NOTES ON THE MICROSTRUCTURE OF THE NAUTILUS SHELL

The shell of the Nautilus was examined using scanning electron microsc opy, transmission electron microscopy and polarized light microscopy. The structure consisted of two major layers: a porcellaneous outer lay er and a nacreous inner layer. Most of the porcellaneous layer was com posed of granular crystals randomly distributed with a substructure su ggestive of bundles of acicular crystallites. A separate prismatic sub layer of the porcellaneous material was composed of a more regular arr angement of crystals. The nacre was composed of alternating crystallin e lamellae and films of organic material. The lamellae were formed of many polygonal crystal platelets. The growth surface of the nacre cons isted mainly of stacks or towers of incomplete platelets but some area s showed a terraced form of growth in which each crystal lamella was e ssentially completed before the next covered it. The porcellaneous mat erial, which is exposed to the external environment, and thus requires a greater erosion resistance, was considerably harder than the nacre. It was found that the internal shell walls showed further layers of m aterial not present in the outermost whorl. These were a thin organic layer, which appeared as a boundary between the existing and added mat erial, and a thick layer of nacre. This extra nacre may be useful in t he shell's buoyancy control. The siphunclar tube, examined with scanni ng electron microscopy and polarized light microscopy, appeared in cro ss-section as a ring of semi-prismatic crystals outside a dark organic hoop. A bisection of the septal neck showed that this ring fits Like a sleeve over the nacre of the septal neck.