Treatise Online no. 93: Part N, Revised, Volume 1, Chapter 3: Periostracum and shell formation in the Bivalvia

with traces of calcite (medAkoviC & others, 1989), although WeiSS and others (2002) identified significant amounts of amorphous calcium carbonate in the larval shells of two bivalves that transformed subsequently into a crystalline phase (see discussion of precursor transient phases below).Together with proteins, chitin has been found to be an important component of the larval shell (CAStilho & others, 1989;WeiSS & SChönitzer, 2006).Shells are made of superimposed layers, each of which can be monomineralic or bimineralic.When calcite is present, it is typically restricted to the outer shell, to outer and innermost layers, or it may comprise all subperiostracal shell layers except for myostracal prisms and ligament fibers, which are invariably aragonitic.Rarely, an outermost shell layer can have both aragonitic and calcitic components, either as distinct sublayers as in some mytilids (CArter & others, 1990) or as spatially isolated, calcitic, mineralogical aberrations-e.g., in some veneroids (CArter, 1980a;CArter, BArrerA, & teveSz, 1998).On these layers, shell-forming crystals organize according to different 3-D (three-dimensional) configurations and arrangements that define the different microstructures.Particular microstructures are characteristic of calcite (e.g., foliated, chalky) or aragonite (e.g., nacre, crossed lamellar), whereas others (e.g., prismatic) have both calcitic and aragonitic counterparts.The shell protects the animal