In the Chalk, as well as in many other calcareous deposits, the shells of the Nautili, Ammonites, &c. are very rarely preserved; even the internal septa are often dissolved, and the stony casts, moulded in the cells, remain distinct, and readily separate (Lign. 153). An entire series, from the innermost cell to the outer chamber, may sometimes be obtained (in the Coralline Oolite); forming, as it were, a dissected model of the internal structure.[412] The beaks or mandibles are occasionally found fossil (Lign. 150, fig. 1).

Clymenia (Lign. 154, Ly. fig. 406).—This genus belongs to the NautilidÆ, and is peculiar to the Devonian deposits. It differs from the allied genera in the siphuncle being situated on the inner margin of the septa. The shell is discoidal, and the septa are very slightly lobed. At Elbersreuth, near Bareuth, in the N. E. of Bavaria, the Devonian strata abound in these shells; thirty-five species have been found, the greater number being peculiar to that locality.

In England they are chiefly found at South Petherwin, Cornwall, and in the Devonshire marbles. (See Phillip’s Pal. Foss. Devonshire.)

Orthoceras (straight shell), Lign. 155.—The shells of this genus may be described as Nautili uncoiled and extended in a straight line. They are straight, elongated, chambered shells, with smooth and gently undulated septa, which are concave towards the opening or upper part, and have the siphuncle either central, or not far removed from the centre. The Orthoceratites more especially belong to the ancient Secondary strata. They first appear in the Silurian, and abound in the Devonian and Carboniferous. They vary in size from a few inches to several feet in length, and eight or nine inches in diameter; and in form, from a slender elongated cone, to a short, massy, almost spherical figure, with a contracted orifice. Some examples have been noticed with upwards of sixty cells. Mr. Sowerby figures and describes O. giganteum (Min. Conch. tab. 246), from Scotland, as exceeding seven or eight feet in length; and I discovered on the beach at Brighton, where it had probably been brought by some vessel, among ballast, a fine fragment of the same species, indicating as great a magnitude. Several species are figured, Lign. 155, to show the structure and appearance of these fossils. The casts of the separate cells are often found. The section, fig. 3, from the red marble of Devonshire, beautifully displays the situation of the siphuncle, and the lines of the septa. The shelly siphuncle, which is moniliform (bead-like), or dilated at each chamber, is replaced by white spar; and the membranous internal tube is filled with a dark substance, probably molluskite.

There are some species in which the internal tube, as well as the external, is calcareous, and the two are connected at regular intervals, by radiating, hollow processes. These Orthoceratites have been principally obtained from the Silurian limestones, at Lake Huron; they also occur in Ireland. Mr. Stokes, who first investigated their structure, has arranged them in a distinct genus, with the name of Actinoceras (radiated-horn).[413]

Slabs of reddish Devonian limestone, containing Orthoceratites, may be seen in some of the pavements at Hampton Court, and in Chelsea College, which when wet present excellent sections of the enclosed shells.

Fig.	1.—	Ammonites varians. Chalk-marl. Hamsey.
	2.—	Ammonites Dufrenoyi.
	2a.—	Shows the keel and septum of the same.
	3.—	Ammonites lautus. Galt. Folkstone.
	3a.—	Keel and septum of the same.

AmmonitidÆ.—The Ammonites, or Cornua Ammonis (so called from a supposed resemblance to the horns engraven on the heads of Jupiter Ammon), are among the most common and well-known fossils of the British secondary strata. In some districts, as in Yorkshire and Somersetshire, where the Ammonites abundantly prevail, they were noticed in very remote times. Local legends, ascribing their origin to swarms of snakes turned into stone by the prayers of some patron saint, are still extant, and are perpetuated by the name of snake-stones, by which these fossils are provincially known. The Lias, near Whitby, in Yorkshire, contains immense numbers of two or three species, one of which (Am. bifrons) is figured in Lign. 127, fig. 7, and another in Lign. 157.

The shells comprehended in this family are either spiral, involute, arched, or straight; their septa are deeply lobed, and have the margins foliated. The siphuncle is dorsal, as shown by the notch in the cast, Lign. 156, fig. 3a. Several hundred species have been described; they are divided into genera which are characterized by essential modifications in the direction of the spire, and the inflections of the septa. Thus, in the Ammonites, Lign. 156, the spire is involute, and all the turns contiguous; in Crioceras (curved-horn), Lign. 160, fig. 2, evolute; in Scaphites, incurved at both extremities, Lign. 162; Hamites, bent like a siphon, or hook, Lign. 161, fig. 1; Turrilites, spiral, round a vertical axis, Lign. 163; and in Baculites, straight, Lign. 161, fig. 2. New genera are continually being added, to embrace modifications of structure which appear to be too important for specific distinctions. I will endeavour to render this arrangement more clear to the student by the following definitions.

