is usually elongated and bullet-shaped (though in some subgroups the rostrum may only exist as a thin layer coating the phragmocone). The hollow region at the front of the guard is termed the alveolus, and this houses a chambered conical-shaped part of the shell (called the phragmocone). The phragmocone is usually only found with the better preserved specimens. Projecting forwards from one side of the phragmocone is the thin pro-ostracum. While belemnoid phragmocones are homologous with the shells of other cephalopods and are similarly composed of aragonite, belemnoid guards are evolutionarily novel and are composed of calcite or aragonite, thus tending to preserve well. Broken guards show a structure of radiating calcite fibers and may also display concentric growth rings. Diagenetic modifications of the shells are complex. Radiating calcitic crystals are thin, or very large, with a shape indicative of a strong alteration. In other samples, the aragonite - calcite boundary is not dependent on growth lines. In a given fossil sites, some specimens are calcite, others are aragonite. The guard, phragmocone and pro-ostracum were all internal to the living creature, forming a skeleton which was enclosed entirely by soft muscular tissue. The original living creature would have been larger than the fossilized shell, with a long streamlined body and prominent eyes. The guard would have been in place toward the rear of the creature, with the phragmocone behind the head and the pointed end of the guard facing backward. The guard of the belemnoid Megateuthis gigantea, which is found in Europe and Asia, can measure up to in length, giving the living animal an estimated length of . Very exceptional belemnoid specimens have been found showing the preserved soft parts of the animal. Elsewhere in the fossil record, bullet-shaped belemnite guards are locally found in such profusion that such deposits are referred to semi-formally as "belemnite battlefields" (cf. "orthocone orgies"). It remains unclear whether these deposits represent post-mating mass death events, as are common among modern cephalopods and other semelparous creatures. Thunderstones The name "thunderbolt" or "thunderstone" has also been traditionally applied to the fossilised rostra of belemnoids. The origin of these bullet-shaped stones was not understood, and thus a mythological explanation of stones created where lightning struck has arisen. Uses The stable isotope composition of a belemnoid rostrum from the Peedee Formation (Cretaceous, southeast USA) has long been used as a global standard (Peedee Belemnite, "PDB") against which