From Fensome et al., 2019:
Impletosphaeridium tenuifilum (Cookson and Eisenack, 1982, p.40, pl.8, fig.14) Islam, 1993, p.87. Holotype: Cookson and Eisenack,
1982, pl.8, fig.14. Originally Cleistosphaeridium, subsequently (and now) Impletosphaeridium?. Questionable assignment: Islam (1993, p.87). Age: Paleocene.

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Original description: [Cookson and Eisenack, 1982] (translated from German):

Cleistosphaeridium tenuifilum n. sp.
Plate 8, Fig. 14

Type: The specimen shown on Plate 8, Fig. 14, preserved in specimen Rivernook 26.
Diagnosis: Shell spherical, with numerous, very thin appendages that are briefly forked at the end.
Dimensions: Spherical shell, approx. 71 μ, total approx. 90 μ; appendages up to approx. 17 μ.
Note: Given the thinness of the appendages, it is difficult to determine whether they are hollow.
Occurrence: Rivernook Beds, Victoria; Paleocene.
Distinguished from other species of the genus Cleistosphaeridium by its very thin appendages.