A conical calcareous fossil, tapering to a point at the
   lower extremity, with a conical cavity at the other end, where it is
   ordinarily broken; but when perfect it contains a small chambered cone,
   called the phragmocone, prolonged, on one side, into a delicate concave
   blade; the thunderstone. It is the internal shell of a cephalopod
   related to the sepia, and belonging to an extinct family. The
   belemnites are found in rocks of the Jurassic and Cretaceous ages.