Belemnites are known from the Jurassic and Cretaceous. In the typical genus Belemnites the shell has three parts; the guard is the largest and most posterior of these. It is solid calcite and forms a massive bullet-shaped cylinder, which tapers posteriorly to a point. The surface of the guard is usually smooth but can be granular or pitted, while the anterior end is indented by a conical cavity called the alveolus. The second part of the shell is the phragmocone and this lies within the alveolus. This is homologous to the nautiloid shell and has a slender siphuncle that threads through the septa at the ventral margin. The pro-ostracum is the third component and is a long flat expanded tongue that projects forwards and protects the anterior part of the body. However this is rarely preserved.