From Fensome et al., 2019:
Impletosphaeridium rugosum Morgenroth, 1966a, p.36, pl.10, figs.2–3. Holotype: Morgenroth, 1966a, pl.10, figs.2–3. Age: Early
Eocene.

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Original description: [Morgenroth, 1966a] (translated from German):

Impletosphaeridium rugosum n. sp.
Plate 10 Fig. 2-3

Derivio nominis: rugosus Latin: wrinkled.
Holotype: The specimen shown on Plate 10 Fig. 2 and 3 and preserved in specimen H101.
Paratypoids: HF No. 15, HF No. 37, HF No. 61.
Locus typicus: Fehmarn, Katharinenhof.
Stratum typicum: Lower Eocene (?2).
Diagnosis: The thin-walled capsule is spherical to distinctly ellipsoidal in shape. Numerous processes rise from the smooth capsule surface, at the base of which 3-5 tiny wrinkles converge in a ridge-like pattern. These wrinkles usually connect the processes to one another, creating a net-like pattern on the capsule surface. The massive processes expand distally into hollow calyces, whose unjagged edges may be folded over.
Remarks: None of the examined specimens possesses a pyloma. The dimensions vary only within narrow limits. The axial ratio can reach a maximum of 1:1.6.
Dimensions: Capsule length 26μ (22μ/28μ), capsule width 20μ (17μ/25μ), process length 8μ - 11μ (15 measurements).
Number of examined specimens: 27.