Chapter 27

[401]The best systematic account of the Cephalopoda and Gasteropoda, both recent and fossil, yet published, is contained in the first part of the very valuable Manual of Mollusca, by Mr. S. P. Woodward, of the British Museum.

FOSSIL DIBRANCHIATE CEPHALOPODA.

TheArgonaut, or Paper Nautilus, whose elegant fragile shell is too well known to require description, is the only living genus of this Order, in which the animal is protected by a hard calcareous external covering. This shell is symmetrical, and convoluted on a vertical plane, and consists of but one cavity or chamber. The other genera are naked, and possess an internal chambered shell (as in the recentSpirula), or some modification of such an apparatus. The last chamber or cell of these enclosed shells is too small to admit any part of the body of the animal; a character by which the fossil species of this Order may be distinguished from those of the other order. Others have a horny or calcareous osselet, as the bone of the Cuttle-fish, and pen of the Calamary or Sea-pen (seeBd.pl. 28); and in an appendage of this kind a conical chambered shell is contained in many of the fossil genera, hereafter to be noticed. These animals have eight arms, with the addition in some genera of two long tentacula, which are furnished with rows of suctorial disks or cups, calledacetabula(seeLign.142,figs.1, 6).

These naked Cephalopoda, devoid of any external defence, possess a very extraordinary means of escape from their enemies. They are furnished with a bag or bladder, containing a dark fluid resembling ink in appearance, which they have the power of ejecting into the surrounding water upon the approach of danger; and by the obscurity t us induced, they foil the pursuit of their adversaries: the Nautilus and other cephalopods, protected by a large external shell, are destitute of such an apparatus. The deep brown colour,sepia, was formerly prepared from the fluid of the ink-bags of different species of Cuttle-fish; a similar substance secreted by extinct naked Cephalopoda, as we shall presently demonstrate, is found in a fossil state. These preliminary remarks on the organization of the recent animalswill prepare us for the investigation of the extinct species. We will first notice those remarkable fossils, calledBelemnites, or thunder-stones.

Lign. 141. Belemnites:1/2nat.Chalk and Oolite.Fig.1.—Belemnitella mucronata.Chalk.Brighton.On the right of the figure is a view of the aperture, and a transverse section.2.—Portion of a Belemnite, containing the internal conical chambered shell, calledphragmocone.Oolite.3.—Belemnitella quadrata.Beauvais, France. The quadrangular cavity is shown in the upper figure on the left. (M. D’Orbigny.)4.—Belemnites dilatatus.Lower Greensand(Néocomien).France.

Lign. 141. Belemnites:1/2nat.Chalk and Oolite.

BELEMNITES

Belemnite(from a supposed resemblance to the head of a dart or javelin).Lign.141to144. Among the innumerable relics of an earlier world, which swarm in the sedimentary deposits, there are perhaps no fossil bodies that have excited more curiosity, and given rise to so many fruitless conjectures as to their nature and origin, as the Belemnites.[402]These are long, cylindrical, or fusiform fossils, more or less pointed at one extremity, and having at the other and larger end a conical cavity, which is either occupied by a chamberedshell, or filled up with the material in which the fossils are imbedded. Their substance is like fibrous calcareous spar, varying in colour from a dark brown to a light amber; many are transparent, others nearly opaque. When broken transversely they present a radiated structure (Lign.141,fig.1) and a minute central cavity, or axis, is seen to extend through the whole length of the solid portion of the stone (seeLign.142,fig.5.). A longitudinal section (Lign.142,figs.4 and 5) shows the conical cavity in the upper part, and that the shaft consists of a series of concentric layers. Such are the characters of these fossils in the examples of most frequent occurrence.

[402]See Park. Org. Rem. vol. iii. p. 122.

The Belemnites vary in size from the small, delicate, transparent species,Lign.142,figs.3 and 4, to massy opaque specimens, several inches in circumference, and from ten to twenty inches in length. They present also considerable variety of form; some are regularly cylindrical, as inLign.141fig.1; others broad and flattened, as infig.4; or subfusiform, as inLign.142, figs. 3 and 4. The small end is slender and pointed in some belemnites, and in others is obtuse, or rounded, with a projecting point. In many there is a longitudinal groove or furrow on the ventral aspect; and some species have a furrow on each side, as in that represented inLign.142,fig.2.

