Stauroforma exiguiformis

Valves are lanceolate with convex central margins and broadly rounded ends. The central sternum is narrow and linear. Striae are distinct and composed of round areolae. Areolae tend to decrease in diameter from valve face/mantle junction to the axial area Striae are parallel at the valve center, becoming slightly radial toward the apices and extending continuously onto the valve mantle, where up to two rows of areolae may be present. Costae are wider than the striae. Spines are tubular and hollow, bifurcating toward the tips. Spines are present along the valve face edge, including the apices, positioned in line with the costae. A row of areolae is located on the apical mantle region, continuous with the mantle areolae from the rest of the valve. Apical pore fields and rimoportulae are absent. Copulae are open and lack ornamentation. Valvocopula is wider than the remaining copulae. In girdle view, frustules are rectangular. Cells form short ribbon-like colonies, joined by linking spines.

A brief summary of the nomenclature may help to understand the current name of this species. Fragilaria (virescens var.) exigua Grunow in Cleve and Möller no. 144 (Cleve and Möller, 1978) lacked a formal description, so it is invalid. Grunow in Van Heurck (1881) illustrated the taxon, again without valid description, and used the name Fragilaria virescens var.? exigua Grunow in Van Heurck. Lange-Bertalot (1993) validated the taxon, with the new name, Fragilaria exiguiformis. At the time, Lange-Bertalot (1993) referred to fig 125: 4 (SEM) and 126: 11-18 (LM) in Krammer and Lange-Bertalot (1991) to illustrate of the taxon. Although the North American specimens conform to the European LM illustrations, the SEM features of North American taxa are quite different, particularly in terms of spines and the mantle areolae. The spines and mantle areolae of the specimens illustrated here are more characteristic of Staurosira in Krammer and Lange-Bertalot´s illustration. The current placement in Stauroforma reflects Flower et al. (1996), who erected the genus Stauroforma as distinct from Fragilariforma, Fragilaria and Staurosira.