The Collaborative International Dictionary of English v.0.48:

Belemnite \Be*lem"nite\, n. [Gr. ? dart, fr. ? dart, fr. ? to
   throw: cf. F. b['e]lemnite.] (Paleon.)
   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. --
   Bel*em*nit"ic, a.
   [1913 Webster]

WordNet (r) 3.0 (2006):

belemnite
    n 1: a conical calcareous fossil tapering to a point at one end
         and with a conical cavity at the other end containing (when
         unbroken) a small chambered phragmocone from the shell of
         any of numerous extinct cephalopods of the family
         Belemnitidae