Ventrally to ventrolaterally, the conotheca consists of (from inside out) an inner prismatic, a nacreous, a thin outer prismatic, and a lamellar layer (Fig. 14.3). The rostrum overlies the outermost lamellar layer. The inner prismatic layer is between 17 ||m and 25 ||m thick. Thickness increases lateralward. The outer prismatic layer is constantly 3-4 |im thick (Fig. 14.3B). A well-developed 20-40 |im thick nacreous layer (platelets-nacre, Nautilus-Type, Type 1) separates the inner and outer prismatic layer. Longitudinal fractures between a phragmocone diameter of 4 mm and 10 mm show that the thickness of nacre is correlated with the ontogenetic stage, i.e., the thickness of nacre increases with the phragmocone diameter. Prisms of the inner and outer layer are clearly delimited from the intermediate nacreous layer. There are absolutely no interspaces between them.
The outside of the outer prismatic layer is covered by a 4-5 | m thick lamellar layer (Figs. 14.3B, 14.4A). Only the rostrum occurs outside this lamellar layer.
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