Frustules usually rectangular in girdle view; when mature pervalvar axis exceeding apical axis. Valves isopolar, their surface flat, mantle vertical, margin sharply stepped hyaline lip.
Primary internal transapical ribs present, sometimes with secondary and tertiary ribs. Sternum with indefinite margins, often internally thickened.
Striae parallel to slightly radiate, extending across mantle to projecting hyaline valve margin. Virgae coarser than vimines, raised above valve surface close to margin, becoming less so towards sternum. Areolae close to sternum more widely spaced than elsewhere, and somewhat irregularly distributed.
Apical pore fields present in some species, consisting of demarcated area with striae closely spaced, as are areolae within them. Rimoportulae often present, simple, varying in number and position.
Cingulum consisting of several (many) open bands, opening at alternate poles; others closed. Cingulum composed of four kinds of bands: valvocopula, with septum; primary copulae, each with ligula and septum; secondary copulae, without septa, their ligulae larger than those of primary copulae; single pleura. Septa arising from abvalvar part of pars exterior of valvocopula and primary copulae, extending variable distances into frustule interior; pore situated at or near base of septum, on abvalvar face communicating via groups of pores on outer face of band.
Type species: Tetracyclus glans (Ehrenb.) Mills 1935, p. 1602 = Tetracyclus lacustris Ralfs 1843a, p. 105, pl. 2, fig. 2.
About 35-40 species known, mostly fossil (extinct), with only three living species known from UK and Europe:
Tetracyclus emarginatus (Ehrenb.) W. Smith (Basionym = Biblarium emarginatum Ehrenb.)
Tetracyclus glans (Ehrenb.) F.W. Mills (Basionym = Navicula glans Ehrenb.; synomym: Tetracyclus lacustris Ralfs)
Tetracyclus rupestris (Kützing) Grunow in Van Heurck (Basionym = Denticula thermalis β [var.] (‘rupestris’) rupestre Kützing)