Beautiful shell fossil, a nautiloid (Cymatoceras sp). High quality! Nautiloids are a subclass of marine cephalopod molluscs whose fossils date back to the Upper Cambrian period. They have a fairly wide depth distribution (up to about 900 m) and are characterised by an aragonitic calcareous shell divided internally by septa into chambers, connected by a partially mineralised organic structure called a “siphunculus”, whose function is to regulate the pressure of the internal fluids (gas and liquid in the chamber) by osmosis. This particular shell structure is an adaptation to the variable pressure conditions resulting from the wide bathymetric distribution range that characterises the group. NOTE: Sand not included