But the fossils above described are only a part of the original structure of the Belemnite. When in a perfect state, the cavity seen inLign.142,fig.5, is occupied by a chambered conical shell, called thephragmocone, composed of a series of shallow concave cells, of a nacreous or pearly substance, which are pierced by a siphuncle at the margin; seeLign.141,fig.2.

The parts of theBelemniteat present known consist of—

1st. The spathose osselet, orguard, having at the larger end a conical cavity, called thealveolus, as inLign.141,fig.1, andLign.142,fig.5.

2dly. A conical, chambered pearly shell, termed thephragmocone, which is situated in thealveolus(as inLign.141,fig.2).

Lign. 142. Belemnites:1/2nat.Fig.1.—A front view of the supposed animal of the Belemnite, byM. D’Orbigny.b.denotes the osselet, to the base of which theBelemnite,a, is attached.2.—Belemnites bipartitus; the figure below shows the form of the aperture.Neocomian Formation.France.(M. D’Orbigny.)3.—Belemnites Listeri(G. A. M.);from the Galt.Ringmer.4.—A longitudinal section ofB. Listeri.5.—Belemnites semicaniliculatus: a longitudinal section; the figure below is the aperture of thealveolus.From the Firestone (Craie tufeau) of France.6.—A side view of fig. 1.

Lign. 142. Belemnites:1/2nat.

3dly. The horny prolongation of thecapsule(the outer investment of theguard), called thereceptacle, as inLign.143.

4thly. Theink-bag, and its inspissated fluid,sepia; (Bd.pl. 44′, figs. 7, 9.)

Lign. 143. The structure of the sherry parts of the Belemnites Puzosianus.Oxford Clay.Christian Malford.(1/4nat. size.)a, a.The dorsal basilar processes of the phragmocone.b, b.Upward extension of the attenuated osselet.c.Siphunculus.d.Phragmocone: the transverse lines indicate the septa.e.The capsule or outer investment of the guard.f.The distal part of the phragmocone.g.The alveolus or cavity in the guard.h.Vertical section of the guard.i.The solid part of the rostrum or guard.k.The sulcus or groove on the ventral aspect of the rostrum.l.Shows the continuation of the capsule, in section, continued from e.m.Diverging parallel striæ observable between the dorsal processes of the phragmocone.n.Transverse section of half the diameter of the radiated structure.

Lign. 143. The structure of the sherry parts of the Belemnites Puzosianus.Oxford Clay.Christian Malford.(1/4nat. size.)

a, a.The dorsal basilar processes of the phragmocone.

b, b.Upward extension of the attenuated osselet.

c.Siphunculus.

d.Phragmocone: the transverse lines indicate the septa.

e.The capsule or outer investment of the guard.

f.The distal part of the phragmocone.

g.The alveolus or cavity in the guard.

h.Vertical section of the guard.

i.The solid part of the rostrum or guard.

k.The sulcus or groove on the ventral aspect of the rostrum.

l.Shows the continuation of the capsule, in section, continued from e.

m.Diverging parallel striæ observable between the dorsal processes of the phragmocone.

n.Transverse section of half the diameter of the radiated structure.

Lign. 144. Very perfect specimen of Belemnites Puzosianus.Oxford Clay.Christian Malford, Wilts.(1/6nat. size.)a, a.Basal processes of the phragmocone.b.The phragmocone.c.The rostrum or guard of the osselet, containing the apex of the phragmocone in its upper part.

Lign. 144. Very perfect specimen of Belemnites Puzosianus.Oxford Clay.Christian Malford, Wilts.(1/6nat. size.)

a, a.Basal processes of the phragmocone.

b.The phragmocone.

c.The rostrum or guard of the osselet, containing the apex of the phragmocone in its upper part.

The invariably radiated crystalline structure of the Belemnite has evidently resulted from the peculiar organization of the original osselet, which is formed of thin concentric laminæ, of very minute prismatic trihedral fibres, arranged at right angles to the planes of the successive layers.[403]

[403]The Belemnitic shell presents the same arrangement of its constituent layers as the Pearl-mussel, Pinna, and otherAviculidæ, viz. the outer layer is prismatic-cellular, the inner nacreous: the first is formed by the free margin of the mantle, the second by the visceral ("peritoneal") part of the mantle.—Mr. Woodward.