A straight tube, or horn, of an elongated conical figure, tapering to a point, and having its cavity divided by transverse partitions, which septa are not straight, but undulated, and their edges, which fit into the walls of the tube, deeply wrinkled, and the whole series pierced by a pipe running along near the outer margin, would be the model of the shell termed Baculites, Lign. 161, fig. 2, (Bd. pl. xliv. fig. 5;) which may be regarded as a straight Ammonite. A similar shell, gently arched or curved, would be a Toxoceras, Lign. 160, fig. 1; the same tube, bent upon itself, like a siphon, into unequal limbs, not contiguous, a Hamites, Lign. 161 (Bd. pl. xliv. fig. 10); bent and approximate, or anchylosed in a straight line, Ptychoceras, Lign. 161, fig. 4; partially convoluted, the whorls contiguous, and the free end recurved, Scaphites, Lign. 162; the same form, but the spire not contiguous, Ancyloceras, Lign. 160, fig. 3; spirally twisted around an axis, Turrilites, Lign. 163 (Bd. pl. xliv. fig. 14); coiled, but the turns not touching each other, Crioceras, Lign. 160, fig. 2; lastly, coiled up in the form of a disk, all the turns being contiguous, Ammonites.

Ammonites. Lign. 156, 157, 158.—Shell discoidal, more or less compressed, whorls of the spire contiguous, and often visible; septa lobed, their margins deeply sinuated; aperture symmetrical, border or lip thickened, often notched and auriculated. Siphuncle dorsal.

The student will be able readily to distinguish Ammonites from Nautili by attention to the above definition. The situation of the siphuncle, the foliated or wrinkled edges of the septa, as shown in the cast, Lign. 156, fig. 2; and when these characters are wanting, the arched ribs and elevations, as in figs. 1 and 3, will serve as discriminating features. Like the fossil Nautili, the Ammonites most commonly occur as casts, the shell having been dissolved. Sometimes these consist of semi-transparent calcareous spar, the cast of each cell being distinct, but held together by the interlocking of the foliations of the septa; such examples are of great beauty and interest (see Bd. pl. xlii. figs. 2, 3); they most frequently occur in the limestones of the Oolite. The siphuncle is often preserved, even in the chalk specimens, in which all traces of the shell are lost. In a large Ammonite from, near Lewes, not only the shelly siphuncle remains, but even the internal membranous tube, converted into dark molluskite. Separate portions of similar siphuncles occur in the chalk, and have been mistaken for tubular shells.

The outer lip, or margin of the aperture, is occasionally found entire. In some species there is a dorsal process, as in a very common Chalk Ammonite (A. varians, Lign. 156, fig. 1), which extends far beyond the margin; in other species, from the Oxford Clay, there are long, narrow, lateral appendages, (Lign. 158). In the collection of S. P. Pratt, Esq., there is a small Ammonite, from the Inferior Oolite of Normandy, in which these processes meet over the aperture, leaving only a circular aperture towards the back of the shell (where the siphuncle of the animal would be situated) and a narrow fissure on the side next the involute spire. The specimen is unique, and can scarcely be regarded as a normal form.[414] (Mr. Woodward.)

From the small size of the cells of the Ammonites, particularly in those species which are of a depressed or flattened form, it was long doubted whether the outer chamber could have been sufficiently capacious to contain the body of the animal; and it was supposed that these shells were internal, like the recent Spirula, or Crosier. But Dr. Buckland has clearly demonstrated, that the outer cell of the Ammonite, if restricted in breadth, is sufficiently ample in length to have contained the soft parts of a Cephalopod, equal in magnitude to the largest known specimens, its proportion to the chambered part being as considerable as in the Nautilus. The outer chamber often occupies more than half, and in some instances the entire circumference of the outer whorl (see Bd. pl. xxxvi.).

In certain argillaceous deposits, as the Galt, and the clays of the Lias, Ammonites with the shell preserved are abundant; generally the outer opaque layer is wanting, or adheres to the matrix when a specimen is removed, leaving only the internal nacreous, or pearly coat. Folkstone, on the coast of Kent, is celebrated for examples of this kind, which may easily be collected from the Galt, which forms the base of the cliffs, at Eastware Bay. Watchett, in Somersetshire, is equally rich in the pearly Ammonites of the Lias; entire layers of these beautiful organic remains occur in the limestones and in the shale exposed at low water.[415]

