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The shell of the Ammonites Heterophyllus (PI. 38, and PI. 39,) affords beautiful exemplifications of the manner in -which the mechanical strength of each transverse plate is so disposed, as to vary its support in proportion to the different degrees of necessity that exist for it in different parts of the same shelL*
complicated, the number of transverse plates is but sixteen in one revo. lution of the shell; in this, as in almost all other cases, the extreme beauty and elegance of the foliations result from the repetition, at regu* lar intervals, of one symmetrical system of forms, viz. those presented by the external margin of a single transverse plate. No trace of these foliations is seen on the outer surface of the external shell. (See PI. 38, c.)
The figures of A. obtusus, (PI. 35 and PI. 36,) show the relations between the external shell and the internal transverse partitions of an Ammonite. PI. 35 represents the form of the external shell, wherein the body occupied the space extending from b- to c, and corresponding with the same letters in PI. 36.
This species has a single series of strong ribs passing obliquely across the shell of the outer chamber, and also across the air-chambers. From c. to the inmost extremity of the shell, these ribs intersect, and rest on the sinuous edges of the transverse plates which form the air-chambers. These edges are not seen where the outer shell is not removed. (PI. 35, e.) A small portion of the shell is also preserved at PI. 35, b.
From d. inwards, these sinuous lines mark the terminations of the transverse plates at their junction with the external shell; they are not coincident with the direction of the ribs, and therefore more effectually co-operate with them in adding strength to the shell, by affording it the support of a series of various props and buttresses, set nearly at right angles to its internal surface.
* Thus on the back or keel, PI. 39, from V. to B., where the shell is narrow, and the strength of its arch greatest, the intervals between the septa are also greatest, and their sinuosities comparatively distant; but as soon as the flattened sides of the same shell, PI. 38, assume a form that offers less resistance to external pressure, the foliations at the edges of the transverse plates approximate more closely; as in the flatter forms of a Gothic roof, the ribs are more numerous, and the tracery more complex, than in the stronger and more simple forms of the pointed arch.
The same principle of multiplying and extending the ramifications of the edges of the transverse plates, is applied to other species of Ammonites, in which the sides are flat, and require a similar increase of support; whilst hi those species to which the more circular form of the sides
At Plate 41. we have a rare and most beautiful example of the preservation of the transverse plates of the Ammonites giganteus converted to calcedony, without the introduction of any earthy matter into the area of the airchambers.*
This shell is so laid open as to show the manner in which each transverse plate forms a tortuous partition between the successive air-chambers. By means of these winding plates, the external shell, being itself a continuous arch, is farther fortified with a succession of compound arches, passing transversely across its internal cavity; each arch being disposed in the form of a double tunnel, vaulted not only at the top, but having a corresponding series of inverted arches along the bottom.
We can scarcely imagine a more perfect instrument than this for affording universal resistance to external pressure, in which the greatest possible degree of lightness is combined with the greatest strength.
The form of the air-chambers in Ammonites is much more complex than in the Nautili, in consequence of the tortuous windings of the foliated margin of the transverse plates.*
gives greater strength (as in A. obtusus, PI. 35.) the sinuosities of the septa are proportionately few.
It seems probable that some improvement might be made, in fortifying the cylindrical air-tube of Masscy's Patent sounding machine, for taking soundings at great depths, by the introduction of transverse plates, acting on the principle of the transverse plates of the chambered portion of the shells of Nautili and Ammonites, or rather of Orthoceratites, and Baculitee, (see PL 44, Figs. 4. and 5.)
* PL 42, Fig. 1, represents the cast of a single chamber of N. Hexagonus, convex inwards, and concave outwards, and bounded, at its margin by lines of simple curvature. In a few species only of Nautilus the margin is undulated, (as in PI . 43, Fig. 3, 4,) but it is never jagged, or denticulated like the margin of the casts of the chambers 6f Ammonites.
In Ammonites, the chambers have a double curvature, and are, at their centre, convex outwards (see PI . 36. d. and PI. 39. d. V.) PI . 42, Fig, 2, represents the front view of the cast of a single chamber of A. excavatus; d, is the dorsal lobe enclosing the siphuncle, and e. f. the auxiliary ventral lobes, which open to receive the inner whorl of the shell. PI. 42. Fig. 3. represents a cast of three chambers of A. catena, having two transverse plates still retained in their proper place between them. The foliated edges of these transverse plates have regulated the foliations of the calcareous easts, which, after the shell has perished, remain locked into one another, like the sutures of a skull.
It remains to consider the mechanism of the Siphuncle, that important organ of hydraulic adjustment, by means of which the specific gravity of the Ammonites was regulated. Its mode of operation as a pipe, admitting or rejecting a fluid, seems to have been the same as that we have already considered in the case of Nautili.*
The substance of the casts in all these cases is pure crystalline carbonate of lime, introduced by infiltration through the pores of the decaying shell. Each species of Ammonite has its peculiar form of air-chambers, depending on the specific form of its transverse plates. Analogous variations in the form of the air-chambers are coextensive with the entire range of species in the family of Nautili.
