Structural relationship between interlamellar organic sheets and nacreous tablets in gastropods and the cephalopodNautilus

The nacreous tablets in gastropods and the cephalopodNautilus are composed of three calcareous layers: a principal, thick, finely granular layer and two thin, coarse-granular layers, one covering the upper surface of the principal layer and another the lower surface of this layer. The granules on the surface layers inNautilus differ from those in gastropods by their much more elongated shape and larger size. The central portion of the nacreous tablet of gastropods andNautilus is more or less elevated forming the central elevation. The granules on this portion usually are larger, irregularly shaped and more crowded than on the main, peripheral portion of the tablet. The untreated, dry interlamellar organic sheets on upper surfaces of immature nacreous tablets are uncalcified and elastic. Narrow thicker parts of the sheet, the trabeculae, Surround large intertrabecular spaces where the sheet is thin. In places it can be observed that each calcareous granula on the surface layer of the nacreous tablet is situated within the intertrabecular space. The size, shape and distribution of the intertrabecular Spaces correspond those of the surface granules. No mineral bridges were observed between the consecutive nacreous tablets.