The shell of the Ammonite is generally thinner and more delicate than that of the Nautilus. However thin these shells may be, they are possessed of great strength of structure. Not only is the shell one continuous arch, but it is moreover provided with transverse arches or corrugated ribs, which, like the flutings on metal pencil-cases, and corrugated sheet-iron, greatly strengthen the thin material. These ribs are further subdivided, so as to multiply supports as the convexity of the shell enlarges, in a manner somewhat similar to the groin-work of vaulted roofs. The spines, tubercles, and bosses, which often arise from the ribs, are so many additions to the strength of the latter. The sides of the shell are also supported by the transverse plates forming the septa, or divisions of the chambers, and, as these plates are very sinuous where they meet the sides of the shell, they distribute their support over a considerable portion of the surface.[416] These complicated edges of the septa form the delicate and intricate lines of foliation (sutures) seen on the casts of Ammonites (Lign. 156, fig. 2), Baculites (Lign. 161, fig. 3), Hamites, &c. In some species the shell is thick and dense, as in the Ammonites of the Kimmeridge Clay near Aylesbury, in which one species (Ammonites biplex, Min. Conch. pl. ccxciii.) in particular abounds, and is very generally invested with the shell, as perfect as if recent. The same Ammonite occurs in the Portland limestone above, in the state of casts, without any vestige of the shell. In some strata the shell is replaced by calcareous spar; in others by silex or flint.[417] In the pyritous clays and shales of the Lias, the shell and all its delicate internal mechanism are coated with or replaced by brilliant sulphuret of iron, forming the most exquisite natural electrotype imaginable. Polished vertical sections of these fossils often exhibit the inner cells filled with transparent white calcareous spar; sometimes with groups of crystals of sulphate of lime. The Ammonites of the Galt, and of the Kimmeridge Clay, are also frequently imbued with the same mineral.

There are about two hundred identified species of Ammonite in the British strata, ranging through all the secondary formations; they have not been found in the Tertiary deposits. They vary in size from half an inch to four feet in diameter.[418]

Certain species are restricted to particular formations, and are therefore oftentimes of essential aid in determining the relations of a deposit; for example, the Galt contains several species not found in the upper division of the Chalk; and in the Chalk-maid are species that have not been discovered in other strata. Certain Ammonites of the Lias are peculiar to that formation (as A. Walcottii, Lign. 127, fig. 7, p. 397; and A. communis, Lign. 157). Ammonites of species allied to those of our Middle and Lower Oolites have been discovered in strata in the Himalaya mountains, several thousand feet above the level of the sea.

Fig.	1.—	Goniatites Listeri.
	2.—	Goniatites striatus.
	3.—	Outline of a suture of Goniatite.
	4.—	Outline of a suture of Ammonites venustus.
The Arrows in figs. 3 and 4 denote the direction of the dorsal line.

Goniatites, Lign. 159.—From the numerous family of Ammonites, a separation has been made of a large division, in which the margins of the septa are not deeply notched or foliated, and are destitute of lateral crenatures or denticulations, so that their outline always presents a continued uninterrupted line. The siphuncle is relatively small. The last or outer cell of the shell extends beyond one turn of the chambered part. The back is occasionally keeled, but in most species is round. In illustration of this genus, which is named Goniatites, I have selected two common species (Lign. 159) from the Carboniferous limestone, and annexed outlines of a suture (the edge of the septum) of a Goniatite, and of an Ammonite, for comparison. The importance of the separation of this type of Ammonites into a distinct genus, relates to the Goniatites being restricted to some of the older sedimentary strata; for although there are sixty British species, none of them have been observed above the Carboniferous system.[419]

In Ceratites the sutures are more simple than in the Ammonite, being lobed rather than foliated; and the alternate lobes have their edge crenulated or finely toothed. (C. nodosus, Bd. pl. xl.) This genus is found in the Muschelkalk and the Keuper formations of Europe and Asia.

Our limits will not permit us to extend this notice of the very numerous family of AmmonitidÆ,[420] except to offer a few illustrations of some of the modifications in form to which we have already alluded, and which will assist the student in discriminating these fossil remains.

Crioceras (coiled-horn), Lign. 160, fig. 2.—This shell differs from the Ammonites in the turns of the spire being distant from each other. The siphuncle is continuous, and the septa are regularly divided into six lobes. I have found specimens of this genus in the Chalk-marl at Hamsey, and in the White Chalk, near Lewes (Foss. South D. tab. xxiii. fig. 9).

In the genus Ancyloceras (incurved horn), Lign. 160, fig. 3, the whorls are separate, and, at first, spiral (like Crioceras); but afterwards the shell is prolonged, and then inflected at the large extremity, like a Scaphite, but the whorls are not contiguous.

A very large species of Ancyloceras occurs in the Kentish Rag, near Maidstone, some specimens of which are eighteen inches in length. It is figured and described, by the name of Scaphites Hillsii, in the admirable Memoir of Dr. Fitton on the Strata below the Chalk (Geol. Trans. vol. iv. pl. xv.); the present genus was not then established. The Shanklin Sand in the Isle of Wight also contains a gigantic species, which is figured and described by Mr. J. D. Sowerby, in the Geol. Trans., as Scaphites gigas. Ancyloceras occurs also in the Oolite.

In Toxoceras (bow-horn), Lign. 160, fig. 1, the shell is slightly curved, like a horn. Two or three species of Toxoceras are found at Hamsey. The tubercles, in the casts, are the bases of spines, with which the back of the shell was armed, as I have ascertained by examples examined in the rock (see Foss. South D. tab. xxiii. fig. 1). The specimens figured of the above two genera occur in the Neocomian strata of France.

