INTRODUCTION Carbon dioxide and calcium play fundamental roles in the metabolism of living creatures. Photosynthetic transformations of carbon dioxide and water into organic compounds, together with the subsequent reoxidation of these compounds and their derivatives, are the basic biological mechanisms for the storage and utilization of solar energy. In many living systems the quantity of carbon dioxide controls the hydrogen ion concentration. In the tissues and body of marine invertebrates the presence of calcium stabilizes intercellular matrices and mucous coverings; in its absence the constituent cells of most invertebrate tissues fall apart. Calcium is essential for the normal activity of the neuro-muscular system, for amoeboid and ciliary movement, chromatopore activity, the clotting of the body fluids, and the control of permeability (Robertson, 1941). Calcium and carbon dioxide are major components of organic hard parts. For this reason they are of particular importance to the geologist and paleontologist, not only because of the high percentage of limestone in sedimentary rocks, but because carbonate structures are the principal remains of the animals and plants that lived during the geologic past. Calcium carbonate customarily occurs in two crystal forms in the sea. Calcite, crystallizing in the rhombohedral Class of the hexagonal system, is usually the most abundant form in cold environments, while aragonite, the orthorhombic form, is most common in warm tropical seas. The two closely related elements, magnesium and strontium, are often associated with calcium in marine carbonates. Under marine conditions, certain classes of organisms tend to take up magnesium carbonate in...