From Fensome et al., 2019:
Homotryblium ssp. breviradiatum (Cookson and Eisenack, 1961b, p.44, pl.2, figs.10–11) Lentin and Williams, 1977b, p.75. Holotype: Cookson and Eisenack, 1961b, pl.2, figs.10–11. Originally Hystrichosphaeridium floripes subsp. breviradiatum, subsequently Cordosphaeridium floripes subsp. breviradiatum, thirdly (and now) Homotryblium floripes subsp. breviradiatum. Age: Late Eocene.

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Original description: [Cookson and Eisenack, 1961b]:

Hystrichosphaeridium floripes subsp. breviradiatum subsp. nov.
(Plate II, Figs. 10, 11; holotype Nat. Mus. Vict. P20542)

Description. - Similar to H. floripes but with considerably shorter and proportionally broader appendages.
Dimensions. - Holotype - overall diameter 86 μ, diameter of shell 66 μ; appendages 10-15 μ.
Comments. - This subspecies occurs in small numbers in the Rottnest sample between 1,480 and 1,541 feet.