Hamites (hook-shaped). Lign. 161, fig. 1.—Shell involute, spiral, the turns not contiguous; spire irregular, elliptical; the large end reflected towards the spire. The term Hamite, proposed by the late Mr. Parkinson, was formerly given to all the fragments of sub-cylindrical chambered shells, that were bent, or slightly hooked; and the genera Ancyloceras, Toxoceras, &c., have been separated from them, by M. D’Orbigny. But from fossils recently obtained from Cretaceous strata in Pondicherry, and other parts of India, it seems probable that these genera will be found to merge into each other; at present it is convenient to keep up the distinction. The Hamites are distinguished from Ancyloceras, which they most resemble, by their elliptical, irregular spire.

Ptychoceras (folded horn). Lign. 161, fig. 4.—This is another genus formed from the Hamites. The shell is bent double in the shape of a siphon, and the limbs are united together. The specimen figured is from the Neocomian strata of the Lower Alps.

Baculites (staff-like). Lign. 161, figs. 2 and 3.—This, as the name implies, is a straight, elongated, conical, chambered shell; the upper part is destitute of septa, and probably contained the body of the animal.

In my early researches in the Chalk-marl of Hamsey, I discovered numerous solid, oval, and cylindrical pipes of marl, with scarcely any vestige of organic structure, whose origin it was impossible to determine. At length I found the specimen, Lign. 161, fig. 2, which showed the perfect aperture of a chambered shell; and afterwards I found portions which displayed the foliated septa. (Foss. South D. tab. xxiii. figs. 5, 6, 7.) The Baculite, when perfect, is elongated to a point; the septa are very numerous and foliated; the siphuncle is situated on the margin. I have a splendid specimen from the Chalk of France, (collected by M. Alex. Brongniart,) which is composed of distinct casts of the cells, held together by the deep inflections of their margins, in the same manner as are the sparry casts of Ammonites, previously described.[421]

The Galt, near Folkstone, abounds in fossils of the above genera, principally of Hamites; and the nacreous substance of the shells is very often preserved. From the Chalk-marl near Dover, Southbourn, Ringmer, and Southerham, near Lewes, and from Clayton, near Hurstpierpoint, in Sussex, I have obtained examples of several species.

Scaphites (boat-like). Lign. 162.—This name was given by Mr. Parkinson (Org. Rem. vol. iii. pl. x. See Pict. Atlas) to some small chambered shells from the Chalk and Shanklin Sand, of a boat-like form, with the inner whorls coiled up in a spire, and half hidden by the outer chamber, which becomes contracted and recurved on itself, is destitute of septa, and terminates in an oval or transverse mouth. The siphuncle is dorsal. An Ancyloceras closely coiled would be a Scaphite. Hamsey marl-pit yielded to my early researches the first Scaphites discovered in the British strata, together with Turrilites, and other cephalopodous shells, previously unknown in England.[422] The Scaphite is of an elliptical form, the spire and the mouth approaching close to each other; the spire occupies about one-half of the shell. Except the thick outer lip or margin of the aperture, which is almost constantly found changed into pyrites, it is rarely that any vestige of the shell remains. The same mineral constitutes casts of the spiral part; and these, when separated from the other portion, might be taken for Ammonites; see fig. 2. There are two varieties at Hamsey; one, with the surface covered by fine transverse striÆ, which arise singly from the inner margins, and bifurcate on the dorsal part; fig. 1. The other is also striated, but has a row of prominent ribs on the inner half of the broad central portion of the shell; fig. 4. Scaphites occur in the Upper Green Sand of Dorsetshire, and in the Chalk-marl in several places in England; and at Rouen, in France; and a large species, S. Cuvieri, has been found in New Jersey.

Fig.	1	and 4.—Scaphites Æqualis. (Parkinson.)
	2.	—Cast in pyrites of the spiral part of a Scaphite. 2a.—Front view of the same specimen.
	3.	—Front view of Scaphites Æqualis.

Fig.	1.—	Turrilites catenatus. (M. D’Orbigny.) France. a.—One of the septa.
	2.—	Turrilites costatus. Hamsey.

Turrilites. Lign. 163.—Shell spiral, more or less conical, coiled obliquely round an axis, and turriculated. Spire sinistral, whorls contiguous, apparent, with a perforated umbilicus. Edges of septa very sinuous. Siphuncle continuous, situated either on the external convexity, or near the suture at the base of the wreath.

The discovery of three species of these elegant shells rewarded my researches in the little marl-pit at Hamsey, already noticed, and were the first examples of the genus found in England.[423] Like the Ammonites, Scaphites, Hamites, &c. with which they are associated, the Turrilites of our Chalk-marl seldom possess any traces of their shells. The specimens are solid and tolerably sharp casts, with occasional indications of the septa, and more rarely of the siphunculus. They vary in size from two or three inches to two feet in length; and are frequently more or less elliptical, from compression. The three species which generally occur in the Sussex and Kentish chalk, are T. costatus, Lign. 163, fig. 2; T. tuberculatus (Foss. South D. pl. xxiv. fig. 7), characterised by its four rows of tubercles; and T. undulatus (Foss. South D. pl. xxiv. fig. 8), the wreaths of which are ornamented with plain, slightly undulated, transverse ribs. These are all reversed, or sinistral shells; that is, the spire is twisted to the left, the aperture being on the right hand of the observer when the shell is placed on its apex, as in fig. 1. Several other species of Turrilites occur in the Chalk of France,[424] one of which, T. catenatus, is represented Lign. 163, fig. 1.

