From Fensome et al., 2019:
Amphidiadema nucula, (Cookson and Eisenack, 1962b, p.486, pl.1, fig.13) Lentin and Williams, 1976, p.61.
Holotype: Cookson and Eisenack, 1962b, pl.1, fig.13.
Originally Deflandrea, subsequently (and now) Amphidiadema. NIA.
Age: Senonian.

Locus typicus: Victoria, Australia

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Original description as Deflandrea nucula: [Cookson and Eisenack, 1962, p. 486]:

Description:
Shell small, slightly flattened, approximately oval in outline, sometimes slightly narrower just above and below the limits of the capsule. Apex broadly rounded with a small pointed projection. Antapex truncate to rounded without horns.
Tabulation, girdle, and longitudinal furrow absent. Capsule circular to oval in outline; when oval, somewhat longer than broad, sometimes extending, as in the type, to the lateral margins of the shell. Wall of both shell and capsule smooth. Pylome square with rounded angles, crossed by the upper limit of the capsule.

Dimensions:
Holotype: 67µm long, 50µm broad, capsule 48µm long, 50µm broad.
Range: 52-72µm long, 48-56µm broad.