The prismatic region of two bivalve molluscs exemplifies, in its structure and organization, one of the types of differentiated calcareous substrates through which boring organisms must penetrate. The oriented inorganic crystals, separated from one another by intercrystalline “spaces”, are structurally organized into well defined prisms. The prisms of each bivalve vary in shape and size and are delineated from one another by electron-lucent, non-calcified regions. The demineralized organic matrix is also structurally organized into prisms, delineated from one another by prism sheaths, and an intraprismatic matrix structurally organized into closely packed sheet-like compartments and subcompartments in which the inorganic crystals are deposited. The non-mineralized intercrystalline “spaces” between the individual inorganic crystals of the same or adjacent rows in a mineralized section are occupied by the walls of the intraprismalic sheet-like compartments. Similarly, the non-calcified electronlucent regions delineating one mineralized prism from the next are occupied by the thick prism sheaths. These portions of the organic matrix which fail to mineralize completely undoubtedly provide ready pathways for the passage of solutes and solvents through these tissues of highly ordered, densely packed, inorganic crystals. Moreover, the framework of the organic matrix, which fails to mineralize in these heavily calcified, molluscan substrates, may provide the primary, not the secondary source of chemical attack during boring, for once the sheaths and compartments surrounding the crystals are broken down or solubilized, the crystals are themselves loosened and freed for mechanical removal by shell-penetrating organisms.