From Fensome et al., 2019:
Amphidiadema rectangularis ssp. rectangularis. Autonym.
Holotype: Cookson and Eisenack, 1962b, pl.1, fig.14; Jan du Chêne et al., 1986a, pl.48, figs.14–15.
Now redundant.
Originally Deflandrea rectangularis subsp. rectangularis, subsequently Amphidiadema rectangularis subsp. rectangularis.

Locus typicus: Perth Basin, Australia

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

Description:
Shell elongate, flat, either nearly rectangular in outline or slightly narrowed above the limits of the capsule. Apex truncate to rounded, sometimes with a low central elevation. Antapex truncate to rounded or with slight indications of horns.
Tabulation, girdle and longitudinal furrow absent. Capsule circular to oval in outline, generally reaching the lateral margins of the shell. Wall of shell and capsule smooth, that of the capsule being thinner.

Dimensions:
Holotype: 120 µm long, 60 µm broad; capsule 4-x58 µm broad; pylome 24x26 µm.
Range: 100-130 µm long, 56-64 µm broad.