References:— Hustedt (1930), Cupp (1943), Rines & Hargraves (1988), Hernández-Becerril (1996), Jensen & Moestrup (1998), Sunesen et al. (2008), Kooistra et al. (2010), Lee et al. (2014b).

Morphometry: —a.a.: 12–40 μm; p.a.: 13–23 μm.

LM: —Cells are usually solitary but occasionally found in short chains composed of two to three cells. Cells contain numerous small chloroplasts located in the central body and in the setae (Figs 5, 6). The valve face is flat or slightly concave; the mantle is moderately high and the girdle is small. Long and thick setae originate from the valve corners and extend perpendicular to pervalvar axis. They are usually straight, but can also be slightly curved in the apical plane. In the same cell, the two valves are generally rotated to each other at ca. 45° and the setae form a typical cross when observed in valve view (Fig. 5). When two cells are found attached to each other, the setae are fused at the valve corners for a short distance without a basal part but the narrow apertures can still be observed due to the slight concave surface of the valves (Fig. 6).

EM: —The valve is ornamented by a small round central annulus with numerous branching costae extending towards the valve edge, and by numerous poroids irregularly scattered over the valve surface, excluding the central part and mantle (Fig. 7). The mantle is constricted near the edge (Fig. 8) and often ornamented by irregular siliceous thickenings (Fig. 9). Every valve possesses a centrally located rimoportula in the form of a simple oval or commashaped hole on the internal side of the valve (Figs 7–9). Setae in cross-section are round at the bases and later become four to six-sided, with a row of arrowhead-shaped spines on each ridge (1–2 spines in 10 μm) (Fig. 10). The setae sides are ornamented by poroids arranged in a grid pattern, 3–5 longitudinal rows of poroids on each setae side and one transverse row between not particularly thick transverse costae (Fig. 11). Girdle bands are ornamented by alternating costae and striae. The band striae are perforated by irregularly scattered small poroids (Fig. 12).

Distinctive features: —Usually single cells, straight and thick setae perpendicular to the pervalvar axis, forming a cross in valve view. Setae mainly tetragonal with arrowhead-shaped spines on the ridges and sides pattern consisting of poroids arranged in a grid, 3–5 longitudinal poroid rows, one transverse row between costae.