| a, a. Basal processes of the phragmocone. |
| b. The phragmocone. |
| c. The rostrum or guard of the osselet, containing the apex of the phragmocone in its upper part. |
The invariably radiated crystalline structure of the Belemnite has evidently resulted from the peculiar organization of the original osselet, which is formed of thin concentric laminæ, of very minute prismatic trihedral fibres, arranged at right angles to the planes of the successive layers.[403]
[403] The Belemnitic shell presents the same arrangement of its constituent layers as the Pearl-mussel, Pinna, and other Aviculidæ, viz. the outer layer is prismatic-cellular, the inner nacreous: the first is formed by the free margin of the mantle, the second by the visceral ("peritoneal") part of the mantle.—Mr. Woodward.
From the obvious analogy of the structure above demonstrated with that of the recent dibranchiate Cephalopoda, several eminent naturalists inferred that the animal of the Belemnite was closely related to the existing types; and the late Mr. Miller, in a communication to the Geological Society of London, gave a restored figure of the original, which, as modified by M. D’Orbigny, is represented [Lign. 142], figs. 1 and 6. The indefatigable and successful researches of the Rev. Dr. Buckland have confirmed the general correctness of this restoration. In the Lias of Dorsetshire two specimens of the Belemnite, with its chambered shell and horny or pearly receptacle, still retaining the ink-bag and its contents, have been discovered, and were figured in the Br. Treatise (Bd. pl. 44′, 44″). A third specimen, showing the ink-bag, is in the British Museum.
The ink-bag of the Belemnite is very small, as might be expected, from the extent to which it is protected by a chambered shell. The mandibles or beaks of the Belemnite are supposed to have been horny, as in the other naked Cephalopoda; since no calcareous beaks have been found associated with their remains.
"The Belemnite having the advantage of its dense, but well-balanced internal shell, must have exercised the power of swimming backwards and forwards, which it possessed in common with the modern decapod (ten-armed) Dibranchiata, with great vigour and precision. Its position was probably more commonly vertical than in its recent congeners. It would rise swiftly and stealthily to infix its claws in the belly of a supernatant fish, and then perhaps as swiftly dart down, and drag its prey to the bottom and devour it. We cannot doubt at least but that, like the hooked Calamaries of the present seas, the ancient Belemnites were the most formidable and predacious of their class."—Owen.
The Belemnites of the oolitic limestones frequently contain the phragmocone, either filled with calcareous spar, or with its cells empty. In the clays the horny sheath or receptacle is sometimes found pressed flat and extending above the alveolus of the osselet, and has often a thin coat of nacre of a pearly lustre, but it is more commonly detached.
The Belemnites abound in the Lias, Oolite, and Chalk, and have not been discovered in any other deposits; there are nearly thirty British species, some of which are restricted to the Chalk, and others to the Oolite and Lias.
A few characteristic forms are represented, [Lign. 141 and 142], in order to illustrate the three groups which, according to M. D’Orbigny, are peculiar to the grand divisions of the Cretaceous formation.