References:— Ikari (1926), Hustedt (1930), Cupp (1943), Hoppenrath et al. (2009), Shevchenko et al. (2006).

Morphometry: —a.a.: 14–21 μm; p.a.: 5–14 μm.

LM: —Cells are united in usually long chains that are very strongly twisted around the chain axis in such a way that some sibling valves position themselves perpendicular to each other (Figs 173–175). In valve view cells are broadly elliptical and compressed in the transapical direction, in girdle view they are rectangular with rounded valve corners. Each cell contains a single parietal plate-like chloroplast positioned around the girdle (Fig. 176). The valve face is flat or slightly convex; the mantle is low and sometimes constricted near the margin, girdle equidimensional or higher than the mantle. Long and thin setae originate within the valve margin and slightly direct outwards forming a short basal part, and then cross slightly inside or at the chain margin (Fig. 175). Apertures are hexagonally shaped. Intercalary setae are generally oriented more or less perpendicularly to the chain axis or appear slightly curved (Fig. 173). In valve view they diverge at an equal angle (30–45°) from the apical plane. Terminal setae are thick like the intercalary ones and extend almost parallel to the colony axis in a U-shaped curve (Fig. 173).

EM: —The valve has a central hyaline annulus from which extends a complex pattern of densely distributed costae, branching dichotomously towards the valve margin, sometimes forming small dots (Figs 177, 178). The marginal ridge does not have a prominent rim; it is only observable as a thickened edge between the valve face and the mantle. Girdle bands are adorned with transverse fine costae and a slightly thickened longitudinal rib near one edge (Fig. 181). A conspicuous character found in this species is the presence of a large elongated pore on the proximal part of the setae, right after the crossing point (Figs 178–180). Setae are circular in cross-section, ornamented with very small spines and poroids spirally positioned along the seta length (Fig. 182).

Distinctive features: —Very strong torsion of the chain. Large elongated pore on the proximal part of the seta.