Holotype: Slide number SZCZ 27552 at repository of University of Szczecin.

Isotype: Slide number SZCZ 28814 at repository of University of Szczecin (Fig. 7S).

Sand and coral boulder of coral reef at Gili Iyang harbour, Bawean Island, East Java, Indonesia.

The species name is derived from the Latin word decus which literally means an ornament, decoration, or embellishment. The new species exhibits a distinct central area.

The diatom species C.decusa has a unique distribution, being found exclusively in Bawean and Sulawesi Islands, Indonesia. Interestingly, both of these locations share a similar habitat, characterized by the presence of coral reef areas.

Light microscopy (Fig. 7A–Q): The valves are semi-lunate or semi-lanceolate dorsiventrally. Thickening silica in the central area is very clear under LM. A row of linear areolae can be distinguished in the dorsal margin (Fig. 7F). Valve length 8.2–16.5 µm, n = 18 and width 1.9–2.9 µm, n = 17. Apices broadly rounded with indistinct, dot-like helictoglossae. The frustule is rectangular in girdle view, 1.1 µm depth. Raphe slits observed in ventral area.

Scanning electron microscopy (Fig. 7R–Y): Frustules semi-circular to semi-lanceolate, dorsiventral; valve face flat, smooth, abruptly transitioning to mantle. Distinct rhomboidal central nodule observed on ventral and dorsal sides (Fig. 7S, T). Valve face devoid of transapical striae; striae present on dorsal and ventral mantle (Fig. 7R). Raphe sternum positioned near ventral margin; raphe slits straight medially, curving towards apices. External proximal raphe ends simple, slightly expanded; distal ends simple, close to apices (x–=0.72 μm, n = 21), deflected dorsally. Central nodules and helictoglossae indistinct. Dorsal mantle striation density 35–40 in 10 μm; ventral mantle 40–50 in 10 μm. Internal raphe filiform, straight (Fig. 7T, U, Y); mantle areolae occluded, flask-shaped (Fig. 7X). Girdle open, comprising multiple unperforated plain bands (Fig. 7R, U).