as Achnanthes marginulata Grunow in Cleve and Grunow and Achnanthes levanderi Hustedt. The typical habitat of species of Psammothidium is on sand grains, where they may grow in abundance.