In this article, we report an analysis of a section of the shell structure of the bivalve Aulacomya Ater. This bivalve is shown to be made up of a calcite layer allocated between two blocks of crystalline layers of aragonite. The measurements performed suggest the presence of alternate layers of prismatic calcite and aragonite (nacre). Composition maps are presented of the crystalline phases of aragonite and calcite for three bivalves and three gastropods. The gastropods studied are Concholepas Concholepas, Fisurella Crassa, Haliotis Rufescens and the bivalves are Argopecten Purpuratus, Choromytilus Chorus and Aulacomya Ater.