The Argonaut, or Paper Nautilus, whose elegant fragile shell is too well known to require description, is the only living genus of this Order, in which the animal is protected by a hard calcareous external covering. This shell is symmetrical, and convoluted on a vertical plane, and consists of but one cavity or chamber. The other genera are naked, and possess an internal chambered shell (as in the recent Spirula), or some modification of such an apparatus. The last chamber or cell of these enclosed shells is too small to admit any part of the body of the animal; a character by which the fossil species of this Order may be distinguished from those of the other order. Others have a horny or calcareous osselet, as the bone of the Cuttle-fish, and pen of the Calamary or Sea-pen (see Bd. pl. 28); and in an appendage of this kind a conical chambered shell is contained in many of the fossil genera, hereafter to be noticed. These animals have eight arms, with the addition in some genera of two long tentacula, which are furnished with rows of suctorial disks or cups, called acetabula (see [Lign. 142], figs. 1, 6).
These naked Cephalopoda, devoid of any external defence, possess a very extraordinary means of escape from their enemies. They are furnished with a bag or bladder, containing a dark fluid resembling ink in appearance, which they have the power of ejecting into the surrounding water upon the approach of danger; and by the obscurity t us induced, they foil the pursuit of their adversaries: the Nautilus and other cephalopods, protected by a large external shell, are destitute of such an apparatus. The deep brown colour, sepia, was formerly prepared from the fluid of the ink-bags of different species of Cuttle-fish; a similar substance secreted by extinct naked Cephalopoda, as we shall presently demonstrate, is found in a fossil state. These preliminary remarks on the organization of the recent animals will prepare us for the investigation of the extinct species. We will first notice those remarkable fossils, called Belemnites, or thunder-stones.
Lign. 141. Belemnites: 1/2 nat. Chalk and Oolite.
| Fig. | 1.— | Belemnitella mucronata. Chalk. Brighton. On the right of the figure is a view of the aperture, and a transverse section. |
| 2.— | Portion of a Belemnite, containing the internal conical chambered shell, called phragmocone. Oolite. | |
| 3.— | Belemnitella quadrata. Beauvais, France. The quadrangular cavity is shown in the upper figure on the left.(M. D’Orbigny.) | |
| 4.— | Belemnites dilatatus. Lower Greensand (Néocomien). France. |
BELEMNITES
Belemnite (from a supposed resemblance to the head of a dart or javelin). Lign. [141] to [144]. Among the innumerable relics of an earlier world, which swarm in the sedimentary deposits, there are perhaps no fossil bodies that have excited more curiosity, and given rise to so many fruitless conjectures as to their nature and origin, as the Belemnites.[402] These are long, cylindrical, or fusiform fossils, more or less pointed at one extremity, and having at the other and larger end a conical cavity, which is either occupied by a chambered shell, or filled up with the material in which the fossils are imbedded. Their substance is like fibrous calcareous spar, varying in colour from a dark brown to a light amber; many are transparent, others nearly opaque. When broken transversely they present a radiated structure ([Lign. 141], fig. 1) and a minute central cavity, or axis, is seen to extend through the whole length of the solid portion of the stone (see [Lign. 142], fig. 5.). A longitudinal section ([Lign. 142], figs. 4 and 5) shows the conical cavity in the upper part, and that the shaft consists of a series of concentric layers. Such are the characters of these fossils in the examples of most frequent occurrence.
[402] See Park. Org. Rem. vol. iii. p. 122.
The Belemnites vary in size from the small, delicate, transparent species, [Lign. 142], figs. 3 and 4, to massy opaque specimens, several inches in circumference, and from ten to twenty inches in length. They present also considerable variety of form; some are regularly cylindrical, as in [Lign. 141] fig. 1; others broad and flattened, as in fig. 4; or subfusiform, as in [Lign. 142], figs. 3 and 4. The small end is slender and pointed in some belemnites, and in others is obtuse, or rounded, with a projecting point. In many there is a longitudinal groove or furrow on the ventral aspect; and some species have a furrow on each side, as in that represented in [Lign. 142], fig. 2.