References: — Hustedt (1930), Cupp (1943), Rines & Hargraves (1988) (as Chaetoceros didymus var. didymus) Hernández-Becerril (1991b), Jensen & Moestrup (1998), Berard-Therriault et al. (1999), Hernández-Becerril & Flores Granados (1998), Shevchenko et al. (2006), Sunesen et al. (2008), Ishii et al. (2011), Lee et al. (2014a).

Synonyms: — Goniothecium gastridium Ehrenberg, Chaetoceros gastridium Ehrenberg, Chaetoceros mamillanum Cleve, Chaetoceros didymus var. anglica (Grunow) Gran, Chaetoceros. didymus var. hiemalis Tempere and Peragallo, Chaetoceros didymus var. genuina Gran and Yendo, Chaetoceros didymus f. adriatica Schussnig, Chaetoceros didymus f. singularis Takano.

Morphometry: —a.a.: 8–33 μm; p.a.: 9–26 μm.

LM: —The cells are united in straight chains (Fig. 128). Cells are rectangular in girdle view with sharp and slightly drawn up corners. There are two large plastids per cell (Fig. 129). Valve face is slightly concave with a prominent hemispherical protuberance in the centre; mantle is low without any visible constriction near its edge. The setae originate at the valve corners and cross over each other at the point outside of the chain margin with a basal part of variable length (Figs 128, 129). The apertures are widely peanut-shaped due to the presence of central protuberances on both sibling valves. The intercalary setae are perpendicular to the colony axis or sometimes slightly curve towards the terminal part of the chain (Fig. 128). In valve view, the setae diverge at about 25–45° from the apical plane (Fig. 130). Terminal setae extend almost parallel to the colony axis (Fig. 128).

EM: —The valve is ornamented with a weak pattern of costae radiating from the central annulus located on the protuberance and it is densely perforated with round small poroids, with the exception of the central part around the protuberance (Fig. 131). The valve edge and the setae bases are ornamented with long and branched capilli (Figs 130, 131). The setae are in cross-section four-sided (not shown), each side perforated with a single longitudinal line of small poroids and scarcely distributed larger elongated pores (Fig. 132). Shark fin spines are arranged longitudinally on the ridges.

Distinctive features: —Valve face with a prominent hemispherical central protuberance. Valve edge and setae bases ornamented with long capilli.