Species Description: This species is broadly lanceolate with very slightly sigmoid valves and bent rostrate apices (Fig. 1, exterior of entire valve, SEM). The raphe is nearly straight or slightly sigmoid, eccentric near the apices, and curved in opposite directions at the two ends. The raphe ends in the center are also bent in opposite directions (Fig. 2, exterior, valve central area, SEM). The axial area is quite narrow and the central hyaline area is small. Cell length (apical axis) of measured specimens is 70-83µm and 19-23µm wide. Transverse and diagonal striae are 23-27 in 10µm, crossing each other at an angle of about 65 degrees. Individual areolae are circular or oval near the central area, and slit-like near the margin (Fig. 2).
Pleurosigma rostratum closely resembles P. lanceolatum (Hustedt 1955), which has not been identified in the IRL system; the latter is often found in more northern environments. These specimens differ slight from the type description in the number of striae and the shape of the central area. It was found in the St. Lucie estuary at Stuart, and has been previously seen in the Gulf of Mexico and the Caribbean.
Hustedt F. 1955. Marine littoral diatoms of Beaufort, North Carolina. Bull Duke Univ Mar Sta 6: 1-67.