Any of various extinct cephalopod mollusks of the order Belemnoidea that lived from the Triassic into the Tertiary Period. Belemnites had a large, cone-shaped internal shell with a complex structure that served as a support for muscles and as a hydrostatic device. Belemnites were closely related to the present-day squids and cuttlefishes.
The fossilized internal shell of one of these cephalopods. Belemnites are used as index fossils.