Tetracyclus Guide
Credit: Gene Stoermer, Ian Bishop
  1. Striae scattered or less regular at sternum
  2. Valve mantle with prominent hyaline step
  3. Copulae with multiple ornamented patterns

Frustules of Tetracyclus are heavily silicified. Valves are isopolar, with a flat surface. The valve mantle is relatively deep and possesses a prominent hyaline step. The valve face may be elliptic to elongate (some taxa are even circular) and the apices are often capitate. Primary internal transapical ribs are present, and some species may possess secondary and tertiary transapical ribs. The valve margin may be expanded centrally. Striae are uniseriate. The striae are parallel to slightly radiate, and extend over the mantle. At the sternum, the stria pattern is scattered, or less regular than other parts of the valve. That is, the axial area has indefinite margins. Apical porefields are present. The cingulum is composed of several open bands with septae. Valves may have up to two rimoportulae (in some cases more than two rimoportulae). The rimoportulae are positioned near the valve center or in the mantle area, making them difficult to see using LM. Cells form zig-zag colonies joined by mucilage pads. Alternatively, there are species which have valve face to face colonies joined by what appear to be very small spines.

According to Williams (1987), the only extant species that possess rimoportulae are T. emarginatus and T. rupestris.

Tetracyclus is considered a close relative of Tabellaria. Extinct species of Tetracyclus occur in the fossil lacustrine deposits of western North America. In cold water habitats of the Northern Hemisphere, the species T. glans, T. emarginatus and T. rupestris occur in high latitude and high altitude oliotrophic lakes and in moss habitats.