The Authors assuming as hypothesis that elements beyond Calcium could be included as carbonate according to crystalline structure of Aragonite and Calcite in the shell of Mytilus g. have verified the selective inclusion of Cr, Mg, Zn, Fe, Cu, Cd, Pb in the biocrystals in relation to their ionic radius. Though the ratio of concentrations of these elements in the shell layers: [Me] calcite/[Me] aragonite ( [C]/[A] ) is significatively greater than 1 for Zn and Mg, significatively less than 1 for Cr and Pb and the no-significative distribution of Cu, Fe, Cd do not allow a sure relationship between the ionic radii of Ca and the investigated elements in the process of shell inclusions. However, authors, do not exclude that these elements may be included differently from the relative aragonitic or calcitic crystalline carbonate structures.