CHAPTER XI.

MICROSCOPICAL EXAMINATION OF FLINT.

Flint.—Flint, and other siliceous stones, require to be chipped into very thin fragments, and immersed in oil of turpentine. A clear, translucent flint should be selected, from which fragments may be shattered off by smart blows of a hammer, over a sheet of white paper: the most transparent flakes are to be selected, and these should be put in oil of turpentine, in a wide-mouth glass bottle. Take out the pieces for examination with forceps, and inspect them as transparent objects, by transmitted light. When good specimens are discovered, mount them in Canada balsam.

It is hazardous to entrust such fossils to the lapidaries; an interesting group of twenty spiniferites was reduced to ten, by one of our best workmen, in whose hands it was placed for polishing, with the view of rendering it more transparent.

FOSSIL TESTACEOUS MOLLUSCA, OR SHELLS.

Lign. 120. Fossil Oyster, from the Chalk.Kemptown, Brighton.

On Fossil Mollusca.—Numerous as are the fossil remains of the various types of animal organization which have already passed under review, they are far exceeded in number and variety by those of the beings whose mineralized relics we now propose to investigate. Although every one is familiar with the external appearance of the shells cast up by the waves on the shores of our island, and of those which, from their varied colours and elegant forms, are preserved inthe cottage of the peasant, and in the mansion of the rich, but few persons are conversant with the nature of the animals that secreted and were protected by these beautiful and enduring structures. The organization even of the oyster, mussel, whelk, &c., is known only to the naturalist. Appearing to the uninstructed eye as a shapeless gelatinous mass, there is nothing to arrest the attention, or excite the curiosity. Yet the beings which inhabited these durable cases, are objects of the highest interest and present a rich field of instructive investigation.

Except as shedding some light on the structure and economy of their inhabitants, the shells, in the estimation of the naturalist, are the least interesting part of the organization of the Mollusca; but to the geologist, from their permanent nature, and the proofs they yield of the conditions under which the strata that contain them were deposited, they are important in the highest degree. It has even been found convenient to classify formations, in which fossil shells largely prevail, by the relative numerical proportion of the recent and extinct species found in the different groups of strata; and the terms, Eocene, Miocene, and Pliocene, (proposed by Sir C. Lyell,) have reference to this character, as we have previously explained (ante,p. 24.).

FOSSIL MOLLUSCA.

TheMollusca, a name indicative of the soft nature of the integuments of these animals, constitute a very comprehensive subdivision of the animal kingdom, and are separable into two principal groups, viz. theAcephalaand theEncephala.

I. TheAcephala(so termed because they are destitute of a head) have neither jaws, tongue, nor a distinct mouth. They are aquatic, and are subdivided into classes, according to the modification of their integument, or of their gills.

a.TheTunicata(from the elastictunic, ormantle, in which they are enclosed) have no shell, and therefore do notcome within the scope of our inquiries: yet it is possible that the soft parts even of these perishable structures may have left some trace, or that markings of their integument on the silt or mud may be preserved;[348]and I would recommend the student to search for such indications on the rippled surface of clays and sandstones.

[348]TheIschadites Königiof the Ludlow rock was supposed to resembleBoltenia, a pedunculated Ascidian.

b.TheBrachiopoda(arm-feet) have two long spiral fleshy arms, or brachia, developed from the sides of the alimentary orifice, are enclosed in bivalve shells, and respire by means of their vascular skin, or mantle. They have not the power of locomotion, but are fixed by a peduncle to other bodies.

c.TheLamellibranchia(plated gills) have also bivalve shells, but their respiration is effected by gills composed of vascular membrane disposed in plates, and attached to the mantle; thebeardof the Oyster is the branchial or respiratory apparatus of that animal. These bivalve Mollusca are subdivided into those which close their shells by one adductor muscle, hence calledmonomyaria, as the Oyster; and those which have two muscles,dimyaria, as the Cockle. As the impressions left on the shells, by the attachment of these adductor muscles, and by the margin of the mantle, are found as perfect in the fossil as in the recent, they constitute important distinctive characters.

Dr. Gray's definition of the respective parts of univalve and bivalve shells is at once clear, concise, and natural, being conformable to the structure of the body of the enclosed mollusk.

Thefrontof the shell is the part which covers the head of the animal; the back of the shell is the part which covers the tail; the left and right sides correspond with the same parts of the mollusk.

