Staurosira binodis

Valves are biundulate in larger specimens and lanceolate to cruciform in smaller ones, 4-6 µm wide and 7-21 µm long. Valves with rostrate to subcapitate ends. The central margin is constricted in larger specimens or inflated in smaller specimens. The valve face is flat, or slightly undulate due to raised costae. The valve face/mantle junction forms a sharp angle. The abvalvar edge of mantle is parallel to the valve face/mantle junction. In girdle view, frustules are rectangular. Cells form ribbon-like colonies joined by linking spines. One frustule within a colony is attached to the substratum by a mucilage pad. Cells may also be planktonic. The axial area is broad and linear or slightly lanceolate. The striae are distinct, alternate, and composed of lineolae which decrease in size from the valve face/mantle edge to both the central sternum and the valve mantle; 13-16 in 10 µm. Lineolae bear finely branched volae. The striae are parallel to radiate in the central area. Near the valve ends, the striae are slightly radiate to parallel. Striae extend deep onto the valve mantle. Costae are broad. Spines are spatulate, hollow and some possess terminal digitations that attach to neighboring valve lineolae. Spines are present along the valve face edge, except at the apices, and always located on the costae between striae. Well-developed, ocellulimbus type, apical pore fields with round poroids are present. Apical pore fields are located on the valve mantle. Rimoportula are absent. Scab-like structures, or blisters, are present along the abvalvar side of valve mantle. Copulae lack perforations. Valvocopulae wider than copulae.