Shells of adult individuals from two different bivalve families, Hyriopsis cumingii and Diplodon chilensis patagonicus, were studied by Micro-Raman spectroscopy and Focussed Ion Beam-assisted TEM. The shells contain amorphous calcium carbonate in a zone at the interface between the periostracum and the prismatic layer. In this area, the initial prism structures protrude from the inner periostracum layer and it is demonstrated that these structures systematically consist of highly disordered and amorphous calcium carbonate. Within this zone, ordered and disordered areas are intermingled discounting the existence of a crystallization front and favouring models of domainal crystallization processes via so-called mesocrystals. These observations are the first documentation of the use of amorphous calcium carbonate as a precursor phase by adult mollusc species and lend further support to hypotheses postulating widespread use of amorphous phases as building material of skeletal tissue in biology. (C) 2010 Elsevier Inc. All rights reserved.