From Fensome et al., 2019:
Spinidinium echinoideum ssp. rhombicum (Cookson and Eisenack, 1974, p.49–50, pl.20, figs.5–9) Lentin and Williams, 1976, p.64. Holotype: Cookson and Eisenack, 1974, pl.20, fig.7. Originally Deflandrea rhombica, subsequently (and now) Spinidinium echinoideum subsp. rhombicum, thirdly Spinidinium rhombicum. Lentin and Williams (1981, p.257) retained this taxon as Spinidinium echinoideum subsp. rhombicum. Age: Albian– Cenomanian.

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Locus typicus: Perth Basin, Australia
Stratum typicum: Albian-Cenomanian

Original diagnosis: Cookson and Eisenack, 1974, p. 49: Deflandrea rhombica
Overall shape rhombic. Periphragm flattened, epicyst triangular with slightly concave to convex flanks. Weakly developed, blunt apical horn. Hypocyst triangular with pointed antapical horn. Hypocyst trapezoidal when second, weakly developed antapical horn present. Usually just one, pointed to thorn-like antapical horn present, which may be displaced towards the girdle. Girdle relatively wide but indistinct. Periphragm with short, pointed spines, strongest developed on girdle folds. Apex with few (2-4) diverging folds. Endophragm circular and clearly distinguished from the periphragm. Archaeopyle usually present, trapezoidal to rounded triangular.
Dimensions: Holotype c. 72-50 µm, range - length 57-90 µm, breadth 38-76 µm; length/breadth ratio 1,4 - 1,6:1.