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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 PI. 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: —Orthoceratite, Lituite, Baculite, Hamite, Scaphite,. Turrilite, Nummulite, Belemnite.*

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 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.

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.!

An Orthoceratite (see PI. 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 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.

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

t See D'Orbigny's Tableau Methodique des Cephalopodos.

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

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 Chambered shells, called Lituites. (PI. 44, Fig. 3.) These are partially coiled up into a spiral form at their smaller extremity, whilst their larger end is continued into a straight tube, of considerable length, separated by transverse plates, concave outwards, and perforated by a siphuncle (a.) As these Lituites closely resemble the shell of the recent Spirula (PL 44, Fig. 2,) their office may have been the same, in the economy of some extinct Cephalopod.

* 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 arc 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. PI.

Baculite.

As in rocks of the Transition series, the form of a straight Nautilus is presented by the genus Orthoceratite, so we find in the Cretaceous formation alone, the remains of a genus which may be considered as a straight Ammonite. (See PI. 44, Fig. 5.)

The baculite (so called from its resemblance to a straight staff) is a conical elongated, and symmetrical shell, depressed laterally, and divided into numerous chambers by transverse plates, like those in the Ammonite, are sinuous, and terminated by foliated dentations at their junction with the external shell; being thus separated into dorsal, ventral, and lateral lobes and saddles, analogous to those of Ammonites.*

It is curious, that this straight modification of the form of Ammonites should not have appeared, until this Family had arrived at the last stage of the Secondary deposites, throughout which it had occupied so large an extent; and that, after a comparatively short duration, the Baculite

* The external chamber (a) is larger than the rest, and swelling; and capable of containing a considerable portion of the animal. The outer shell was thin, and strengthened, like the Ammonite, by oblique ribs. Near the posterior margin of the shell, the transverse plates arc pierced by a Siphuncle (PI. 44, 5b, c.) This position of the Siphuncle, and the sinuous form and denticulated edges of the transverse plates, are characters which the Baculite possesses in common with the Ammonite.

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should have become extinct, simultaneously with the last - of the Ammonites, at the termination of the Chalk formation.

Hamite.

If we imagine a Baculite to be bent round near its centre, until the smaller extremity became nearly parallel to its larger end, it would present the most simple form of that cognate genus of chambered shells, which, from their frequently assuming this hooked form, have been called Hamites. At PI. 44, Fig. 9, 11, represent portions of Hamites which have this most simple curvature; other species of this genus have a more tortuous form, and are either closely coiled up, like the small extremity of a Spirula, (PI. 44, Fig. 2,) or disposed in a more open spiral. (PI. 44, Fig. 8.*)

It is probable that some of these Hamites were partly internal, and partly external shells; where the spines are present, the portion so armed was probably external. Nine species of Hamites occur in the single formation of Gault or Speeton clay immediately below the chalk, near Scar

• Both these forms of Hamite bear the same relation to Ammonites that Lituites bear to Nautili; each being nearly such as shells of these genera would respectively present, if partially unrolled. See Phillips' Geol. Yorkshire, PI. 1, Figs. 22, 29, 30.

Baculites and Hamites have two characters which connect them with Ammonites; first, the position of the Siphnncle, on the back, or outer margin of the shell, (PI. 14, Figs. 5b, c. 8% a. 10,11, a. 12, a. 13, a.;) secondly, the foliated character of the margin of the transverse plates, at the junction with the external shell. (PI. 44, Fig. 5, 8, 12, 13.) The external shell of Hamites is also fortified by transverse folds or ribs, increasing the strength both of the outer chambers and of the air-chambers, upon the same principles that we have pointed out in the case of Ammonites. (See PI. 44, Fig. 8, 9,11, 12, 13.)

In certain species of Hamites, as in certain Ammonites, the marginal Siphunclc has a keel-shaped pipe raised over it. Others have a series of spines on each side of the back. (PI, 44, Fig, 9, 10.) VOL. I.—24

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