LTHOUGH. it has long been known (for example, ref. 1) that keratins of varying degrees of hardness are responsible for the shape and function of claws and nails by the differential wear of the tissue during use, the structural classification of these organs has been limited to the general description of a “harder” unguis and a “softer” sub-unguis. It is customary to regard the hardness of keratin as being due to the compactness of its cells, its fibrils, microfibrils and other protein units and to the presence of abundant cystine residues. The purpose of this article is to present evidence in support of the hypothesis that a principal factor in determining the form and function of hairs, hoofs, claws, beaks and enamel is the pattern of hardening of keratins with calcium and other salts. The broad picture of an exoskeleton of keratin, mineralized according to its position and use is in con-formity with the situation in other epidermal structures; there is a parallel case in the crustacean cuticle, for example, where calcium carbonate and phosphate and other salts are laid down to strengthen the tissue. A biophysical, biochemical and cytological survey (some aspects of which will be reported elsewhere) has been carried out on a wide variety of natural and calcined mammalian, avian and reptilian keratins, and the results suggest that all keratins contain insoluble calcium and other salts which are deposited by, and usually within, the epidermal cells in selected parts of each tissue. In the case of enamel, the X-ray diffraction evidence points to some affinity of the mature enamel proteins with the keratins, and enamel may be regarded as the most highly calcified but least keratinized member of the group. It is well known that X-ray diffraction patterns of some keratins contain “salt” reflexions, and it has been shown that in baleen the inorganic pattern is given by bone saltsº laid down in an oriented partnership with keratin instead of collagen". Recently, similar “apatite” reflexions have been reported by Earland, Blakey and Stell in Indian rhinoceros horn “and in platypus hair, lion whiskers, goose feather calamus and in some human fingernails". In the survey presented here, the nature and extent of mineralization varied widely, ranging from sparse deposits rich in calcium and phosphorus giving clear X-ray diffraction patterns of hydroxyapatite to dense deposits poor in phosphorus giving no inorganic pattern. In the air-dried native tissues, the calcium salt varied in composition, concentration, crystallinity, location and orientation. Those keratins which did not give an X-ray diffraction pattern of inorganic salt were calcined at 900 C for 2 h; with a few exceptions (some hairs) the diffraction patterns of the water-insoluble part of the ash contained reflexions characteristic of one or more of the hydroxyapatite/CaO/Ca,(PO,), lattices described by Carlström º for calcined bone and enamel; added reflexions suggesting other salts were frequently present. The analyses of a representative selection of specimens is set out in Table 1. For convenience, the keratins have been divided broadly into three groups according to the nature of the mineral which is deposited.