Some of the Turrilites attain a considerable magnitude. The largest found in England is a specimen of T. tuberculatus (Min. Conch. tab. lxxiv.), from Middleham, in the parish of Ringmer, near Lewes; when perfect, it must have been full two feet in length: it consists of six wreaths, the siphuncle, in the state of pyrites, appearing in three or four; portions of the nacreous internal layer of the shell remain.[425] In some specimens in my possession, the form of the aperture, and the termination of the columella, are distinctly shown; as in the fine example the last whorl of which is represented in Lign. 164.

The Chalk-marl of Lewes, of the Sussex coast, and of the cliffs near Dover, and the Upper Green Sand of Dorsetshire, have yielded the principal British specimens of this genus. Several species occur in the lower cretaceous strata, at St. Catherine’s Mount, near Rouen, associated, as in England, with Scaphites, Hamites, and other allied genera.[426]

Aptychus, Meyer. (Trigonellites, Parkinson.) Lign. 165.—Associated with the remains of Ammonites in several localities, are found flattened triangular bodies, from less than an inch to an inch and a half in diameter, the nature of which is still somewhat problematical. A good figure is given by Mr. Parkinson of one species (Org. Rem. vol. iii. pl. xiii. figs. 9, 10, 12. See Pict. Atlas), with the name Trigonellites latus. These bodies frequently occur in pairs and in apposition, as in the specimen figured in Lign. 165. Their structure is cellular; one surface is slightly concave and striated, and the other covered with minute circular pores. Altogether their appearance is that of bodies enclosed in vascular integuments. It is supposed that they are the opercula of Ammonites. These fossils are commonly found in the last or body chamber of Ammonites, in the Oxford Clay, near Chippenham, the Coral Oolite of Malton, the Lias of Lyme Regis, and the lithographic limestone of Solenhofen. M. Ewald states that they may be found in the Chalk Scaphites by making a longitudinal section of the body chamber: but I have not succeeded in detecting them in the examples from the chalk-marl which I have broken up for that purpose. As these bodies (alluded to by authors as Trigonellites, Aptychus, Munsteria, &c.) will probably come under the observation of the collector, especially among the fossils of the Kimmeridge Clay, these remarks are introduced to suggest diligent research, in the hope that the origin of these fossils may at length be discovered.

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Geological Distribution of Fossil Cephalopoda.—Even from this short review of the principal types of the fossil Cephalopoda, the great interest which attaches to the study of this class of organic remains is strikingly demonstrated. Their geological distribution is alike replete with phenomena of an important character. In the Lower PalÆozoic (Lower and Upper Silurian) strata, the chambered mollusks belong (with a very few doubtful exceptions) to the NautilidÆ, namely, Nautilus, Lituites, Cyrtoceras, Orthoceras, &c. The Devonian and Carboniferous systems contain Nautilus, Clymenia, Gyroceras, Cyrtoceras, and Orthoceras, together with a peculiar group of AmmonitidÆ, the Goniatites. The Trias in general is extremely poor in Cephalopoda; the Permian group affording but two species of Nautilus, and the Muschelkalk two other species: but, in addition to these, the Muschelkalk contains Ceratites, which is a genus peculiar to the Triassic group, and chiefly abounds in the St. Cassian beds (in the Austrian Alps), where it is accompanied by Nautilus, Orthoceras, Ammonites, and Goniatites. In the Lias and Oolite Nautili abound, and we meet for the first time with Belemnites. The same families, viz. NautilidÆ, AmmonitidÆ, and BelemnitidÆ, prevail throughout the Cretaceous strata. The Tertiary formations contain a few NautilidÆ only; no vestiges of the AmmonitidÆ and true BelemnitidÆ, which, as we have seen, swarmed in the ancient seas, are perceptible, while in the existing oceans, the Nautilus and Spirula are the sole representatives of the numerous shell-bearing cephalopoda of the ancient geological eras. Thus, the NautilidÆ extend from the oldest to the newest fossiliferous strata, the genus being still in existence: the AmmonitidÆ, on the other hand, though less ancient in origin, do not pass beyond the limits of the cretaceous epoch.

In the following tabular arrangement these facts are placed in a more distinct point of view:—

Tabular View of the Distribution of Cephalopoda through the Geological Epochs.

