Belemnitida (or Belemnite) is an extinct order of Cephalopods (today Cephalopods are squids, octopuses, cuttlefish, etc.) which existed during the Mesozoic era, from the Hettangian age of the lower Jurassic to the Maastrichtian age of the upper Cretaceous.
The fossil of a Belemnite comes from half of their body called the guard. If you look at the picture of the squid below the red arrow is pointed at the guard. The Belemnite got it's name because Belemnite in Greek means dart. The fossils listed for sale on this site are thought to be up to 150 million years old.