Apertural constrictions in some oncocerid cephalopods

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Abstract
In some oncocerid cephalopods the shape of the aperture, siphuncle and the general outline of the shell have long served as generic characters. The aperture is mostly elaborated into a certain number of sinuses which take their final shape only in the adult. Therefore, knowledge of the relative age of the animal is required. The last chamber may serve as an indicator of age. A last chamber smaller than the 2nd last indicates a mature specimen. This is because continued growth would have caused the death of the animal as the buoyancy turned negative. It is of great importance to study the growth lines along the peristome to observe whether growth has ceased or not. Growth variations were compared with growth stages. A constricted or contracted aperture can only be determined on specimens with the shell still preserved. Functional parallels are drawn between the Silurian Aptychopsis operculum and the restricted aperture.