From the obvious analogy of the structure above demonstrated with that of the recent dibranchiate Cephalopoda, several eminent naturalists inferred that the animal of the Belemnite was closely related to the existing types; and the late Mr. Miller, in a communication to the Geological Society of London, gave a restored figure of the original, which, as modified by M. D’Orbigny, is representedLign.142,figs.1 and 6. The indefatigable and successful researches of the Rev. Dr. Buckland have confirmed the general correctness of this restoration. In the Lias of Dorsetshire two specimens of the Belemnite, with its chambered shell and horny or pearly receptacle, still retaining the ink-bag and its contents, have been discovered, and were figured in the Br. Treatise (Bd.pl. 44′, 44″). A third specimen, showing the ink-bag, is in the British Museum.

The ink-bag of the Belemnite is very small, as might be expected, from the extent to which it is protected by a chambered shell. The mandibles or beaks of the Belemnite are supposed to have been horny, as in the other naked Cephalopoda; since no calcareous beaks have been found associated with their remains.

"The Belemnite having the advantage of its dense, but well-balanced internal shell, must have exercised the power of swimming backwards and forwards, which it possessed incommon with the modern decapod (ten-armed) Dibranchiata, with great vigour and precision. Its position was probably more commonly vertical than in its recent congeners. It would rise swiftly and stealthily to infix its claws in the belly of a supernatant fish, and then perhaps as swiftly dart down, and drag its prey to the bottom and devour it. We cannot doubt at least but that, like the hooked Calamaries of the present seas, the ancient Belemnites were the most formidable and predacious of their class."—Owen.

The Belemnites of the oolitic limestones frequently contain the phragmocone, either filled with calcareous spar, or with its cells empty. In the clays the horny sheath or receptacle is sometimes found pressed flat and extending above the alveolus of the osselet, and has often a thin coat of nacre of a pearly lustre, but it is more commonly detached.

The Belemnites abound in the Lias, Oolite, and Chalk, and have not been discovered in any other deposits; there are nearly thirty British species, some of which are restricted to the Chalk, and others to the Oolite and Lias.

A few characteristic forms are represented,Lign.141 and 142, in order to illustrate the three groups which, according to M. D’Orbigny, are peculiar to the grand divisions of the Cretaceous formation.

1.Belemnitella mucronata.Lign.141,fig.1.—The nameBelemnitellais given to those Belemnites which have a slit, or crevice, on the anterior margin of the alveolus or cavity, and two lateral impressions. The surface is sometimes granulated, and often has vascular markings, produced by the investing integument of the living animal. The form of the aperture is shown in the middle dextral figure; and the radiated structure, as seen by a transverse section, in the sketch.

This species is abundant in theWhite Chalk, particularlyin certain localities in Norfolk and Devonshire. It is more frequent in the chalk of Kent than in that of Sussex; and in the cretaceous strata around Brighton, than in those near Lewes. I have never been able to detect the least vestige of the phragmocone, or chambered shell, in the alveolus. This Belemnite is occasionally imbedded in flint nodules; and such examples possess the calcareous crystalline structure of the chalk and limestone specimens. In the chalk of Ireland, the Belemnites which have been corroded, or perforated by marine borers (cliona), are often injected with flint; and if the calcareous substance be removed by immersion in dilute hydrochloric acid, exquisite siliceous casts may be obtained (see also page 403). It is not unusual to find flints with a cavity, occasioned by the solution and removal of the calcareous guard, and having a siliceous conical cast of the alveolus, occupying the upper part of the interspace. The reader will recollect that the pulley-stones of the Derbyshire Encrinites were produced by a similar process (seep. 285, vol. i.).

The American cretaceous sands abound in a species of Belemnitella, nearly related toB. mucronata.

2.Belemnites Listeri.Lign.142,fig.3.—This small elegant Belemnite has two lateral grooves, and is generally as transparent as amber; it has frequently a nacreous or calcareous pellicle partially investing the guard. It seldom exceeds two inches in length. It is abundant in, and peculiar to, theGalt, or blue marl of the Chalk, and is constantly associated with the Inocerami, previously described as common at Folkstone, Bletchingley, Ringmer, and other localities of that deposit. The Red Chalk of Norfolk contains the same species (Min. Conch.tab. 589).

3.Belemnites dilatatus.Lign.141,fig.4.—This species is distinguished by its flattened form, and by the longitudinal furrow being situated on the margin oppositeto the siphuncle of the phragmocone, instead of being on the same side, as is most usual. It is supposed by M. D’Orbigny to be characteristic of the Neocomian beds, or lowermost division of the Shanklin Sand.

