Belemnites are a fascinating group of Cephalopods and among my favorite fossils. They looked like modern squid and were related to Ammonites but they had a hardened internal shell called a rostrum. Their (possibly) closest modern relative is the cuttlefish which has a similar internal structure. They were most abundant during the Jurassic and Cretaceous periods and died out about 65 mya. Here are a couple of specimens of Belemnopsis sp. from near Olkusz, Poland.
Specimen #1 is a small guy that I suspect is a juvenile since the fossil tapers more rapidly and is shorter than the other two.
Specimen #2 seems to be a typical size and length. note the divot that runs down the center of the shell surface.
Specimen #3 is the longest of the specimens I have and is interesting because it has oyster spats preserved on it's exterior. In order for this to occur the rostrum would need to be exposed on the sea floor after the organic tissue surrounding it had rotted away. The exposed rostrum then would be an ideal hard surface for an oyster to attach.
These fossils come from rocks dated to the Callovian stage of the Jurassic. There is a really great article posted by Phil Eyden here that goes into much more detail about Belemnites. I thought it was interesting to note that studies have indicated that many Belemnites only lived for a handful of years. This would explain why they are so common in the Mesozoic fossil record.