Pleurosigma ambrosianum Sterrenburg, Tiffany et Lange
This rare species was originally found in the Salton Sea, California (Sterrenburg et al., 2000). As in the original description, the valve is flat sigmoid and lanceolate, tapering to acute apices (Fig. 1, SEM of fractured valve). On the valve interior, the areolae are mostly bisected distal to the raphe area (Fig. 2, detail of Fig. 1) and the valve center has a small hyaline area surrounded by convex raised bars. The central raphe ends are straight, though the original description found that the raphe ends are undulate on the valve exterior, which was not seen. Only one damaged specimen was seen, slightly larger than the Salton Sea material, at about 123µm long and about 17.5µm wide, with 31-33 transverse striae in 10µ and 32-34 oblique striae in 10µm, with the striae crossing at an angle of approximately 55 degrees. The measurements are approximate because of damage to the valve. The specimen was found in the IRL near Vero Beach.
Sterrenburg FAS, Tiffany MA, Lange CB. 2000. Studies on the genera Gyrosigma and Pleurosigma (Bacillariophyceae). Species from the Salton Sea, California, U.S.A., including Pleurosigma ambrosianum, nov. sp. Proc Acad Nat Sci Philadelphia 150: 305-313.