Secondly, a conical thin horny sheath, or cup, commencing from the base of the hollow cone of the fibro-calcareous sheath, and enlarging rapidly as it extends outwards to a considerable distance. Pl. 44', Fig. 7, b, e, e', e″. This horny cup formed the anterior chamber of the Belemnite, and contained the ink-bag, (c,) and some other viscera.[1]
Thirdly, a thin conical internal chambered shell, called the Alveolus, placed within the calcareous hollow cone above described. (Pl. 44, Fig. 17, a. and Pl. 44', Fig. 7, b, b'.)
This chambered portion of the shell is closely allied in
form, and in the principles of its construction, both to the
Nautilus and Orthoceratite. (See Pl. 44, Fig. 17, a, b. and
Fig. 4.) It is divided by thin transverse plates into a series
of narrow air-chambers, or areolæ, resembling a pile of
watch-glasses, gradually diminishing towards the apex.
is made up of a pile of cones, placed one within another, having a common
axis, and the largest enclosing all the rest. (See Pl. 4-4, Fig. 17.) These cones are composed of crystalline carbonate of lime, disposed in fibres that radiate from an eccentric axis to the circumference of the Belemnite. The crystalline condition of this shell seems to result from calcareous infiltrations (subsequent to interment,) into the intervals between the radiating calcareous fibres of which it was originally composed. The idea that the Belemnite was a heavy solid stony body, whilst it formed part of a living and Hosting sepia, would be contrary to all analogies afforded by the internal organs of living Cephalopods. The odour, resembling burnt horn, produced on burning this part of a Belemnite, arises from the remains of horny membranes interposed between each successive fibro-calcareous cone.
An argument in favour of the opinion that Belemnites were internal organs, arises from the fact of their surface being often covered with vascular impressions, derived from the mantle in which it was inclosed. In some species of Belemnites the back is granulated, like the back of the internal shell of Sepia officinalis.
- ↑ This laminated horny sheath is rarely preserved in connexion with the fibro-calcareous shelly sheath; but in the Lias at Lyme Regis it is frequently found without the shell. Certain portions of it are often highly nacreous, whilst other parts of the same sheath retain their horny condition.
