Beautiful fossil shell, a polished nautiloid (Cymatoceras sp). High quality! Nautiloids are a subclass of marine cephalopod molluscs whose fossil record dates back to the Upper Cambrian. They have a fairly wide depth distribution (up to about 900 m) and are characterised by a calcareous aragonitic shell divided internally by septa into chambers, which are connected by a partly mineralised organic structure called a 'sifunculus', whose function is to regulate the pressure of the internal fluids (gas and chamber liquid) by osmosis. This particular structure of the shell constitutes an adaptation to the variable pressure conditions resulting from the wide bathymetric distribution range that characterises the group.