Assorted belemnite fossils. The belemnites are a group of extinct marine molluscs, which flourished in the Mesozoic era (230 to 65 million years ago). They were cephalopods, closely related to present- day octopus, squid & cuttlefish. They were similar in appearance to modern cuttlefish, possessing ten tentacles equipped with a double row of horny hooks, an ink sac and an internal shell. The internal shells, as seen here, are usually the only parts to survive in fossil form.