In the coleoid literature, the terminology of shell elements that are deposited on the external
surface of the primary shell wall is inconsistent and confusing. Morpho-and phylogenetic
interpretations have been therefore controversial. A strict layer-by-layer comparison of
seventeen species from seven coleoid subgroups suggests that the coleoid shell is covered
by either one (Diplobelida, Spirulida, Sepiida, Vampyropoda) or two (Aulacocerida,
Belemnitida, Belemnoteuthidida) outer shell formations. The confusion has been caused …