Pleurosigma Guide
Credit: Mark Edlund, David Burge
  1. Valves sigmoid
  2. Raphe sigmoid
  3. Striae decussate

Valves are elongate and sigmoid. Striae are composed of evenly spaced decussate rows. The rows are arranged in 3 patterns: a trans-apical row and two opposing oblique rows. Internally, the areolae are occluded by hymenes. The raphe is strongly sigmoid, with a narrow axial area. The sternum is thickened equally on both sides. The raphe terminates proximally in a small oval, expanded central area defined by two curved ridges. Proximal raphe ends are slightly inflated and straight, or unilaterally bent. The distal raphe ends are hooked and deflected to opposite sides on the mantle. The valve mantle is relatively shallow and the valve / mantle interface is broad and curved. Two to four ribbon-like plastids extend from apex to apex.

Pleurosigma is primarily a marine genus, with several species found in brackish water and and other high conductivity habitats. It is typical of epipelic habitats of North American saline lakes, large rivers and estuaries (Smith 1852, Patrick and Reimer 1966, Czarnecki and Blinn 1977, Grimes and Rushforth 1982, Kaczmarska and Rushforth 1983, Sterrenburg et al. 2000). Pleurosigma is closely related to Gyrosigma, but differs in plastid structure, decussate striae and the prominent sternum on both sides of the raphe.