From Fensome et al., 2019:
Impagidinium? simplicium (Cookson and Eisenack, 1961b, p.42, pl.2, figs.3–4; text-figs.1e–f) Stover and Evitt, 1978, p.166. Holotype: Cookson and Eisenack, 1961b, pl.2, figs.3–4; text-figs.1e–f; Jan du Chêne et al., 1986a, pl.54, figs.1–5. Originally Rottnestia, subsequently Psaligonyaulax, thirdly (and now) Impagidinium?. Questionable assignment: Stover and Evitt (1978, p.166). Age: Eocene.

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Locus typicus: Rottnest Bore, W. Australia
Stratum typicum: Eocene

Original diagnosis: Cookson and Eisenack, 1961, p.42: Rottnestia simplicia
Shell without appendages. Apical and antapical expansions of the same construction as those of R. borussica but less strongly developed, apical horn well marked. Longitudinal furrow distinct, broadening towards the antapex, Girdle, tabulation and pylome similar to those of R. borussica. The internal body is elongate-oval in outline.
Dimensions: Type - Length 83 µm, breadth 56 µm, internal body 60 x 38 µm.