Belemnitidae

Belemnitidae is one of the three families of that section of the ten-armed Cephalopoda, known as the Phragmophora owing to the possession of a "phragmocone." This structure consists of a series of septa or partitions slightly separated from one another, but communicating by a narrow tube known as the siphuncle. (This should be compared with the structure of a Nautilus shell.) In the forms with the simplest shells the phragmocone consists simply of a series of septa with the chambers between them closed in at the sides by a thin calcareous wall; in the Belemnites it is enclosed in a cavity (alveolus) at one end of a dense, long, and usually conical shield, known as the "guard." At the alveolar end the guard is continued as a horny tube (the proostracum). Like the squid they possessed an ink sac. The family is extinct; the principal genus was Belemnites, which was common throughout the Mesozoic (q.v.) era; a few species occurred earlier and a few survived till later. The "guards" of Belemnites, owing to their indestructibility are very common as fossils, and have long been known as "thunderbolts." Other important genera are Aulacoceras, an early form found in the Tyrol, and Spirulirostra from the Miocene, in both of which the guard is small in comparison with the phragmocone.