From Fensome et al., 2019:
Cleistosphaeridium microcystum (Bujak in Bujak et al., 1980, p.88,90, pl.22, figs.2–5) Lentin and Williams, 1981, p.49. Holotype: Bujak et al., 1980, pl.22, figs.2–3; Fauconnier and Masure, 2004, pl.64, figs.4–5. NOW Sentusidinium. Originally Tenua Eisenack, subsequently Cleistosphaeridium, thirdly (and now) Sentusidinium, fourthly Pilosidinium. Age: Middle Eocene (see Aubry, 1986).

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Original description as Tenua microcysta: [Bujak in Bujak et al., 1980, p. 88-89]:

Diagnosis:
Autocyst ovoidal, sometimes with two weakly developed antapical lobes.
Autophragm smooth to chagrinate with abundant processes. The processes are nontabular and are of constant length on a specimen, but vary in length intraspecifically. The processes are mostly expanded proximally, hollow, and are distally capitate or bifid. Occasional processes are solid or simple.
The archaeopyle is apical, tetratabular, and has well-developed accessory archaeopyle sutures between the precingular paraplates. The operculum is detached.

Dimensions:
Autocyst length (without operculum) = 28-36 µm, breadth = 24-34 µm. Process length is up to 6 µm.