Existing Genera	{	Argonauta. Octopus, &c.	}	Octopoda.			}	Dibranchiata.
		Loligo, Cranchia. Sepiola, Onychoteuthis, &c.	}	(TeuthidÆ).	}	Decapoda.
		Sepia (SepiadÆ).
		Spirula.
		Nautilus (NautilidÆ). Tetrabranchiata.
General fossil in the Tertiary Formations	{	SepiadÆ	{	Sepia, Spirulirostra, Beloptera, Belemnosis.
		NautilidÆ		Nautilus and Aturia
Cretaceous Deposits	{	BelemnitidÆ	{	Belemnites, Belemnitella, and Conoteuthis.
		NautilidÆ		Nautilus.
		AmmonitidÆ	{	Ammonites, Crioceras, Scaphites, Ancyloceras, Toxoceras, Hamites. Ptychoceras, Helicoceras, Turrilites, Baculites.
Oolite and Lias	{	SepiadÆ		Sepia.
		TeuthidÆ	{	Teudopsis, Beloteuthis, Geoteuthis, Leptoteuthis, Ommastrephes.
		BelemnitidÆ		Belemnites. Acanthoteuthis, and Belemnoteuthis.
		NautilidÆ		Nautilus.
		AmmonitidÆ		Ammonites, Ancyloceras.
Trias	{	NautilidÆ		Nautilus.
		AmmonitidÆ[427]		Ammonites, Goniatites, and Ceratites.
Carboniferous System	{	NautilidÆ	{	Nautilus, Gyroceras, Cyrtoceras, Gomphoceras, and Orthoceras, Actinoceras, &c.
		AmmonitidÆ		Goniatites.
Devonian System	{	NautilidÆ	{	Nautilus, Clymenia, Cyrtoceras, Phragmoceras, Gomphoceras, Orthoceras, Actinoceras, &c.
		AmmonitidÆ		Goniatites.
Upper and Lower Silurian Systems	{	NautilidÆ	{	Nautilus, Lituites, Gyroceras, Cyrtoceras, Phragmoceras, Gomphoceras, Oncoceras, Ascoceras, Orthoceras, Actinoceras, &c.

With regard to the zoological affinities between the living and extinct species of testaceous Cephalopoda, Dr. Buckland remarks, "that they are all connected by one plan of organization; each forming a link in the common chain which unites the existing species with those that prevailed among the earliest conditions of life upon our globe; and all attesting the identity of the design that has effected so many similar ends, through such a variety of instruments, the principle of whose construction is, in every species, fundamentally the same.

"Throughout the various living and extinct genera of these beings, the use of the air-chambers and siphuncle of their shells, to adjust the specific gravity of the animals in rising and sinking, appears to have been identical. The addition of a new transverse plate within the coiled shell added a new air-chamber, larger than the preceding one, to counterbalance the increase of weight that attended the growth of the shell and body of these animals." (Bd. p. 380.)

The occurrence of the Nautilus, and its congeners, among the earliest traces of the animal kingdom, and their continuance throughout the immense periods during which the family of AmmonitidÆ was created, flourished, and became extinct, and the existence of species of the same genus at the present time, are facts too remarkable to have escaped the notice even of those who are not professed cultivators of geological science; and I am induced to quote the following beautiful lines, by Mrs. Howitt, to impress this interesting phenomenon more strongly on the mind of the youthful reader.[428]

"TO THE NAUTILUS.

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On the Collection of British Fossil Cephalopoda.—In the Tertiary formations of England, the remains of but seven species of Nautilus (comprising Aturia) have been noticed; the large species (N. imperialis) is the most common. These are generally in a good state of preservation, and only require the careful removal of the surrounding clay or marl. When pyrites largely enters into the composition of the specimens, the investing matrix can seldom be effectually cleared off: if the outer surface, and general form, be not well displayed, breaking the specimen will often expose the inner cells, with the siphunculus, in a beautiful state. The Nautilus imperialis is occasionally imbedded in the septaria of the Isle of Sheppey, and of Bognor and Bracklesham, on the Sussex coast. Sections of such examples, in the vertical direction of the enclosed shell, afford, when polished, very brilliant and interesting fossils; the septa and the shelly tube of the siphunculus are often preserved.

The Cephalopods of the Cretaceous formation, with the exception of those in the argillaceous strata of the Galt, are generally destitute of their shells, and only occur in the state of casts; and the Chalk Nautili are liable to separate at the divisions of the septa, and an entire series of the casts of the chambers may sometimes be obtained, so as to display the entire form of the original shell. The Ammonites of the White Chalk, although mere casts, yet retain their configuration, the foliated margins of the septa dove-tailing them together. I have already mentioned that search should be made along the back of these specimens for the siphuncle, the shelly tube of which is sometimes well defined. In the Chalk-marl the casts are sharper than in the White Chalk, and generally of a deep ochreous colour, with the lines of the sinuous septa clearly defined. The siphuncle is occasionally preserved in pyrites, in the Ammonites, Nautili, Turrilites, and Scaphites; and the outer lip or margin of the mouth, or aperture, of the latter, and of the Ammonites, is frequently replaced by the same mineral.

