Discussion

Our results partly confirm earlier observations by Bandel and Kulicki (1988) in which the conotheca of Belemnotheutis consists of an inner and an outer prismatic layer separated by a nacreous layer (Fig. 14.7). In contrast to Bandel and Kulicki (1988), we found a fourth lamellar layer, which is external to the outer prismatic layer. It occurs all around the phragmocone. Most probably, the laminae of this layer were primarily organic because in some places they are penetrated by borings (fungi? algae?), disintegrated or replaced by secondary calcite.

However, a four-layered construction of the conotheca does not occur all around the whole phragmocone circumference (Fig. 14.7). Whereas Bandel and Kulicki (1988) assumed a persistent conotheca, we conclude from our observations that in the genus Belemnotheutis only two-thirds of the phragmocone circumference consist of four layers. The remaining dorsal one-third of the circumference is built of only two layers. Ventrolaterally to dorsolaterally the thickness of the nacreous layer decreases continuously until it wedges out. As a result, inner and outer prismatic layers merge together. Consequently, only a single inner prismatic layer and an outer lamello-organic layer constitute the dorsal conotheca.

Because of its outermost position, it seems reasonable to interpret the lamello-organic layer as the periostracum. As characteristic for most shelled molluscs the periostracum covers the inner mineralized layers. Dorsally, where the nacreous layer is absent, the rather thin periostracum is modified into a comparably thick layer. Where the sheath-like rostrum has been removed, this sudden shift from a nacre-dominated conotheca to a periostracum-dominated conotheca is even visible macroscopically (Fig. 14.2). Weak, forwardly curved growth lines on the dorsal black surface of the periostracal layer are typical for a belemnoid proostracum. This led us to assume that the periostracum forms the proostracum.

Fig. 14.6 (A) Dorsal cross-fracture of specimen MC-8 (phragmocone diameter = 1 cm) to show inner prismatic layer (ipl), spindle-like swelling of the lamellar layer (arrows), and rostrum (ro); 100x enlarged. (B) Detail of (A). The lamellar layer (ll) between the inner prismatic layer (ipl) and the rostrum (ro) is comparatively thick and replaced by secondary calcite; 1,000x enlarged.

Fig. 14.6 (A) Dorsal cross-fracture of specimen MC-8 (phragmocone diameter = 1 cm) to show inner prismatic layer (ipl), spindle-like swelling of the lamellar layer (arrows), and rostrum (ro); 100x enlarged. (B) Detail of (A). The lamellar layer (ll) between the inner prismatic layer (ipl) and the rostrum (ro) is comparatively thick and replaced by secondary calcite; 1,000x enlarged.

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