Remnants of decalcified nacreous layers from Ordovician, Devonian, Carboneferous (including Pennsylvanian), Permian, Jurassic and Cretaceous, nautiloid and ammonoid shells, examined in the electron microscope, consists of various, biuret-positive structures, which reflect degrees in degradation of the original conchiolon sheets. Identification of these structures as altered nacreous concholin and discrimination from contaminating structures of foreign organisms, a frequent finding in fossil shells, were based on detection of similar structural modifications in the Recent Nautilus shell, in well preserved shells of Pennsylvanian and Mesozoic cephalopods and in alterations produced artificially in experimental diagenesis of the Recent Nautilus shell. Nacreous organic remnants subsisted in recrystallized shells. The mechanism of the conchiolin alterations, the significance of variations in the structural patterns of nautiloid conchiolin, the structural pattern of ammonoid nacreous conchiolin, and modifications in configuration of the nacreous layers in shells with preserved original aragonite and in recrystallized shells, have been discussed
