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The uses ascribed by Von Buch to the lobes of Ammonites in affording attachment to the base of the mantle around the margin of the transverse plates, would in no way interfere with the service we have assigned to the same lobes, in supporting the external shell against the pressure of deep water. The union of two beneficial results from one and the same mechanical expedient, confirms our opinion of the excellence of the workmanship, and increase our admiration of the Wisdom in which it was contrived.
On examining the proofs of Contrivance and Design that pervade the testaceous remains of the family of Ammonites, we find, in every species, abundant evidence of minute and peculiar mechanisms, adapting the shell to the double purpose of acting as a float, and of forming a protection to the body of the inhabitant.
All the lobes, as they dip inward, are subdivided by numerous dentations, which afford points of attachment to the mantle of the animal; thus each lobe is flanked by a series of accessory lobes, and these again are provided with farther symmetrical dentations, the extremities of which produce all the beautiful appearances of complicated foliage, which prevail through the family of Ammonites, and of which we have a striking example on the surface of PI. 38.
The extremities of the dentations are always sharp and pointed, inwards, towards the air-chamber, (PI. 38, d. 1.;) but are smooth and rounded upwards towards the body of the animal, (PI. 38, S. S.,) and thus the jagged terminations of these lobes may have afforded holdfasts whereby the base of the mantle could fix itself firmly, and as it were take root, around the bottom of the external chamber.
No such dentations exist in any species of Nautilus. In the N. Pompilius, Mr. Owen has shown that the base of the mantle adheres to the outer shell, near its junction with the transverse plate by means of a strong horny girdle; a similar contrivance probably existed also in all the fossil species of Nautili. The sides of the mantle also of the N. Pompilius are fixed to the sides of the great external chamber by two strong broad lateral muscles, the impressions of which are visible in most specimens of this shell.
As the animal increased in bulk, and advanced along the outer chamber of the shell, the spaces left behind it were successively converted into air-chambers, simultaneously increasing the power of the float. This float, being regulated by a pipe, passing through the whole series of the chambers, formed an hydraulic instrument of extraordinary delicacy, by which the animal could, at pleasure, control its ascent to the surface, or descent to the bottom of the sea.
To creatures that sometimes floated, a thick and heavy shell would have been inapplicable; and as a thin shell, enclosing air, would be exposed to various, and often intense degrees of pressure at the bottom, we find a series of provisions to afford resistance to such pressure, in the mechanical construction both of the external shell, and of the internal transverse plates which formed the air-chambers. First, the shell is made up of a tube, coiled round itself and externally convex. Secondly, it is fortified by a series of ribs and vaultings disposed in the form of arches and domes on the convex surface of this tube, and still farther adding to its strength. Thirdly, the transverse plates that form the air-chambers, supply also a continuous succession of supports, extending their ramifications, with many mechanical advantages, beneath those portions of the shell which, being weakest, were most in need of them.
If the existence of contrivance proves the exercise of mind; and if higher degrees of perfection in mechanism are proof of more exalted degrees of intellect in the Author from whom they proceeded; the beautiful examples which we find in the petrified remains of these chambered shells, afford evidence coeval and coextensive with the mountains wherein they are entombed, attesting the Wisdom in which such exquisite contrivances originated, and setting forth the Providence and Care of the Creator, in regulating the structure of. every creature of his hand.
NAUTILUS SYPHO, AND NAUTILUS ZIC ZAC.
The name of Nautilus Sypho* has been applied to a very curious and beautiful chambered shell found in the Tertiary strata at Dax, near Bourdeaux; and that of Nautilus Zic Zac to a cognate shell from the London clay. (See PI. 43,. Figs. 1, 2, 3, 4.)
These fossil shells present certain deviations from the ordinary characters of the genus Nautilus, whereby tfiey in some degree partake of the structure of an Ammonite.
These deviations involve a series of compensations and. peculiar contrivances, in order to render the shell efficient in its double office of acting as a float, and also as a defence and chamber of residence to the animal by which it was constructed.
Some details of these contrivances, relating to the Nautilus Sypho will be found in the subjoined note.f
* This Bhell has been variously described by the names of Ammonites Atun, Nautilus Sypho, and N. Zonarius. (See M. de Basterot. Mem. Gcol. de Bourdeaux.)
t The transverse plates, (PI. 43, Fig^l, a. a1, a2,) present a peculiarity of structure in the prolongation of the collar, or siphuncular aperture entirely across the area of the air-chambers, so that the whole series of transverse plates are connected in one eontinuous spiral chain. This union is effected by the enlargement and elongation of the collar for the passage of the siphuncle into the form of a long and broad funnel, the point of which b. fits closely into the neck of the funnel next beneath it, c. whilst its inner margin, resting upon the arch of the subjacent whorl of the shell, transfers to this arch a portion of the external pressure upon the transverse plates, thereby adding to their strength.