* In the family of Ammonites, the place of the Siphuncle is always upon the exterior, or dorsal margin of the transverse plates. (See PI. 36. <1. e. f. g. h. i., and PI. 42, Fig. 3. a, b.) It is conducted through them by a ring, or collar, projecting outwards; this collar is seen, well preserved, at the margin of all the transverse plates in PI. 36. In Nautili, the collar projects uniformly inwards, and its place is either at the centre, or near the inner margin of the transverse plates. (See PI. 31, Fig. 1. y. and PI. 42. 1.)
The Siphuncle represented at PI. 36, is preserved in a black carbonaceous state, and passes from the bottom of the external chamber M.) to the inner extremity of the shell. At e. f. g. 1). its interior is exposed by section, and appears filled, like the adjacent air-chainbers, with a cast of pure calcareous spar. At PI. 42. Fig. 3. b. a similar cast fills the tube of the Siphuncle, and also the air-chambers. Here again, as in PI. 36, its diameter is contracted at its passage through the collar of each transverse plate, with the same mechanical advantages as in the Nautilus,
The shell engraved at PI. 42. Fig. 4. from a specimen found by the Marquis of Northampton in the Grcensand of Earl Stoke, near Devizes, and of which Figs. 5. 6. are fragments, is remarkable for the preservation of its Siphuncle, distended and empty, and still fixed in its place along the interior VOL. I.—23
The universal prevalence of such delicate hydraulic con» trivancesin the Siphuncle, and of such undeviating and sys-; tematic union of buoyancy and strength in the air-chambers, throughout the entire family of Ammonites and Nautili, are among the most prominent instances of order and method, that pervade these remains of former races that inhabited the ancient seas; and strange indeed must be the construction of that mind, which can believe that all this order and method can have existed, without the direction and agency of some commanding and controlling Intelligence.
Theory of Von Buck.
Besides the uses we have attributed to the sinuous arrangement of the transverse septa of Ammonites, in giving strength to the shell to resist the pressure of deep water, M. Von Buch has suggested a farther use of the lobes thus 26?
of the dorsal margin of the shell. This Siphuncle, and also the shell and transverse plates, are converted into thin chalcedony, the pipe retaining in these empty chambers the exact form and position it held in the living shell.
The entire substance of the pipe, thus perfectly preserved in a state that rarely occurs, shows no kind of aperture through which any fluid could have passed to the interior of the air-chambers. The same continuity of the Siphuncle appears at PI. 42, Fig. 3. and in PI. 36, and in many other specimens. Hence we infer, that nothing could pass from its interior into the air-chambers, and that the office of the Siphuncle was to be more or less distended with a fluid, as in the Nautili, for the purpose of adjusting the specific gravity, so as to cause the animal to float or sink.
Dr. Prout has analyzed a portion of the black material of the Siphuncle, which is so frequently preserved in Ammonites, and finds it to consist of animal membrane penetrated by carbonate of lime. He explains the black colour of these pipes, by supposing that the process of decomposition, in which the oxygen and hydrogen of the animal membrane escaped, was favourable to the evolution of carbon, as happens when vegetables are converted into coal, under the process of mineralization. The lime has taken the place of the oxygen and hydrogen which existed in the pipe before decomposition.
formed around the base of the. outer chamber, as affording points of attachment to the mantle of the animal, whereby it was enabled to fix itself more steadily within its shell. The arrangement of these lobes varies in every species of Ammonite, and he has proposed to found on these variations, the specific character of all the shells of this great family.*
* The most decided distinction between Ammonites and Nautili, is founded on the place of the siphon, to the Ammonite, this organ is always on the back of the shell, and never so in the Nautilus. Many other distinctions emanate from this leading' difference; the animal of the Nautilus having its pipe usually fixed near the middle, (See PI. 31, Fig. 1,) or towards the ventral margin (PI. 32, Fig. 2, and PI. 42. Fig. 1.) of the transverse plates, is thereby attached to the bottom of the external chamber, which is generally concave, and without any jagged termination, or sinuous flexure, of its margin. As the siphon in Ammonites is comparatively small, and always placed on the dorsal margin (PI. 36, d. and Fl. 39, d,) it would have less power than the siphuncle of Nautili to keep the mantle in its place at the bottom of the shells another kind of support was therefore supplied by a number of depressions along the margin of the transverse plate, forming a series of lobes at the junction of this plate with the internal suiface of the shell.
The innermost of these, or ventral lobe, is placed on the inner margin of the shell (PI. 39, V.;) opposite to this, and on the external margin, is placed the dorsal lobe (D,) embracing the siphon and divided by it into two divergent arms. Beneath the dorsal lobe are placed the superior lateral lobes (L,) one on each side of the shell; and still lower, the inferior lateral lobe, (1,) next above the ventral lobe.
The separations between these lobes form seats, or saddles, upon which the mantle of the animal rested, at the bottom of the outer chamber; these saddles are distinguished in the same manner as the lobes—that between the dorsal and superior lateral lobe, forming the dorsal saddle (S. d.,) that between the superior and inferior lateral lobes, forming the lateral saddle (S. L.,) and that between the inferior lateral and ventral lobe, the ventral saddle (S. V.) This general disposition, variously modified, pervades all forms of Ammonites; but when, as in PI. 39, the turn of the shell increases rapidly in width, so that the last whorl nearly, or entirely, covers the preceding whorls, the additional part is furnished with small auxiliary lobes, varying with the growth of the Ammonite to the number of three, four, or five pairs. (PL 39, a. 1, a. 2, a. 3, a. 4, a. 5.)