4. The Chalk-marl contains a Belemnite of a more elongated form than those above described, the apex gradually tapering to a point, with a slight double furrow on each side. It is namedB. lanceolatus(Sow. Min. Conch.tab. 600, figs. 8, 9), and is very common in the marl-pits at Steyning, Clayton, and Hamsey, in Sussex.

At the base of Golden Cap Hill, near Charmouth, there are two strata of marl-stone observable on the shore, which are literally paved with Belemnites. Great numbers of these fossils have Serpulæ and other extraneous shells attached to them, a proof that the ink-bags and other soft parts of the mollusks had decomposed, and that the guards had lain uncovered at the bottom of the sea.

M. De Koninck has discovered in the Devonian limestone of Belgium, at Couvin and Visé, a small fossil body which closely resembles in form and structure the rostrum or guard of a belemnite; it is, however, too fragmentary to admit of positive detemination.[404]

[404]Bulletin del’Académie Roy ale de Bruxelles, tome x. No. 3. p. 207.

BELEMNOTEUTHIS

Belemnoteuthis[405](J. C. Pearce).Lign.145.—Within the last few years much additional knowledge has been obtained regarding the nature of the extinct Cephalopoda, by the discovery in the Oxford clay, at Christian Malford, not only of several examples with the receptacle and ink-bag in their natural relative positions, but also with the remains and impressions of the mantle, body, tentacula with their hooks, and the fins!

[405]For the history of this interesting Cephalopod consult Phil. Trans. 1848, and 1850; Ann. Nat. Hist. June 1850; Petrif. p. 459, &c.

Lign. 145. Belemnoteuthis antiquus.(Pearce.)Oxford Clay.Christian Malford.(1/2nat. size.)a.The uncinated arms and tentacles.b.Remains of the head and eyes.c.The mantle, with indications of fins.d.The pigmental sac or ink-bag.e.The osselet: the transverse lines indicate the septa of the phragmocone, which is covered by a horny sheath or capsule.f.The solid terminal apex of the osselet.

Lign. 145. Belemnoteuthis antiquus.(Pearce.)Oxford Clay.Christian Malford.(1/2nat. size.)

a.The uncinated arms and tentacles.

b.Remains of the head and eyes.

c.The mantle, with indications of fins.

d.The pigmental sac or ink-bag.

e.The osselet: the transverse lines indicate the septa of the phragmocone, which is covered by a horny sheath or capsule.

f.The solid terminal apex of the osselet.

BELEMNOTEUTHIS ANTIQUUS

Certain argillaceous strata of the Oolite, as well as of the Lias, appear to have been peculiarly favourable for the preservation of the muscular tissue and integuments, and in many specimens ofBelemnoteuthis, the arms, the large sessile eyes, the funnel, a great proportion of the muscular parts of the mantle, remains of the two lateral fins, the ink-bladder and duct, and the phragmocone, are well displayed, as in the beautiful example,Lign.145, for the drawing of which I am indebted to S. P. Woodward, Esq., of the British Museum. (See alsoLond. Geol. Journ.pl. xv. and xvi.)

Lign. 146. Horny rings and hooks of Belemnoteuthis antiquus.Fig.1, 3.—Detached hooks (natural size).2.—Three hooks with attached horny rings: from a specimen in the possession of Mr. Cunnington.4.—Part of one of the arms, showing four hooked spines.5.—Transverse section of the distal part of the osselet ofBelemnoteuthis, exposing the apex of the chambered shell in the centre, surrounded by the radiated osselet,a(magnified four diameters).

Lign. 146. Horny rings and hooks of Belemnoteuthis antiquus.

From the extraordinarily perfect condition of the Belemnoteuthis here figured, which of itself exemplifies the essential parts of its structure, a brief description will suffice. The body is of an elongated form, with a pair of lateral fins, two large sessile eyes, eight uncinated arms, and a pair of armed tentacles; each arm was furnished with from twenty to forty pairs of hooks, placed alternately. Like the Sepia it had a pigmental sac or ink-bag, which is generally found filled with the inspissated secretion. The inferior part of the body is of a conical form, and contains a brown hornyosselet, with a siphunculated phragmocone, that terminates in a guard or rostrum of a fibrous structure.

