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

Synonyms: — Chaetoceros borealis var. densa Cleve, Chaetoceros borealis var. brigthwellii Cleve, Chaetoceros brightwellii (Cleve) Mills.

Morphometry: —a.a.: 17–38 μm; p.a.: 13–63 μm

LM: —Cells are united into robust, straight and long chains. Cells are rectangular in girdle view with diagonally cut valve corners (Fig. 13). Cells and setae contain numerous small chloroplasts. The valve face is flat or slightly concave; the mantle is high and shows a visible constriction at its edge (Fig. 13). A simple hole-shaped rimoportula is present in the centre of each valve (Fig. 14). The long, thick and straight setae originate inside the valve margin. Apertures are narrow and slit- or hexagonally shaped (Fig. 13). In valve view, the intercalary setae diverge from the apical plane at equal angles (ca. 30–45°) (Fig. 14). Terminal setae are almost parallel to the chain axis.

EM: —The setae are four-sided (not shown) with the rows of sharp stipule-shaped spines on the ridges. Each side is perforated by small poroids arranged in a grid pattern, comprising 12–15 longitudinal rows and a single row of poroids between transverse costae (Fig. 15).

Distinctive features: —Very narrow apertures. Four-sided setae with each side ornamented with small poroids arranged in a grid pattern, with 12–15 longitudinal rows of poroids.