Valves linear or lanceolate; usually sigmoid; axial area very narrow. Central area small; striae punctate, in transverse and longitudinal rows. Chromatophores two, in long and narrow bands, one on each valve, the margins serrated in marine forms. Pyrenoids are present in all species (Cupp, 1943).
Navicula baltica Ehrenberg; Pleurosigma balticum (Ehrenberg) Smith
Valves linear, sides more or less parallel in the middle, sigmoid toward the ends, with obtuse apices. Apical nodules small. Axial area very narrow, raphe sigmoid, central, having the same sigmoid curvature as the valve margin, particularly toward the ends, but often curved or flexuose as it approaches the central nodule. Central area is small, elliptical, sometimes slightly oblique. Valve surface striate; striae finely punctuate, and arranged in transverse and longitudinal lines. Transverse and longitudinal striae equidistant, about 11-13 in 10 µm; striae distinct; girdle narrow, plain.
Chromatophores two dentate plates lying under the valves. Size very variable, especially in length; 236-500 µm, mostly between 280-330 µm; breadth 28-32 µm (Hendey, 1964).
Cells length 240-300 µm; width 30-33 µm.
Mainly littoral species, but is frequently found in the plankton.
Sites of occurrence in RSA:
In Winter 2006 Cruise, it was present in small numbers at one station (refer site map).