Lingula, a living brachiopod, has a shell composed of organic substances and an inorganic substance which is crystallochemically identical with francolite, a carbonate fluorapatite. X-ray powder diffraction photographs are essentially identical for central and marginal portions of the shell and resemble those of synthetic carbonate hydroxyapatite and human dentin (Pl. 1, fig. 1). These microcrystalline substances evidently are composed of a single inorganic crystalline phase. Fossil inarticulate brachiopods may have been accumulators of fluorine as well as phosphorus.

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