Ehrenberg, 1844 :
Frustules in short or long filaments, sometimes free, oblong, separated by foramina and united by setae crossing each other near their insertion. Valves convex, concave or plane, usually elliptical, sometimes nearly circular. From each valve extend two setae, one on each side, which cross the setae of the opposing valve. Setae robust or slender, variable in length, with or without spines, punctate or transversely striated, often hollow. Chromatophores of various kinds, either laminate, single, on the wall of the zone, double, one on each valve, or granular and more or less numerous, either on the wall of the cell or scattered through the cell and the setae. Pyrenoids occasionally found.  Spores occur in some species with more or less equal and convex valves, sometimes furnished with spines or with the setae of the parent cell attached. All species are marine or occur in saline water. Gran classifies the genus according to the chromatophores. The variation in number and form of the chromatophores is very great and as they cannot always be determined it is best to rely more upon the structure of the frustule, not omitting, however, a consideration of the cell contents.