References:— Rines and Hargraves (1988) (as C. compressus), Jensen & Moestrup (1998), Bérard-Therriault et al. (1999), Rines (1999), Sunesen et al. (2008), Kooistra et al. (2010), Shevchenko & Orlova (2010), Lee & Lee (2011), Lee et al. (2014a), Chamnansip et al. (2015).

Morphometry: —a.a.: 6–23 μm; p.a.: 7–24 μm.

LM: —Cells are united in long and straight chains, often twisted about the chain axis (Fig. 83). Cells are cylindrical, in girdle view rectangular with markedly rounded corners, in valve view nearly circular. Cells contain 5–12 small discoid-like chloroplasts (Fig. 84). The valve face is slightly convex with a central inflation. The mantle is more or less high with the girdle equidimensional or wider than the mantle. The setae originate within the valve margin and cross each other at the chain margin after a basal part of medium length (Fig. 84). The apertures are relatively wide, and have a hexagonal shape (Figs 84, 86). There are two types of intercalary setae: common and special. Common setae are thin, generally oriented almost perpendicularly to the chain axis, and slightly curved (Fig. 83). Special setae are thick, heavily silicified and bend in a bell-like curve towards the nearest extremity of the chain (Fig. 85). The terminal setae resemble the normal intercalary setae but they are directed outwards from the terminal valve forming a V-like shape. Resting spores are positioned centrally within the parent cell, with both valves smooth and dome-shaped (Fig. 86).

EM: —The valve has a central annulus from which extends a complex pattern of radiating costae, branching dichotomously towards the valve margin and becoming parallel on the valve mantle (Figs 87, 88). Some costae originate from small spirals scattered on the valve face. The costae are often interconnected with transverse short connections. The central annulus is not hyaline but filled with an irregular reticulate pattern, and the only hyaline area on the valve surface is observed around the bases of the setae (Figs 87, 88). The marginal ridge possesses a hyaline rim, often extending into somewhat rectangular-shaped siliceous projections (Figs 87, 89, 90). A rimoportula is present only in terminal valves. It is in the form of a round hole from the internal view and externally appears as a short clublike tube (Figs 90–92). Girdle bands are adorned with transverse fine costae and with a longitudinal rib near one edge (Fig. 93). Common setae are circular in cross-section and ornamented with shark fin-shaped spines arranged in a spiral pattern (Fig. 94). The longitudinal costae extend parallel with the seta axis but can also be arranged in a slight spiral pattern. Transverse costae are found perpendicular to each other and form a regular reticulate ornamentation pattern across the setae. Special setae were not observed in EM.

Distinctive features: —Cylindrical cells united in very long and slightly twisted chains. Special, thick intercalary setae often present. Valve face, including central annulus, having a complex ornamentation pattern with transverse connections between branching costae and presence of small spirals. Setae showing a regular reticulate ornamentation pattern.