The Ammonites, Hamites, &c. of the Galt have their pearly coat remaining, but this investment is extremely delicate; and although when first removed from the marl it is beautifully iridescent, the vivid hues are very evanescent, and the shell becomes opaque and of a light fawn colour. Very commonly the shell flakes off, wholly or in part, leaving a cast of indurated pyritous marl. I have preserved specimens with the shell many years, by applying a thin coat of mastic varnish with a soft camel-hair pencil, before the marl had become dry, and while the shells were entire. The Galt Ammonites, like the Nautili of the London Clay, are often invested with pyrites, and have the inner cells and siphuncle well preserved.

The argillaceous strata of the Oolite and Lias contain Ammonites, &c. in much the same state of mineralization as those of the Galt. The Kimmeridge Clay, in some localities, particularly around Aylesbury (and especially at Hartwell Park, the seat of Dr. Lee), abounds in Ammonites with the shell as perfect and beautiful as if just dredged up from the sea. But, like the fossils of the Galt, few of the specimens are durable; although in many examples the shell may be preserved by the application of mastic varnish. The most common Ammonite at Hartwell is A. biplex (Sow. Min. Conch.), which varies from three inches to one foot in diameter; the surface is covered by very strong ribs that encircle the whorls. The shell is thick, and composed of several laminÆ.[429]

The sparry casts of the separate cells of Ammonites which occur in some of the calcareous beds of the Oolite, will not fail to be observed by the collector. It is convenient to preserve such specimens either on a tray or board, in which a groove is made for their reception, or in a mould of gutta percha.

In collecting BelemnitidÆ, the caution already given, of examining the surrounding clay or marl, must not be disregarded; the student should remember, that traces of the soft parts of the animals, even of mere impressions of the body and head, with the tentacula and their acetabula, or little horny rings and hooks, are more important than the most splendid examples of the spathose durable osselet. The guards should be selected with especial reference to their containing the phragmocone (see Lign. 141, fig. 2), or chambered conical shell, in the alveolus or cavity of the upper and larger end. An apparently worthless fragment of a Belemnite will often be found to possess this part of the structure, as in the example figured, which, until fractured longitudinally, had been thrown by among useless duplicates. The search for the remains of the fossil naked Cephalopoda, as the TeuthidÆ and SepiadÆ, and their ink-bags, must be made in a like cautious manner. In the Lias marls, the ink-bag and its duct is often found partially covered by a pellicle of nacre, without any trace of the other parts of the animal. A reference to Dr. Buckland’s plates (Bd. pl. xxviii. xxix) will familiarize the student with the appearance of these fossil remains.

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A FEW BRITISH LOCALITIES OF FOSSIL CEPHALOPODA.

Abingdon, Berks. Ammonites, fine casts in spar and limestone; Middle Oolite.

Aylesbury, Bucks. Ammonites, several species; splendid examples of A. biplex, with the shell remaining, in the Kimmeridge Clay.

Aymestry. Upper Silurian; Gomphoceras, Orthoceras, &c.

Bath. Fine Ammonites in the Oolite.

Beachy Head. Along the shore, gigantic Ammonites in the Chalk, at low-water.

Benson, Oxfordshire. Fine Hamites, in Chalk-marl.

Blackdown, Devonshire. Beautiful siliceous casts of Ammonites; Green Sand.

Bognor, Sussex. Nautili, in the Tertiary Clays and sandy Limestones; also, along the neighbouring coast, in Septaria.

Bolland, Yorkshire. Mountain Limestone; Goniatites.

Boreham, near Warminster, Wilts. Nautili and Ammonites in Green Sand.

Bracklesham Bay, Sussex. Nautili in Tertiary Clay.

Bridport, Dorset. Ammonites; Inferior Oolite.

Brighton. In the Chalk, Ammonites, BelemnitellÆ, &c.

Brill, Lucks. Ammonites, as at Aylesbury.

Buxton, Derbyshire. Goniatites; Mountain Limestone.

Charmouth, Dorsetshire. Ammonites, Belemnites, &c.; Lias.

Cheltenham. Ammonites, Belemnites, Nautili, &c. in abundance; Inferior Oolite and Lias.

Chicksgrove, Tisbury, Wilts. Ammonites, several species; some chalcedonic; Upper Oolite.

Christian Malford, near Chippenham. In Oxford Clay, Belemnites, Belemnoteuthis, and Geoteuthis; very fine.

Clayton, near Hurstpierpoint, Sussex. In Chalk-marl, Ammonites, Nautili, and Turrilites; very fine specimens.

Closeburn, Dumfriesshire. Orthocerata, large species; Silurian.

Comb Down, near Bath. Ammonites and Nautili; Oolite.

Connaught, Ireland. Goniatites; Mountain Limestone.

Cork. Orthocerata; Mountain Limestone.

Crockerton, near Warminster. Ammonites, in Galt.

