From Fensome et al., 2019:

Subtilidinium, Morgenroth, 1968, p.539.
Taxonomic senior synonym: Phanerodinium, according to Below (1987b, p.36) -- however, Lentin and Williams (1989, p.354,622) retained Subtilidinium.
Type: Morgenroth, 1968, pl.42, figs.8–9, as Subtilidinium minutum.

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

Original description: [Morgenroth, 1968]: (Translation: Stover and Evitt, 1978, p. 82):

Description:
Theca very small, weakly ellipsoidal, without processes. Tabulation formula: ?2`, ?6a or 7a, 6``, 6c, 6```, p, 1````. Cingulum is equatorial, often faintly spiral to the left. Ventral longitudinal furrow is undifferentiated. Archaeopyle: in the form of an apical opening with a elliptical outline.

-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Modified description:

Stover and Evitt, 1978, p. 82-83:

Synopsis:
Cyst proximate, subsphaerical; autophragm with parasutural ridges; paratabulation formula: 2`?, ?6-7a, 6``, 6c, 6```, 1p, 1````; archaeopyle apical, type uncertain

Description:
Shape: Subsphaerical.
Wall relationships: Autophragm only.
Wall features: Parasutural ridges smooth, as is the rest of the cyst.
Paratabulation: Indicated by parasutural ridges; formula provisional, given as ?2`, ?6-7a, 6``, 6c, 6```, 1p, 1````, but archaeopyle type is uncertain, and the presence of six or seven intercalary paraplates is unusual.
Archaeopyle: Apical, type uncertain.
Paracingulum: Indicated by six paraplates.
Parasulcus: Longitudinally elongate area between paraplates 1` and 1````, which widens antapically; epicystal part may be sudivided.
Size: Small.

Affinities:
Subtilidinium may be congeneric with either Cladopyxidium or Microdinium.