At Pl. 35, from d. to e. we see the edges of the same transverse plates which, in Pl. 36, are simple curves, becoming foliated at their junction with the outer shell, and thus distributing their support more equally beneath all its parts, than if these simple curves had been continued to the extremity of the transverse plates. In more than two hundred known species of Ammonites, the transverse plates present some beautifully varied modifications of this foliated expansion at their edges; the effect of which, in every case, is to increase the strength of the outer shell, by multiplying the subjacent points of resistance to external pressure. We know that the pressure of the sea, at no great depth, will force a cork into a bottle filled with air, or crush a hollow cylinder or sphere of thin copper; and as the air-chambers of Ammonites were subject to similar pressure, whilst at the bottom of the sea, they required some peculiar provision to preserve them from destruction,* more especially as most zoologists agree that they existed at great depths, "dans les grandes profondeurs des mers."+

Here again we find the inventions of art anticipated in

* Captain Smyth found, on two trials, that the cylindrical copper air tube, under the vane attached to Massey's patent sounding machine, collapsed, and was crushed quite flat under a pressure of about three hundred fathoms. A claret bottle filled with air, and well corked, was burst before it had descended four hundred fathoms. He also found that a bottle filled with fresh-water, and corked, had the cork forced at about a hundred and eighty fathoms below the surface; in such cases, the fluid sent down is replaced by salt water, and the cork which had been forced in, is sometimes inverted.

Captain Beaufort also informs me, that he has frequently sunk corked bottles in the sea more than a hundred fathoms deep, some of them empty, and others containing a fluid. The empty bottles were sometimes crushed, at other times, the cork was forced in, and the bottle returned full of sea water. The cork of the bottles containing a fluid was uniformly forced in, and the fluid exchanged for sea water; the cork was always returned to the neck of the bottle, sometimes, but not always, in an inverted position.

† See Lamarck, who cites Bruguières with approbation on this point.— Animaux sans: Vert: vol. vii. p. 635.

the works of nature, and the same principle applied to resist the inward pressure of the sea upon the shells of Ammonites, that an engineer makes use of in fixing transverse stays beneath the planks of the wooden centre on which he builds his arch of stone.

The disposition of these supports assumes throughout the family of Ammonites a different arrangement from the more simple curvature of the edges of the transverse plates within the shells of Nautili; and we find a probable cause for this variation, in the comparative thinness of the outer shells of many Ammonites; since this external weakness creates a need of more internal support under the pressure of deep water, than was requisite in the stronger and thicker shells of Nautili.

This support is effected by causing the edges of the transverse plates to deviate from a simple curve, into a variety of attenuated ramifications and undulating sutures.. (See Pl. 38. and Pl. 37, Figs. 6, 8.) Nothing can be more beautiful than the sinuous windings of these sutures in many species, at their union with the exterior shell; adorning it with a succession of most graceful forms, resembling festoons of foliage, and elegant embroidery. When these thin septa are converted into iron pyrites, their edges appear like golden filigrane work, meandering amid the pellucid spar, that fills the chambers of the shell.*

*The A. Heterophyllus, Pl. (38,) is so called from the apparent occurrence of two different forms of foliage; its laws of dentation are the same as in other Ammonites, but the ascending secondary saddles (Pl. 38. S. S.) which, in all Ammonites are round, are in this species longer than ordinary, and catch attention more than the descending points of the lobes, (Pl. 38. d. 1.)

The figures of the edge of one transverse plate are repeated in each suc. cessive plate. The animal, as it enlarged its shell, thus leaving behin 1 it a new chamber, more capacious than the last, so that the edges of the plates never interfere or become entangled.

Although the pattern on the surface of this Ammonite is apparently so

The shell of the Ammonites Heterophyllus (Pl. 38, and Pl. 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.

38, c.)

(See Pl.

The figures of A. obtusus, (Pl. 35 and Pl. 36,) show the relations between the external shell and the internal transverse partitions of an Ammonite. Pl. 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 Pl. 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. (Pl. 35, e.) A small portion of the shell is also preserved at Pl. 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, Pl. 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, Pl. 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 in 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, Pl. 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 Massey'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 Baculites, (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 Pl. 43, Fig. 3, 4,) but it is never jagged, or denticulated like the margin of the casts of the chambers of Ammonites.

In Ammonites, the chambers have a double curvature, and are, at their centre, convex outwards (see Pl. 36. d. and Pl. 39. d. V.) Pl. 42, Fig. 2, represents the front view of the cast of single chamber of A. exca

Siphuncle.

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.*

vatus; 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. Pl. 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 casts, which, after the shell has perished, remain locked into one another, like the sutures of a skull.

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 co-extensive 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 Pl. 36. d. e. f. g. h. i., and Pl. 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 Pl. 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 Pl. 31, Fig. 1. y. and Pl. 42. 1.)

The Siphuncle represented at Pl. 36, is preserved in a black carbonaceous state, and passes from the bottom of the external chamber (d.) to the inner extremity of the shell. At e. f. g. h. its interior is exposed by section, and appears filled, like the adjacent air-chambers, with a cast of pure calcareous spar. At Pl. 42. Fig. 3. b. a similar cast fills the tube of the Siphuncle, and also the air-chambers. Here again, as in Pl. 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 Pl. 42. Fig. 4. from a specimen found by the Mar. quis of Northampton in the Greensand 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