Comparing the shell of Belemnite, with that of Nautilus,
we find the agreement of all their most important parts to
be nearly complete;[1] and the same analogies might be
traced through the other genera of chambered shells.[2]
strata was deposited upon them, in a soft and fmuid state. (See Allan's Paper on Belemnite, Trans. Royal Soc. Edin., and Miller's Paper, Geol. Trans. Lond, 1826, p. 53.)
Thus of the millions of Belemnites which crowd the Secondary formations, only the fibro-calcareous sheath and chambered alveoli are usually preserved; whilst in certain shale beds this sheath and shell have sometimes entirely disappeared, and the horny or nacreous sheath or ink-bag alone remain. See Pl. 44″, Fig. 1, 2, 3, 4, 5, 6, 7, 8. In the rare case, Pl. 44′, Fig. 7, which has afforded the clue to this hitherto unexplained enigma, we have all the three essential parts of a Belemnite preserved in their respective places nearly entire. The ink-bag (c) is placed within the anterior horny cup (e, e', e″;) and the chambered alveolus, (b b') within the hollow cone of the posterior fibro-calcareous shell, or common Belemnite.
- ↑ The air-chambers and siphuncle are, in both these families, essentially the same.
In Belemnites, the anterior extremity of the fibro-calcareous shell, which forms a hollow straight cone, surrounding the transverse plates of the chambered alveolus, represents the hollow, coiled-up cone containing all the transverse plates, which make up the alveolus of the Nautilus.
The anterior horny cup, or outer chamber of the Belemnite, surrounding the ink bag, and other viscera, represents the large anterior shelly chamber which contains the body of the Nautilus.
The posterior portion of the Belemnite, which is elongated backwards into a fibrous pointed shaft, is a modification of the apex of the straight cone of this shell, to which there seems to be no equivalent in the apex of the coiled-up cone of Nautilus. The cause of this peculiar addition to the ordinary parts of shells, seems to rest in the peculiar uses of the shall of the belemnite, as an internal shell, acting like the internal shell of the Sepia Officinalis, to support the soft parts of the animals, within the bodies of which they were respectively enclosed. The fibrous structure of this shaft is such as is common to many shells, and is most obvious in the Pinnæ.
- ↑ Comparing the Belemnite, or internal shell of Belemno-sepia with the Sepiostaire, (Blainville,) or internal shell of the Sepia Officinalis, we have the following analogies. In the Sepiostaire, (Pl. 44', Fig. 2, a. e. and Figs. 4, 4', 5,) the small conical apex (a) represents the apex of the long calcareous posterior sheath of the Belemnite, (Fig. 7, a.,) and the
