Belemnites are members of an extinct form of cephalopods that lived from the 250-201 million years ago becoming extinct around the same time as dinosaurs (~66 million years ago). These cephalopods were common in the ancient oceans. Belemnite comes from the Greek word “belemnon” which means javelin or dart referring to their bullet-shaped rostrum that survives in fossils.
For more information on Belemnites see our blog post Belemite details and folklore