Dover. In the cliffs, and along the shore, in Chalk and Chalk-marl, Turrilites, Ammonites, Nautili, &c.

Dowlands, near Lyme. Fine Ammonites, &c. in the Lias.

Dundry, near Bristol. Ammonites, &c. Inferior Oolite.

Earlstoke, Wilts. Hamites, Ammonites, &c. in Green Sand.

Faringdon, Berks. In the gravel-pits, Nautili, Ammonites, &c. In the Coral Rag, beautiful casts in limestone and spar of Ammonites, Belemnites, &c.

Folkstone, Kent. In the Galt, at Eastware Bay, in the cliff, and along the shore at low-water, Belemnites, Hamites, Ammonites, &c. in profusion.

Hamsey, near Lewes, Sussex. Chalk-marl; Turrilites, Scaphites, Hamites, Baculites, Crioceratites, Ammonites, Nautili; Belemnites, very rare.

Hartwell, Bucks, seat of Dr. Lee. Splendid Ammonites, with their shells, in Kimmeridge Clay.

Heytesbury, Wilts. Nautilus elegans, and other Chalk-marl Cephalopoda.

Horncastle. Very fine Ammonites.

Hythe, Kent. In Green Sand, large Ancyloceratites, Ammonites, &c.

Ilminster, Somerset. Upper Lias, Ammonites; Marlstone, Belemnites and Ammonites.

Kelloway. Many beautiful Ammonites, &c.; Middle Oolite.

Keynsham, near Bristol. Splendid Nautili and Ammonites; the large A. giganteus, two or three feet in diameter; and specimens with the chamber filled with spar, of surpassing beauty; Lias.

Lewes, Sussex. Nautili, Ammonites, &c. in the Chalk and Marl quarries of the vicinity.

London. Tertiary strata in the vicinity. Highgate Hill, fine Nautili, and Aturia ziczac, Beloptera, &c.

Ludlow. Upper Silurian; Lituites, Orthoceras, Phragmoceras, &c.

Lyme Regis, Dorsetshire. Ammonites, Nautili, Belemnites, SepiÆ, &c. in profusion in the Lias; and Scaphites and Turrilites in the Chalk.

Lympne, Kent. Ammonites, Ancyloceratites, &c. in Green Sand.

Maidstone, Kent. Ammonites, of large size, in Shanklin Sand.

Malton. Ammonites, several large species. Lower Oolite.

Marsham, near Abingdon. Ammonites; Oolite.

Marston Magna, near Ilchester. Ammonite-marble; Lias.

Newton Bushel, Devonshire. Nautilus, Orthoceras, Cyrtoceras, Goniatites, &c. in the Devonian rocks.

Norwich. In Chalk, Belemnites in profusion; Ammonites, &c.

Nutfield, Surrey. Fuller’s-earth pits: beautiful Nautili (N. undulatus, and A. Nutfieldiensis), and Ammonites.

Offham, near Lewes. In the Chalk-pits, large Ammonites; Chalk-marl in a pit, on the right-hand side of the road, a quarter of a mile north of the village, Hamites, Turrilites, Scaphites, Nautili, rare species of Ammonites, &c.

Oxford. Quarries in the vicinity, Ammonites, Belemnites, &c.

Petherwin, Cornwall. Clymenia, Goniatites, Orthoceras, &c. in the Upper Devonian rocks.

Portland. Upper Oolite; gigantic Ammonites.

Roak, near Benson, Oxfordshire. In Chalk-marl, Hamites, Ammonites, &c.

Scarborough. Kelloway Rock; Ammonites, &c.

Scarlet, Isle of Man. Nautili, &c.; Mountain Limestone.

Settle, Yorkshire. Goniatites; Mountain Limestone.

Sherbourn, Somersetshire. Ammonite-marble; Lias.

Southerham, near Lewes. In the Chalk-pits, large Ammonites; in the Marl, Nautili, Ammonites, Turrilites, &c.

South Petherton, Somerset. Marlstone; Belemnites and Ammonites in profusion.

Speeton, Yorkshire. Galt; Crioceras, Ancyloceras, &c.

Steyning, Sussex. In Chalk-marl near the town, Belemnites (B. lanceolatus), Nautili, Ammonites, &c.

Swindon, Wilts. In the Portland-stone quarries, Ammonites, in abundance; principally casts of A. biplex, and A. triplicate. In the Kimmeridge Clay in the vicinity, Ammonites with the shell preserved.

Tisbury, Wilts. In Portland-stone, fine Ammonites, often chalcedonic (see Bd. pl. xli.).

Trowbridge, Wilts. In Oxford Clay, Ammonites, Belemnites, &c. were obtained in great numbers during the railway cuttings.

Watchett, Somersetshire. Ink-bags of SepiadÆ, &c.; splendid Ammonites; Lias.

Whitby, Yorkshire. Ammonites, Belemnites, &c. in abundance; Nautili, &c.; Lias.

Yeovil, Somersetshire. Nautili and Ammonites; Inferior Oolite.

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