Electron backscatter diffraction is used to characterize the transition interface between aragonite prisms and nacre in two species of freshwater molluscs, Anodonta anatina (Linnaeus, 1758) and Anodonta cygnea (L. 1758). In the duck mussel, Anodonta anatina, the shell thickness is comprised mainly of nacre with only a thin outer prismatic layer, whereas in the swan mussel, A. cygnea, the shell comprises a greater thickness of aragonite prisms relative to the nacreous layer. In A. anatina, the prism−nacre interface is flat and featureless. In A. cygnea, the nacre below the base of the prisms is characterized by distinct concave footholds. In both species, the c-axis of aragonite nanogranules comprising the prisms is parallel with the prism length. This crystallographic orientation, with the c-axis perpendicular to the shell exterior, occurs in the nacreous layer and thus is conserved throughout the shell thickness. In A. cygnea alternate prisms have a crystallographic orientation with the c-axis deviating from the prism long axis by approximately 20 deg. The initial nacreous layer adopts the crystallographic orientation of the preceding prisms and subsequent nacreous laminae are oriented in register with the bulk nacre. This is consistent with the hypothesis that nacre evolved through horizontal partitioning of prisms.
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