Imágenes de página
PDF
ePub

was analogous to that of the inhabitant of the Nautilus Pompilius. (See Pl. 31, Fig. 1.)

Mr. De la Beche has shown that the mineral condition of the outer chamber of many Ammonites, from the Lias at Lyme Regis, proves that the entire body was contained within it; and that these animals were suddenly destroyed and buried in the earthy sediment of which the lias is composed, before their bodies had either undergone decay, or been devoured by the crustaceous Carnivora with which the bottom of the sea then abounded.*

As all these shells served the double office of affording protection, and acting as floats, it was necessary that they should be thin, or they would have been too heavy to rise to the surface: it was not less necessary that they should be strong, to resist pressure at the bottom of the sea; and accordingly we find them fitted for this double function, by the disposition of their materials, in a manner calculated to combine lightness and buoyancy with strength.

* In the Ammonites in question, the outer extremity of the first great chamber in which the body of the animal was contained, is filled with stone only to a small depth, (see Pl. 36, from a. to b.;) the remainder of this chamber from b. to c., is occupied by brown calcareous spar, which has been ascertained by Dr. Prout to owe its colour to the presence of animal matter, whilst the internal air-chambers and siphuncle are filled with pure white spar. The extent of the brown calcareous spar, therefore, in the outer chamber, represents the space which was occupied by the body of the animal after it had shrunk within its shell, at the moment of its death, leaving void the outer portion only of its chamber, from a. to b., to receive the muddy sediment in which the shell was imbedded.

I have many specimens from the lias of Whitby, of the Ammonites Communis, in which the outer chamber thus filled with spar, occupies nearly the entire last whorl of the shell, its largest extremity only being filled with lias. From specimens of this kind we also learn, that the animal inhabiting the shell of an Ammonite, had no ink-bag; if such an organ existed, traces of its colour must have been found within the cavity which contained the body of the animal at the moment of its death. The protection of a shell seems to have rendered the presence of an ink-bag supurfluous.

First, The entire shell, (Pl. 35,) is one continuous arch, coiled spirally around itself in such a manner, that the base of the outer whorls rests upon the crown of the inner whorls, and thus the keel or back is calculated to resist pressure, in the same manner as the shell of a common hen's egg resists great force if applied in the direction of its longitudinal dia

meter.

[ocr errors]

Secondly, besides this general arch-like form, the shell is farther strengthened by the insertion of ribs, or transverse arches which give to many of the species their most characteristic feature, and produce in all, that peculiar beauty which invariably accompanies the symmetrical repetition of a series of spiral curves. (See Pl. 37, Figs. 110.)

From the disposition of these ribs over the surface of the external shell, there arise mechanical advantages for increasing its strength, founded on a principle that is practically applied in works of human art. The principle I allude to, is that by which the strength and stiffness of a thin metallic plate are much increased by corrugating, or applying flutings to its surface. A common pencil-case, if made of corrugated or fluted metal, is stronger than if the same quantity of metal were disposed in a simple tube. Culinary moulds of tin and copper are in the same way strengthened, by folds or flutings around their margin, or on their convex surfaces. The recent application of thin plates of corrugated iron to the purpose of making self-supporting roofs, in which the corrugations of the iron supply the place, and combine the power of beams and rafters, is founded on the same principle that strengthens the vaulted shells of Ammonites. In all these cases, the ribs or elevated portions, add to the plates of shell, or metal, the strength resulting from the convex form of an arch, without materially increasing their weight; whilst the intermediate depressed parts between these arches, are suspended and supported

by the tenacity and strength of the material. (See Pl. 37, Figs. 1-10.*)

The general principle of dividing and subdividing the ribs, in order to multiply supports as the vault enlarges, is conducted nearly on the same plan, and for the same purpose, as the divisions and subdivisions of the ribs beneath the groin work, in the flat vaulted roofs of the florid Gothic Architec

ture.

Another source of strength is introduced in many species of Ammonites by the elevation of parts of the ribs into little dome-shaped tubercles, or bosses, thus superadding the strength of a dome to that of the simple arch, at each point where these tubercles are inserted.t

The figures engraved at Pl. 37, afford examples of various contrivances to give strength and beauty to the external shell. The first and simplest mode, is that represented in Pl. 35 and Pl. 37, Fig. 1 and 6. Here each rib is single, and extends over the whole surface, becoming gradually wider, as the space enlarges towards the outer margin, or back of the shell.

The next variation is that represented (Pl. 37, Figs. 2, 7, 9,) where the ribs, originating singly on the inner margin, soon bifurcate into two ribs that extend outwards, and terminate upon the dorsal keel.

In the third case, (Pl. 27. Fig. 4.) the ribs originate simply, and bifurcating as the shell enlarges, extend this bifurcation entirely around its circular back. Between each pair of bifurcated ribs, a third or auxiliary short rib is interposed, to fill up the enlarged space on the dorsal portion where the shell is broadest.

