The shell of the Nautilus was examined using scanning electron microscopy, transmission electron microscopy and polarized light microscopy. The structure consisted of two major layers: a porcellaneous outer layer and a nacreous inner layer. Most of the porcellaneous layer was composed of granular crystals randomly distributed with a substructure suggestive of bundles of acicular crystallites. A separate prismatic sublayer of the porcellaneous material was composed of a more regular arrangement of crystals. The nacre was composed of alternating crystalline lamellae and films of organic material. The lamellae were formed of many polygonal crystal platelets. The growth surface of the nacre consisted mainly of stacks or towers of incomplete platelets but some areas showed a terraced form of growth in which each crystal lamella was essentially completed before the next covered it. The porcellaneous material, 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 material not present in the outermost whorl. These were a thin organic layer, which appeared as a boundary between the existing and added material, and a thick layer of nacre. This extra nacre may be useful in the shell's buoyancy control. The siphunclar tube, examined with scanning electron microscopy and polarized light microscopy, appeared in cross-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.
Mitchell, P. R. and Phakey, P. P.
"Notes on the Microstructure of the Nautilus Shell,"
Scanning Microscopy: Vol. 9
, Article 16.
Available at: https://digitalcommons.usu.edu/microscopy/vol9/iss1/16