From Fensome et al., 2019:

"Planinosphaeridium", Eisenack, 1965b, p.151.
Taxonomic senior synonym: Pentadinium, according to Stover and Evitt (1978, p.179–180).
Taxonomic junior synonym: Ataxiodinium, by implication in Wall et al. (1977, p.149), who transferred the "type species" of Ataxiodinium, Ataxiodinium choane, to Planinosphaeridium --however, Edwards and Andrle (1992, p.265) retained Ataxiodinium.
Type: Eisenack, 1965b, pl.14, fig.4; text-fig.1, as Planinosphaeridium membranaceum.

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Original description: [Eisenack, 1965]:

Diagnosis:
Central body nearly sphaeroidal, surrounded by an irregular membrane. The shell is fine to coarsely granulate or vermiculate. Type consists of a large, trapezoidal pylome with rounded corners.

Dimensions:
Type: central body 54x56 µm, allover 70-82 µm, pylome 30x32 µm.