From Fensome et al., 2019:
Impletosphaeridium selseyense (Islam, 1983b, p.337, pl.2, figs.3,7) Islam, 1993, p.87. Holotype: Islam, 1983b, pl.2, fig.3. Originally
Cleistosphaeridium, subsequently (and now) Impletosphaeridium. Age: Middle Eocene.

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Paratype: Islam, 1983
Locus typicus: Hampshire Basin, S England
Stratum typicum: Eocene, Tertiary

Original diagnosis: Islam, 1983, p. 337: Cleistosphaeridium selseyense
Autocyst subspherical; surface mildly retlculate; skolochorate to proximochorate; numerous nontabular solid processes a third to a quarter of average cyst diameter in length, proximally wide and taeniate and sometimes joined proximally to adjacent ones, tapering rapidly to thin, slender forms at or below mid-length and continue as such to distal bifurcations; archeopyle apical type tA, operculum free or attached. In some forms, a thin psilate proximal membrane connects most processes, a feature found in some species of Impletosphaeridium (Islam 1983).
Dimensions: Holotype: cyst diameter 30 µm, length of processes 4-7 µm. Range: cyst diameter 31 (26)22 µm, length of processes 3-8 µm. Specimens measu red: 10.

Affinities:
Islam, 1983, p. 337: Cleistosphaeridium selseyense
This species differs from others of the genus in possessing proximally taeniate and distally thin solid processes, and smaller size.