As this structure renders it impossible for the flexible siphuncle to expand itself into the area of the air-chambers, as in other Nautili and in Ammonites, the diameter of each funnel is made large enough to allow space within it for the distension of the siphuncle, by a sufficient quantity of fluid to cause the animal to sink.
As the place of the siphon in this species is upon the internal margin of the transverse plates (PI. 43, Fig. 2, b',b2,b3,) it had less power than the more central siphuncle of the Nautilus to attach the mantle of the animal to the bottom of the outer chamber. For this defect we find a compensation^ resembling that which Von Buch considers to have been afforded by the lobes of Ammonites to the inhabitants of those shells. This compensation will be illustrated by a comparison of the lobes in N. Sypho (PI. 43, Fig. 2.,) with a similar provision in the Nautilus Zic Zac (PI. 43, Figs. 3, 4.*)
At each articulation of the funnels, the diameter of the siphuncl* is contracted, as the siphuncles of Ammonites and Nautili are contracted at their passage through the collars of their transverse plates.
Another point in the organization of the siphuncle is illustrated by this shell, namely, the existence of a soft calcareous sheath, (PI. 43, Fig. 1, b, c. d.,) analogous to that of the N-. Pompilius, (PI. 31, Fig. 1, a. b. c. d„) between each shelly funnel and the membranous pipe or siphuncle enclosed within it. At PI. 43, Fig. 1, b, we have a section of this sheath folding round the smaller extremity of the funnel a'. From c. to dfit lines the inside of the subjacent funnel a*; and from d. continues downwards to the termination of the funnel a2, on the inside of e. At e, and f, we sec the upper termination of two perfect sheaths, similar to that of which a section is represented at b. c. d. This sheath, from its insertion between the point of the upper siphon and mouth of the lower one, (Fig. 1, c.,) must have acted as a collar, intercepting all communication between the interior of the shelly siphuncnlar tube and the air-chambers. The area of this shelly tube probably may be sufficient, not only to have contained the distended siphuncle, but also to allow it to be surrounded with a volume of air, the elasticity of which would act in forcing back the pericardial fluid from the siphuncle, in the same manner as we have supposed the air to act within the chambers of the N. Pompilius.
* On each side of the transverse plate in both these species there is an undulation, or sinus, producing lobes (PI. 43, Fig. 2. ai. a2, a9, Fig. 3. a. and Fig. 4. a. b.) There is also a deep backward curvature of the two ventral lobes, Fig. 4. c. c. All these lobes may have acted conjointly with the siphuncle, to give Arm attachment to the mantle of the animal at the bottom of the outer chamber. The shell Fig. 1. is broken in such a manner, that no portion of any lateral lobe is visible on the side here represented. At Fig. 2. a1, we see the projection of the lateral lobes, on each side of the convex internal surface of a transverse plate; at a2 we see the interior of the same lobes, on the concave side of another transverse plate; and at a3 the points of a third pair of lobes attached to the sides of the largest air-chamber that remains in this fragment..
A still more important use of the lobes formed by the transverse plates both of the N. Sypho and N. Zic Zac, may be found in the strength which they impart to the sides of the external shell (see PI. 43, Figs. 1, 2, 3, 4.,) underpropping their flattest and weakest part, so as to resist pressure more effectually than if the transverse plates had been curved simply, as in N. Pompilius. One cause which rendered some such compensation necessary, may be found in the breadth of the intervals between each transverse plate; the weakness resulting from this distance, being compensated by the introduction of a single lobe, acting on the same principle as the more numerous and complex lobes in the genus Ammonite.
The N. Sypho and N. Zic Zac seem, therefore, to form Links between the two great genera of Nautilus and Ammonite, in which an intermediate system of mechanical contrivances is borrowed, as it were, from the mechanics of the Ammonite, and applied to the Nautilus. The adoption of lobes, analogous to the lobes of the Ammonite, compensating the disadvantages, that would otherwise have followed from the marginal position of the siphuncle in these two species, and the distances of their transverse plates.*
* In some of the most early forms of Ammonites which we find in the Transition strata, e. g. A. Henslowi, A. Striatus, and A. Sphericus, (PI. 40, Figs. 1, 2, and 3,) the lobes were few, and nearly of the same form as the single lobe of the Nautilus Sypho, and of N. Zic zac; like them also the margin was simple and destitute of fringed edges. The A. nodosus (PI. 40, Figs. 4 and 5.,) which is peculiar to the early Secondary depositcs of the Muschelkalk, offers an example of an intermediate state,