From Williams et al., 2017:

[Planoperidinium, de Coninck, 1986b, p. 18

Type Species: Planoperidinium gracile, de Coninck, 1986b (pl.8, figs.1–3)]

-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Original description: [de Coninck, 1986]:

Diagnosis:
Small, dorsoventrally flattened pericyst with peridinoid ambitus. Pericyst and endocyst separate from each other in the antapical horns and often in the apical horn as well unless it is very poorly developed. Between horns and cingulum peri- and endocyst generally not separate; in some specimens however a narrow separation is visible.
Cingulum mainly marked at the lateral margin. Sulcus generally invisible or slightly marked.
The archeopyle is intercalary and relatively wide.
No ornamentation of the periphragm, hence no tabulation indicated.