[In the recent genusOnychoteuthis, the tentacles alone are armed with claws;Enoploteuthishas claws both on the arms and on the tentacles, but the latter are long and feeble, and the hooks are confined to their extremities. The extinctBelemnoteuthis(like theAcanthoteuthisof Solenhofen,Lign.140) had eight nearly equal arms, the dorsal pair being rather smaller than the rest; each arm was furnished with twenty to forty pairs of hooks, forming a double, alternating row. The tentacles were not longer than the arms, and like them had a double series of hooks extending from their bases to the points. In all essential points of structure, the Belemnoteuthis is most nearly related to the Calamaries (Teuthidæ), but, in consequence of the prolongation of its pointed shell posteriorly, the fins become lateral (as inSepiolaandSepia), instead of terminal. Whilst the complicated (chambered) structure of its shell, and the peculiar character of thetentacles, show that it must be regarded as a type distinct from and equal in importance to the Calamaries. It cannot be doubted that theBelemniteandConoteuthispresent similar conditions of the soft parts; and the four genera will form the FamilyBelemnitidæ. The normal position of these animals in the sea is horizontal, whilst that of the Nautiloid genera must have been vertical, with the head downwards.—Mr. Woodward.]

The fossils which have afforded this unexpected and highly interesting illustration of the nature of the extinct animals of this Order have been obtained by closely examining the shales in which they abound, and, before removing the solid osselet, carefully searching the surrounding stone for traces of the more perishable parts. The attention of the collector can scarcely be too often directed to the necessity of examining the surrounding matrix before extricating a fossil from its bed.

Lign. 147. Osselets of extinct dibranchiate Cephalopoda.(Woodward,Manual, p. 76, pl. 1 & 2.)

BELOPTERA

Beloptera.(Bd.pl. xliv.fig.15.Min. Conch.tab. 591.)—Under this name Mr. Sowerby figures and describes a very curious fossil, from the London Clay at Highgate, which seems to hold an intermediate place between the Cuttle-fish and the Spirulirostra. The guard, which is of an oblong form, with an obtuse apex, has the structure of the osselet of the Sepia, and contains in its upper part a phragmocone, the cells of which are very narrow. In strata of the same age, in France, three species have been discovered by M. Deshayes. I allude to these shells, that the attention of the collector may be directed to the search after other examples in our tertiary deposits.

Fossil Calamary, or Squid. Geoteuthis.[406](Bd.pl. xxviii.xxix.)—The common Calamary (Loligo vulgaris) is so often seen on our shores, that its general aspect must be familiar to all who frequent the sea-side. In this animal, the osselet, or internal support, is a cartilaginous elongated body, which, from its form, is called Sea-pen (Bd.pl. xxviii.); and even this delicate structure is found in a fossil state. In the Lias of Lyme Regis, Miss Mary Aiming first discovered specimens ofSea-pensin juxtaposition with the ink-bag, as in the recent Calamary; and subsequently many similar examples have been found, both in England and on the Continent. Dr. Buckland has given some exquisite figures of these fossils; and his collection contains a matchless series of these most interesting organic remains. In some specimens the ink-bag and its tube or duct, but little compressed, are occasionally met with, having a brilliant nacreous pellicle, the remains of the sheath, attached to the surface. The ink-bag is sometimes of considerable magnitude; specimens have been found at Lyme Regis nearly a foot in length.[407]The circumstance of the ink-bags being generally full of sepia admits of the inference (as Dr. Buckland with his wonted acumen remarks), that these individuals died suddenly; for their living analogues reject the inky fluid upon the least approach of danger. The perfect condition of the bag proves also their instantaneous enclosure in the deposit, for the distended membrane would otherwise have burst from decomposition, and the contents would have escaped. The fossil marine reptiles, the Ichthyosauri, &c., with which these fossils are associated, present similar phenomena, as we shall hereafter have occasion to remark, and strengthen the probability, that swarms of the inhabitants of the Liassic ocean were suddenly destroyed, and imbedded, on the area now occupied by their remains.

[406]Geoteuthis has hooks on its arms; hut, being a Calamary (Teuthid), it would probably have unequal arms.—Mr. Woodward.[407]The large ink-bags figured by Dr. Buckland (Br. Tr. vol. i. pp. 372-379, pl. xliv′.) belonged to the great Geoteuthis Bollensis, of Schuble.—Mr. Woodward.

