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.