References:— Hustedt (1930), Cupp (1943), Rines & Hargraves (1988), Hernández-Becerril (1996), Jensen & Moestrup (1998), Hernández-Becerril & Flores Granados (1998), Berard-Therriault et al. (1999), Shevchenko et al. (2006), Lee et al. (2014a).

Morphometry: —a.a.: 5–18 μm; p.a.: 7–17 μm.

LM:—Cells are united in very straight and usually short chains (Fig. 79). The valve surface is concave and the valve corners are sharp and drawn up, touching between sibling valves (Fig. 81). The aperture is quite large and distinctively elliptically shaped (Figs 79, 81). Valve mantle is high with a distinct constriction near the margin (Fig. 81). Intercalary setae are long and rigid; originate from the valve apices and immediately cross each other at the chain margin without the basal part. In girdle view they are usually proximally perpendicular towards the chain axis and distally curved (Fig. 79). Terminal setae are usually characteristically oriented, long and straight, extending parallel to the chain axis and sometimes crossing each other distally (Fig. 79).

EM: —The valve face is ornamented with numerous small spines which are more pronounced in terminal valves (Fig. 82). The marginal ridge has a distinct hyaline rim (Fig. 82). A rimoportula, bearing an external short, flattened and wide tube, is present only in terminal valves (Fig. 82). Setae are quadrangular in cross-section, perforated with longitudinal rows of small slit-shaped poroids and small spines (Fig. 80).

Distinctive features: —Valve mantle markedly constricted near the advalvar margin, observable in LM as a marked constriction near the girdle. Intercalary setae long, straight and rigid, terminal setae extending parallel to the chain axis sometimes distally crossing each other. Valve face ornamented with many small spines.