Approximately 100 recent molluscan shells have been analyzed spectrographically for Mg, Sr and Ba; their calcite-aragonite ratios were determined by X-ray diffraction. The methods are described in detail. The most important parameter controlling the concentration of these trace elements in calcium carbonate shells appears to be generic association rather than water temperature or calcite-aragonite ratio of the shell. As a group, snails are higher in Mg and lower in Sr and Ba than clams....