From Fensome et al., 2019:
Impletosphaeridium labyrinthus Morgenroth, 1966a, p.35, pl.9, figs.6–7. Holotype: Morgenroth, 1966a, pl.9, figs.6–7. NIA. Age:
Early Eocene.

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

Impletosphaeridium labyrinthus n. sp.
Plate 9 Fig. 6-7

Derivation: labyrinthus lat.: The labyrinth (cf. pattern formed by the projections on the capsule surface!).
Holotype: The specimen depicted on Plate 9 Fig. 6 and 7 and preserved in specimen H99.
Paratypoids: H71, H98, H101, H102.
Locus typicus: Fehmarn, Katharinenhof.
Stratum typicum: Lower Eocene (?2).
Diagnosis: The thin-walled capsule is usually spherical in shape. Its smooth surface is densely covered with flat, massive projections, which are more or less severely fractured distally and usually end in very fine hairs (cf. Fig. 7).
Remarks: No pylomas were observed! The specimens presented show no significant variation in their dimensions.
Dimensions: Capsule diameter 25μ (22μ/28μ), process length 11μ-14μ (15 measurements). Number of specimens examined: 15.