Twinning in crystals is of importance for crystal growth, phase transformation and mechanical, electrical, optical properties of materials. We highlight and discuss twinning characteristics of foraminiferal calcite together with patterns of biocarbonate crystal assemblies for shells of benthic and planktonic rotaliid foraminifera. Crystal organization and twin characterization was done with electron backscattered diffraction (EBSD) and sub-micrometer to nanometer scale FE-SEM and STEM imaging of the mineral and the organic components (Fig. 1). We describe (i) the pattern of calcite orientation within the shells (Fig. 1A), the very extensive {001} twinning of calcite (Fig. 1B), (ii) demonstrate for the twinned crystals a new microstructure (Figs. 1C, D) and (iii) address twin development from crystal nucleation within an organic template to outer shell surfaces [7]. We show that (iv) all rotaliid foraminifera, investigated with EBSD so far, are twinned, however, (v) to different degrees (Fig. 1E) and (vi) show that shell morphology, in particular shell/chamber sphericity, is linked to the strength of calcite twinning (Fig. 1E and [7]).