In the fourth modification, (Pl. 37, Fig. 3,) the ribs originating singly on the internal margin, soon become trifurcate, and expand outwards, around the circular back of the shell. A perfect mouth of this shell is represented at Pl. 37, Fig. 3. d.

A fifth case is that (Pl. 37, Fig. 5,) in which the simple rib becomes trifurcate as the space enlarges, and one or more auxiliary short ribs are also interposed, between each trifurcation. These subdivisions are not always maintained with numerical fidelity through every individual of the same species, nor over the whole surface of the same shell; their use, however, is always the same, viz. to cover and strengthen those spaces which the expansion of the shell towards its outer circumference, would have rendered weak without the aid of some such compensation.

†These places are usually either at the point of bifurcation, as in Pl. 37, Figs. 2, 7, 9, 10, or at the point of trifurcation, as in Fig. 3.

The bosses thus often introduced at the origin, division, and termination of the ribs, are placed like those applied by architects to the intersections of the ribs in Gothic roofs, and are much more efficient in producing strength.* These tubercles have the effect of little vaults or domes; and they are usually placed at those parts of the external shell, beneath which there is no immediate support from the internal transverse plates (see Pl. 37, Fig. 8. Pl. 42. Fig. 3. c. d. e. and Pl. 40. Fig. 5.)†

* The ribs and bosses in vaulted roofs project beneath the under surface of the arch; in the shells of Ammonites, they are raised above the convex surface.

+ In Pl. 37, Fig. 9. (A. varians,) the strength of the ribs and proportions of the tubercles are variable, but the general character exhibits a triple series of large tubercles, rising from the surface of the transverse ribs. Each of these ribs commences with a small tubercle near the inner margin of the shell. At a short distance outwards is a second and larger tubercle, from which the rib bifurcates, and terminates in a third tubercle, raised at the extremity of each fork upon the dorsal margin.

Many species of Ammonites have also a dorsal ridge or keel, (Pl. 37, Figs. 1. 2. 6.) passing along the back of the shell, immediately over the siphuncle, and apparently answering, in some cases, the farther purpose of a cut-water, and keel (Pl. 37, Figs. 1, 2.) In certain species, e. g. in the A. lautus (Pl. 37, Fig. 7, a. c.) there is a double keel, produced by a deep depression along the dorsal margin; and the keels are fortified by a line of tubercles placed at the extremity of the transverse ribs. In the A. varians (Pl. 37, 9. a. b. c.) which has a triple keel, the two external ones are fortified by tubercles, as in Fig. 7, and the central keel is a simple convex arch.

Pl. 37, Fig. 8. offers an example of domes, or bosses, compensating the weakness that, without them, would exist in the A. catena, from the minuteness of its ribs, and the flatness of the sides of the shell. These flat parts are all supported by an abundant distribution of the edges of the transverse plates directly beneath them, whilst those parts which are elevated into bosses, being sufficiently strong, are but slightly provided with any other support. The back of this shell also, being nearly flat, (Pl.. 37, Fig. 8. b. c.) is strongly supported by ramifications of the transverse plates.

In Pl. 37. Fig. 6, which has a triple keel, (that in the centre passing over the siphuncle,) this triple elevation affords compensation for the weakness that would otherwise arise from the great breadth and flatness of the dorsal

Similar tubercles are introduced with the same advantage of adding Strength as well as Beauty in many other cognate genera of chambered shells. (Pl. 44, Fig. 9. 10. 14. 15.)

In all these cases, we recognise the exercise of Discretion and Economy in the midst of Abundance; distributing internal supports but sparingly, to parts which, from their external form, were already strong, and dispensing them abundantly beneath those parts only, which without them, would have been weak.

We find an infinity of variations in the form and sculpture of the external shell, and a not less beautiful variety in the methods of internal fortification, all adapted with architectural advantage, to produce a combination of Ornament with Utility. The ribs also are variously multiplied, as the increasing space demands increased support; and are variously adorned and armed with domes and bosses, wherever there is need of more than ordinary strength.

Transverse Plates, and Air Chambers.

The uses of the internal air-chambers will best be understood by reference to our figures. Pl. 36 represents a longitudinal section of an Ammonite bisecting the transverse plates in the central line where their curvature is most simple. On each side of this line, the curvature of these plates become more complicated, until, at their termination in the external shell, they assume a beautifully sinuous, or foliated arrangement, resembling the edges of a parsley leaf, (Pl. 38,) the uses of which, in resisting pressure, I shall farther illustrate by the aid of graphic representations.

portion of this species. Between these three keels, or ridges, are two de pressions or dorsal furrows, and as these furrows form the weakest portion of the shell, a compensation is provided by conducting beneath them the denticulated edges of the transverse plates, so that they present long lines of resistance to external pressure..

« AnteriorContinuar »