From Fensome et al., 2019:
Diconodinium fehmarnense (Morgenroth, 1966a, p.4–5, pl.1, fig.1) Lentin and Williams, 1973, p.46. Holotype: Morgenroth, 1966a, pl.1, fig.1. Originally Gymnodinium (Appendix B), subsequently (and now) Diconodinium?. Questionable assignment: Lentin and Williams (1973, p.46). Age: Early Eocene.

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

Gymnodinium fehmarnense n. sp.
Pl. 1 Fig. 1

Derivation nominis: - From the locality of Fehmarn Island.
Holotype: The specimen shown on Pl. 1 Fig. 1 and preserved in specimen H 88.
Paratypoid: In specimen no. H 85.
Locus typicus: Fehmarn, Katharinenhof.
Stratum typicalum: Lower Eocene (?2).
Diagnosis: A species of the genus Gymnodinium with a delicate, smooth capsule membrane. Pear-shaped outline. Epitheca rounded. Hypotheca extended into small lobes. Transverse furrow strongly pronounced.
Description: The pear-shaped body consists of a delicate membrane with a smooth surface. The epitheca is rounded at the apex. It is larger than the hypotheca. The hypotheca is distinguished by a distinct tip, similar to Gymnodinium dabendorfense ALBERTI. Next to this is the suggestion of a second pointed bulge. The transverse groove is 5-6 μm wide and strongly pronounced.
It appears to be circular.
Comparisons: The new species shows a similar structure of the hypotheca to Gymnodinium dabendorfense ALBERTI and Gymnodinium nelsonense COOKSON, but differs – apart from the dimensions – from the former by a rounded epitheca and from the latter by a smaller, pointed hypotheca relative to the epitheca.
Dimensions: Holotype: Length 70 μm, width 61 μm. Paratypoid: Length 65 μm, width 50 μm. Number of specimens examined: 2.