In Table 1, we have summarized important ultrastructural analyses on the belemnoid conotheca. It seems that the sequence of layers within the conotheca is highly variable.
h- proostracum -m h- proostracum -m
Interests in the ultrastructure of the belemnoid conotheca started with Christensen (1925) and Muller-Stoll (1936). Jeletzky (1966) revised older literature and postulated a belemnoid conotheca consisting of a thin inner prismatic layer, a thin central mineralized layer, and a thick outer prismatic layer. Jeletzky (1966: 110) also added: "Our ideas about microscopic structure of phragmocone and conotheca of the Belemnitida are now in a state of confusion." Since microscopic magnifications have proved to be insufficient for ultrastructural analyses, these older investigations are negligible. Modern SEM analyses with adequate magnifications started with Barskov (1972). In Conobelus, Pachyteuthis, and Mesohibolithes, Barskov observed an inner prismatic and an outer nacreous layer in the conotheca. Observations are in agreement with those of Jeletzky (1966), at least concerning the presence of an innermost prismatic conothecal layer.
Especially concerning the presence of an outer prismatic layer, observations are inconsistent. As is well documented by Bandel et al. (1984) and Doguzhaeva et al. (1999, 2003b), the layer forming the primordial rostrum continues in a layer along the outside of the conotheca. It is possible that this layer was sometimes interpreted as the outer prismatic layer of the conotheca (Table 14.1), because Bandel and Kulicki (1988) identified a comparably thin outer prismatic layer within the conotheca of Belemnotheutis not belonging to the primordial rostrum.
Likewise it is difficult to correlate intermediate layers. Interpretations vary between a thick nacreous layer (Mutvei, 1964; Barskov, 1972; Hewitt and Pinkney, 1982; Bandel and Kulicki, 1988; Doguzhaeva et al., 1999, 2002) and thin organic sheets (Doguzhaeva et al., 2003b).
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