Forms belonging to the Allosigmostaura pseudobisphenoidea have as their only symmetry element a 4-fold rotoinversion axis. Crystals of the Tetragonal-bisphenoidic Crystal Class of the tetragonal Crystal System have this basic form. This form can geometrically be represented by an apical 2-fold gyroid bipyramid, in which the composing pyramids are rotated by 900 with respect to each other. No organisms having this basic form are recorded.
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