From Fensome et al., 2019:
Isabelidinium thomasii (Cookson and Eisenack, 1961a, p.71–72, pl.11, figs.7–10) Lentin and Williams, 1977a, p.168. Holotype: Cookson and Eisenack, 1961a, pl.11, fig.8. Originally Deflandrea, subsequently Isabelia (combination illegitimate), thirdly (and now) Isabelidinium. Age: Senonian.

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Paratype: Cookson and Eisenack, 1961
Locus typicus: Belfast No. 4 Bore, Victoria
Stratum typicum: Senonian

Original description: Cookson and Eisenacik, 1961, p.71-72: Deflandrea thomasii
Shell longer than broad, roughly oval with a median girdle with low borders which, sometimes, is only indicated at the lateral margins. The epitheca has a short, solid pointed horn which terminates in a minute hyaline prominance.
The antapex is variable in outline, it may be indented, obliquely truncate with a single short, pointed, lateral horn or broadly rounded. The shell membrane is 2 layered; the inner layer is thin and hyaline, the outer is formed of densely arranged minute granules or rods; it is from this layer that the apical horn is formed. In poorly preserved specimens the outer layer tends to be destroyed. The internal body is oval in outline and thin-walled; it appears to be absent from some specimens. The pylome is trapezoid in shape.

Dimensions: Type--length 108 µm, breadth 82 µm. Range--length 85-114 µm, breadth 50-90 µm.