[406]Geoteuthis has hooks on its arms; hut, being a Calamary (Teuthid), it would probably have unequal arms.—Mr. Woodward.

[407]The large ink-bags figured by Dr. Buckland (Br. Tr. vol. i. pp. 372-379, pl. xliv′.) belonged to the great Geoteuthis Bollensis, of Schuble.—Mr. Woodward.

In the cream-coloured limestone, of Solenhofen, so rich in organic remains of the highest interest (Wond.p. 578), the soft parts of naked Cephalopoda have also been discovered. I have figured,Lign.140, a beautiful specimen obtained by the late Count Münster, which exhibits an imprint of the body, the arms and tentacles being represented by ten double rows of horny hooks, which precisely resemble those of Belemnoteuthis. M. D’Orbigny supposes that the original animal closely resembled a recent decapod calledEnoploteuthis leptura.

Lign. 148. Fossil Shells related to the Argonaut.[408]

[408]Some naturalists consider the Bellerophon to be allied to the Carinaria (Heteropod).

Bellerophon.—It has been already stated, that the animals of one genus of the existing dibranchiate Cephalopoda are protected by a thin, flexible, symmetrical, keeled shell, convoluted on a vertical plane, and having but one chamber—this is theArgonaut, orPaper Nautilus, an inhabitant of the Mediterranean. This animal belongs to the Octopoda, or those which have eight arms; and in one pair of these processes the extremities expand into broad and thin membranes, by which the delicate, elastic, calcareous envelopement, or shell,is secreted. There membranes usually encompass the shell, and meet and overlap each other along its keel; and by them chiefly the shell is retained in its position. When these membranes are withdrawn, or the animal dies, the shell, having no muscular connexion with the soft parts, readily separates from the body. Hence the doubts so long entertained as to the relation between the animal of the Argonaut and its shell, but which are now set at rest; the observations on the living animal by Madame Tower, and the anatomical demonstrations by M. Sander Rang, having removed the obscurity in which the subject was formerly involved.

In the Silurian, Devonian, and Carboniferous deposits there are several species of a genus of shells, the animals of which are by some considered to have been analogous to the recent Argonaut. It is named Bellerophon. I have figured two species; one from the Mountain Limestone,Lign.148,fig.1; the other from the Silurian System. There are about thirty British species, most of which are of small size; some of them are keeled, others have a slight dorsal depression, as infig.1. and many have the back rounded, and the sides lobed, as inLign.148,fig.2.

FOSSIL TETRABRANCHIATE CEPHALOPODA.

I am not aware of the existence of any British fossils analogous to Spirula (dibranchiate); for the minute fossil polythalamia, formerly referred to this class, are now known to have belonged to animals possessing an organization altogether different, as we have already explained (seep. 369). I therefore proceed to notice the fossil remains of thoseCephalopodawhich were furnished with an external shell having its cavity divided by cells, which are perforated by a hydraulic tube or siphuncle; and of which group the recentNautilusis the type.

Lign. 149.Nautilus pompilius in its shell.a.The animal, occupying the last or body chamber of the shell.b.The shell cut vertically through the middle, and showing the air-chambers and the siphuncle.

Lign. 149.Nautilus pompilius in its shell.

a.The animal, occupying the last or body chamber of the shell.

b.The shell cut vertically through the middle, and showing the air-chambers and the siphuncle.

The appearance and structure of the recent shell are familiar to every one; a correct knowledge of the nature of the original animal has, however, been obtained but very recently. In its general characters the animal of the Nautilus, which is an inhabitant of the seas of hot climates, resembles the naked Cephalopoda; it possesses fourbranchiæ, or gills, and numerous hollow arms and retractile tentacula. Its head is furnished with a muscular flattened disk, which serves as an operculum to the shell when the animal is retracted. The beaks are horny, and coated at their tips by calcareous matter. It has no ink-bag, and is destitute of fins or other organs for swimming. The body occupies the ample outer cell of the shell, to which it is firmly attached by two lateral muscles; and it has a siphuncle, that passes from the posterior part of the animal through the shelly tube, and by which communication is maintained with the entire series of cells or chambers. The siphuncle is provided with a small artery and vein, and traverses the entire series of chambers, thus maintaining the vitality of the shell. Mr. Edwards considers that "it may be looked upon as an elongated cæcum, and that it is not under any circumstances used by the animal as a hydrostatic balance."[409]

[409]The reader interested in this subject should consult theMemoirsby M. Valenciennes, Mem. de l’Inst.; M. Vander Hoven, in the Proceedings of the Zoological Society; Prof. Owen’sMemoir on the Pearly Nautilus, Mr. Gray’s paper in the An. Nat. Hist., Mr. Edward’s Monograph in the Palæont. Soc., and Mr. Woodward’sManual.

