. Bulletin of the British Museum (Natural History). Zoology . 2 2 J. D. TAYLOR, W. J. KENNEDY & A. H species, area of shell and stage of growth. In Pinctada martensi, Wada (i960, 1961b) records crystals between 1 and 4 microns across near the shell margins, and 3-8 microns across in the central region of the shell ; most crystals are between 2 and 10 microns across. Light microscopy and electron diffraction indicate that the flat horizontal surfaces of the tablets correspond to the basal pinacoid faces of aragonite {001}, and the sides to the {010} and {100} forms (text f
. Bulletin of the British Museum (Natural History). Zoology . 2 2 J. D. TAYLOR, W. J. KENNEDY & A. H species, area of shell and stage of growth. In Pinctada martensi, Wada (i960, 1961b) records crystals between 1 and 4 microns across near the shell margins, and 3-8 microns across in the central region of the shell ; most crystals are between 2 and 10 microns across. Light microscopy and electron diffraction indicate that the flat horizontal surfaces of the tablets correspond to the basal pinacoid faces of aragonite {001}, and the sides to the {010} and {100} forms (text figs. 6 and 7). The {no} form is usually dominant, often giving rise to rhombic crystals. One interesting feature seen on the {001} surfaces of single crystals is the presence of stepped dextral and sinistral spirals, with smooth, curved outlines in rounded crystals, and straight, polygonal outlines in euhedral crystals (Plate 7, fig. 3 ; text figs. 8-9). These successively higher layers are interpreted as being the result. Fig. 8. Screw dislocation in a rounded nacre tablet in Pinctada martensi. (Based on Wada 1961b.) X6500. Fig. 9. Screw dislocation in euhedral nacre tablet of Pinctada martensi. (Based on Wada 1961b.) X4250. of screw dislocations within the crystals, a feature well known in inorganic crystal growth (Read 1953, Frank 1952, Dekeyser and Amelinckx 1955). The origin of these dislocations is largely speculative. They may be caused by the foreign inclusions which are abundant in nacre in the form of the inter- and intracrystalline organic matrix. Development of sheet nacre begins with the appearance of minute, elongate, rounded aragonite seed crystals on, and in, an underlying interlamellar matrix, (Wada 1961b, fig. 6). The seeds are usually oriented parallel to one another and to the structure within the interlamellar matrix. The seeds grow and develop as rounded or euhedral tablets (Plate 1, fig. 2) or euhedral crystals, and with continued growth link up along par
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