As the place of the siphon in this species is upon the internal margin of the transverse plates (Pl. 43, Fig. 2, b1, 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 (Pl. 43, Fig. 2.,) with a similar provision in the Nautilus Zic Zac (Pl. 43, Figs.. 3, 4.*).

space within it for the distension of the siphuncle, by a sufficient quantity of fluid to cause the animal to sink.

At each articulation of the funnels, the diameter of the siphuncle 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, (Pl. 43, Fig. 1, b, c.. d.,) analogous to that of the N. Pompilius, (Pl. 31, Fig. 1, a. b. c. d.,) between each shelly funnel and the membranous pipe or siphuncle enclosed within it.. At Pl. 43, Fig. 1, b, we have a section of this sheath folding round the smaller extremity of the funnel a'. From c. to d, it lines the inside of the subjacent funnel a2; and from d. continues downwards to the termination of the funnel a2, on the inside of e. At e, and f, we see 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 siphuncular 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 (Pl. 43, Fig. 2. a1. a2, a3, 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 firm attachment to the mantle of the animal at the bottom of the outer chamber.. The shell Fig. 1. is broken

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 Pl. 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

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. 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 such a manner, that no portion of any lateral lobe is visible on the side here represented. At Fig. 2. al, 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 frag

ment..

* In some of the most early forms of Ammonites which we find in the Transition strata, c. g. A. Henslowi, A. Striatus, and A. Sphericus, (Pl.. 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 (Pl. 40, Figs. 4 and 5.,) which is peculiar to the early Secondary deposites of the Muschelkalk, offers an example of an intermediate state,

It is a curious fact, that contrivances, similar to those which existed in some of the most early forms of Ammonite, should have been again adopted in some of the most recent species of fossil Nautili, in order to afford similar compensation for weakness that would otherwise have been produced by aberrations from the normal structure of the genus Nautilus. All this seems inexplicable on any theory which would exclude the interference of controlling Intelligence.

SECTION VI.

CHAMBERED SHELLS ALLIED TO NAUTILUS AND AMMONITE.

WE have reason to infer, from the fact of the recent N.. Pompilius being an external shell, that all fossil shells of the great and ancient family of Nautili, and of the still more numerous family of Ammonites, were also external shells, enclosing in their outer chamber the body of a CephalopodWe farther learn, from Peron's discovery of the shell of a Spirula partially enclosed within the body of a Sepia,* (see Pl. 44, Fig. 1, 2,) that many of those genera of fossil chambered shells, which, like the Spirula, do not terminate externally in a wide chamber, were probably internal, or partially enclosed shells, serving the office of a float, constructed on the same principles as the float of the Spirula. In the class of fossil shells thus illustrated by the discovery of the animal enclosing the Spirula, we may include the following extinct families, occurring in various positions from the earliest Transition strata to the most recent Secondary formations:

in which the fringed edge is partially introduced, on the descending or inward portions only, of the lobated edge of the transverse plates.

*The uncertainty which has arisen respecting the animal which constructs the Spirula, from the circumstance of the specimen discovered by

-Orthoceratite, Lituite, Baculite, Hamite, Scaphite, Turrilite, Nummulite, Belemnite.*

Orthoceratite, Pl. 44, Fig. 4.

The Orthoceratites (so called from their usual form,that of a straight horn) began their existence at the same early period with the Nautili, in the seas which deposited the Transition strata; and are so nearly allied to them in structure, that we may conclude they performed a similar function as floats of Cephalopodous Mollusks. This genus contains many species which abound in the strata of the Transition series, and is one of those which, having been called into existence amongst the earliest inhabitants of our planet, was at an early period also consigned to almost total destruction.t

An Orthoceratite (see Pl. 44, Fig. 4) is, like the Nautilus,. a multilocular shell, having its chambers separated by transverse plates, concave externally, and internally convex; and pierced, either at the centre or towards the margin, by a Siphuncle, (a.) This pipe varies in size, more than that of any other multilocular shell, viz. from one-tenth to one-half of the diameter of the shell; and often assumes a tumid form, which would admit of the distension of a membranous

Peron having been lost, is in some degree removed by Captain King's discovery of another of these shells, attached to a fragment of the mantle of an animal of unknown species resembling a Sepia, which I have seen in the possession of Mr. Owen, at the Royal College of Surgeons, London.

* In the genus Lituite, Orthoceratite, and Belemnite, Pl. 44, f. 3, 4, 17, the simple curvature of the transverse plates resembles the character of the Nautilus. In the Baculite, Hamite, Scaphite, and Turrilite, Pl. 44, Fig. 5, 8, 12, 13, 14, 15, the sinuous foldings and foliated edges of the transverse plates resemble those of the Ammonites.

+ See D'Orbigny's Tableau Méthodique des Céphalopodos.

There are, I believe, only two exceptions yet known to the general fact, that the genus Orthoceratite became extinct before the deposition of the Secondary strata had commenced. The most recent rocks in which they have been noticed, are a small and problematical species in

siphon. The base of the shell beyond the last plate presents a swelling cavity, wherein the body of the animal seems to have been partly contained.

The Orthoceratites are straight and conical, and bear the same relation to the Nautili which Baculites (see Pl. 44, Fig. 5) bear to Ammonites; the Orthoceratites, with their simple transverse septa, resembling straight Nautili; and the Baculites, with a sinuous septa, having the appearance of straight Ammonites. They vary considerably in external figure, and also in size; some of the largest species exceeding a yard in length, and half a foot in diameter. A single specimen has been known to contain nearly seventy air-chambers. The body of the animal which required so large a float to balance it, must have greatly exceeded, in all its proportions, the most gigantic of our recent Cephalopods; and the vast number of Orthoceratites that are occasionally crowded together in a single block of stone, shows how abundantly they must have swarmed in the waters of the early seas. These shells are found in the greatest numbers in blocks of marble, of a dark red colour, from the transition Limestone of Oeland, which some years ago was imported largely to various parts of Europe for architectural purposes.*

Lituite.

Together with the Orthoceratite, in the Transition Limestone of Oeland, there occurs a cognate genus of Cham

the Lias at Lyme, and another species in Alpine Limestone of the Oolite for mation, at Halstadt, in the Tyrol.

* Part of the pavement in Hampton Court Palace, that of the hall of University College, Oxford, and several tombs of the kings of Poland in the cathedral at Cracow, are formed of this marble, in which many shells of Orthoceratites are discoverable. The largest known species are found in the Carboniferous limestone of Closeburn, in Dumfrieshire, being nearly of the size of a man's thigh. The presence of such gigantic Mollusks seems to indicate a highly exalted temperature, in the then existing climate of these northern regions of Europe. See Sowerby's Min. Con. Pl.