Upon making a vertical section of the shell, the inner volutions are exposed, and the cavity is seen divided at regular intervals into cells, by smooth, concave, nacreous septa; these vary in number according to the age of the individual; there are about thirty-five in an adult specimen. The partitions are pierced in thecentreby a shelly tube, which traverses each cell to within a short distance of the next partition; and this tube is rendered a continuous channel in the living animal, by the membranous siphuncle. This series of air-chambers constitutes an apparatus which renders the Nautilus nearly of the same specific gravity as the surrounding water, and enables it to rise to the surface of the sea, or sink to the bottom, by a very small amount of muscular exertion. The Nautilus swims, like the Cuttle-fish, by expelling the water from its respiratory chamber; the walls of which are very thick and powerful muscles.

From this very general description of the only living representative of the numerous genera of tetrabranchiate Cephalopoda, which swarmed in such prodigious numbers in the ancient seas, we may pass to the consideration of the fossil Nautili, and their related congeners. Our remarks must be limited to the genera that will serve to demonstrate the most important modifications of structure, and explain the nature of the fossil remains of this extensive class of extinct beings.

The genera into which these shells are distributed are founded upon the mode in which the shell is coiled, its form, the character of the partitions or septa, and the situation of the siphuncle. A little reflection will enable the student to understand the principles of this classification. The essential character of all the shells of this class, is to have an external chamber larger than the inner chambers, and which contains the body of the animal; to be divided internallyinto different compartments, by partitions (concavo-convex, with the concavity outwards); and to have a pipe or tube extending from the outer open chamber to the innermost cell. They are divided into three groups or families.

1.The Nautilidæ(Bd.pl. xxxi.): in these the septa are smooth, or but slightly undulated, and the siphuncle either traverses the centre of the cell-partitions, or is situated towards the inner margin or turn of the spire.

2.The Orthoceratidæ(Lign.155): in these the siphuncle is complicated in its structure; it is central or lateral; the septa are smooth. (Woodward’s Man. Moll.p. 87.)

3.The Ammonitidæ(Bd.pl. xxxv-xlii.): in these the septa are more or less waved, and their margins foliated or crenated, that is, indented; and the siphuncle is situated at or near the outer margin.

In the Nautilus, the shell is convoluted on the same plane, in spiral whorls, all of which are contiguous, and the siphuncle is central.

The British strata contain about sixty species of Nautili. The Tertiary formations have yielded five or six; the Cretaceous a like number; the Lias and Oolite ten or eleven; the Carboniferous about thirty species; and the Devonian two species. In the London Clay a large and beautiful species is abundant (Nautilus imperialis.Min. Conch.tab. i.), having the shell very commonly entire; but the outer opaque coat frequently flakes off, and exposes the pearly or nacreous internal layer. The septa generally retain their original nacreous structure, and the cells are either occupied by clay or marl, or are partially filled or lined with calcareous spar, brilliant pyrites, or other mineral matter. These Nautili are often found constituting the nuclei of theseptaria, or clay nodules, with which this deposit abounds.[410]The small species,N. centralis(Ly.fig.179), andNautilus(Aturia)ziczac(Wond.p. 247), occur in the same strata. The London Clay of the Isle of Sheppey and of the coasts of Hants and Sussex is productive of these fossils.

[410]Three other well-marked species are figured and described by Mr. Edwards (Monog. Pal. Soc.) from the English eocene strata: viz.N. Sowerbyi,N. urbanus, andN. regalis.

Lign. 150. Fossil Nautili.Chalk marl.Fig.1.—Beak of a Nautilus, (Rhyncolite.) Back view.1a.—Anterior view of the same.1b.—Profile of the same.2.—Vertical section ofNautilus pseudo-elegans.Hamsey.a.The siphuncle.3.—Front view ofN. Deslongchampsii. (M. D’Orbigny.)Hamsey.3a.—Lateral view of the same.

Lign. 150. Fossil Nautili.Chalk marl.

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