Crystalline structure,orientation and nucleation of the nacreous tablets in the cephalopod <Emphasis Type="Italic">Nautilus</Emphasis> |
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Authors: | Harry Mutvei Elena Dunca |
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Institution: | (1) Department of Palaeozoology, Swedish Museum of Natural History, Box 50007, 10405 Stockholm, Sweden |
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Abstract: | The nacreous tablets in the Nautilus shell have similar crystalline structure as the tablets in the gastropod Gibbula shell. Etching with Mutvei’s solution reveals that each tablet is composed of vertical crystalline columns that are structurally
similar to the acicular crystallites in the outer spherulitic-prismatic layer of the shell wall. The columns are attached
to each other to form numerous vertical crystalline lamellae, oriented parallel to the longitudinal axis of the tablet. It
is still unknown whether or not the orientation of the vertical lamellae corresponds to that of the crystallographic a- or
b-axis. The orientation of the crystalline lamellae in the adjacent tablets is parallel in some nacreous laminae, but random
in other laminae. Similar large variation was found in the nacreous tablets of the gastropod and bivalve shells. The nucleation
sites of the nacreous tablets are predominantly situated on the peripheral portion of the upper surface of the preceding tablet,
both in the shell wall and septa. The “aragonite-nucleating proteins” in the central portion of the crystal imprints of the
organic interlamellar sheets, described by several writers, have therefore a negative correlation with the nucleation sites
of the nacreous tablets. |
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