Belemnites (Gr. belemnon, 'a dart' or 'arrow'), an interesting genus of fossil cephalopodous Mollusca, the type of a family called Belemnitidae, to the whole of which the name Belemnites is very generally extended, and which is closely allied to the Sepiade, or Cuttle (q.v.) family. No recent species of Belemnites is known: fossil species are very numerous, and are found in all the jurassic and cretaceous strata, from the lowest lias to the upper chalk, some of which are filled with myriads of their remains. These remains are generally those of the shell alone, which is now known to have been an internal shell, entirely included within the body of the animal, like that of the cuttle. The shell, as seen in the most perfect specimens, is double, consisting of a conical chambered portion (the phragmacone), inserted into a longer, solid, somewhat conical or tapering, and pointed sheath, which is termed the rostrum or guard. The space between the phragmacone and sheath is occupied either with radiating fibres or conical layers. The chambers of the shell are connected by a tube (siphuncle), so that the animal probably had the power of ascending and descending rapidly in the water. From some well-preserved specimens we learn that its body was furnished with lateral fins, and that it had eight arms and two longer tentacles. The suckers seem to have been provided with horny hooks; and these it probably fixed upon a fish, and descended with its prey to the bottom, like the hooked Calamary (q.v.) of the present seas. Horny mandibles and remains of an ink-bag, like that of the cuttle, have been found in the last and largest chambers of the belemnites; but remains of this chamber, which must have contained all the viscera of the animal, are very rarely preserved, the shell having been very thin at this part. The part most commonly found, and generally known by the name of belemnite, is the solid guard, or point into which the sheath was prolonged behind the chambered shell. These have received such popular names as Arrowheads, Petrified Fingers, Spectre-candles, Picks, Thunderstones, &c., from their form, or from the notions entertained of their nature and origin. Belemnites appear to have been of very different sizes; in some of the largest, the mere mucro is 10 inches long, and the entire animal, with its arms outstretched, must have been several feet in length.