In univalves, the apex of the shelly cone whether it besimply conical or spiral (except in Patella) is over the hinder part of the animal: when the shell is placed on its mouth with the apex towards the observer, the parts of the shell correspond with the position of the person looking at it.

FOSSIL BIVALVE SHELLS.

Lign. 121. Illustration of Fossil Bivalve Shells;nat.Petricola Patagonica.D'Orb.Interior of right valve, and the same valve with the animal as seen on the removal of the left valve.a.Labial Palpi.b.Mantle.c.Margin of shell.d.Branchiæ.e.Anal siphon.f.Branchial siphon.g.Foot.h.Retractor muscle ofsiphons.i.Posterior adductor.j.Anterior adductor.k.Ligament.l.Umbo.m.Lunule.n.Base, or ventral margin.o.Anterior side.p.Posterior side.q.Pallial line.

(The length of the shell is estimated fromotop, its breadth fromlton.)

In bivalves (Lign.121) the ligament is always on the dorsal surface of the animal, and the mouth in front of the apex or umbo of the valves, before the ligament. A bivalve placed with the hinge side uppermost and the ligamenttowards the observer is in the same relative position as the person looking at it; viz. theheadin front, and the right and left valves in their natural relations. The length of the shell is therefore from the front to the back of the animal: the width or transverse diameter is from the umbo to the margin. Much confusion has arisen from many conchologists having described the length and width of a shell diametrically opposite to the proper position of its inhabitant.

II.The Encephalous Mollusca.—These possess a head, with feelers or soft tentacula, eyes, and a mouth with jaws; they are arranged in classes, according to the modification of their locomotive organs; for, with but few exceptions, they are free animals, and can crawl, climb, or swim. Their shells are, for the most part, composed of one piece, or valve, hence they are termedUnivalves. In some genera the shell is a simple cavity, spirally disposed, as in the Snail; in others, it is conical, consisting of one or many pieces, as in theLimpetandChiton. In the Cephalopoda it is internally divided into cells, or chambers, as, for example, in the Nautilus.

The Encephalous Mollusca are subdivided into the following classes; viz.—

a.Pteropoda(wing-feet).—In these the organs of progression are two wing-like muscular expansions, proceeding from the sides of the neck, by which they can swim and float in the open sea: all the species are of small size.

b.Gasteropoda(feet under the body).—These crawl by means of a muscular disk, or foot, which is attached to the under-part of the body; most of the species are marine, but some are terrestrial, and others inhabit fresh-water. They are very widely distributed; the garden snail is a familiar instance of a terrestrial Gasteropod.

c.Cephalopoda(feet around the head).—The mollusca ofthis order have powerful muscular arms, or tentacula, which surround the head, or upper part of the body; some genera have no shell, but possess an internal skeleton, as the recent Sepiadæ and the fossil Belemnitidæ. Most of the testaceous Cephalopoda have a discoidal, univalve shell, which is divided internally by septa or partitions; as the Nautilus.

FOSSIL MOLLUSCA.

In many univalves the aperture or opening is entire, that is, without any notch or groove; in others it is notched or extended into a canal, or siphon, and this character has relation to the respiratory organs: thus the Gasteropods, in which the water is conducted to the interior by a muscular tube, or siphon, have the margin of the aperture of the shell channelled; as in the Whelk, orBuccinum. Many of the land and fresh-water species have entire openings, and are, for the most part, herbivorous; while the greater number of the marine univalves have the aperture indented or notched, and are carnivorous.[349]Some of these mollusca, too, have a retractile proboscis, armed with minute teeth, by which they can rasp or bore into the shells of the species on which they prey. There are some exceptions to the above rules, but the prevalence of the characters specified afford pretty certain indications of the fluviatile or marine nature of the originals. The application of these data to geological investigations will be considered hereafter.

[349]The form of the aperture does not necessarily indicate fresh-water genera.Melanopsis,Pirena, and most of theMelaniæhave a channelled or notched aperture. Fresh-water univalves frequently have the spire corroded; in a fossil state they can only be determined [to be fresh-water species] by their analogy to recent genera and sub-genera.—Note by Mr. Woodward.

In the generic distinctions of the simple univalves, the form of the mouth is an important character; while in the bivalves, the configuration of the hinge affords an equally convenient aid for their classification.

Some tribes of testaceous mollusca are exclusively marine; many are restricted to the brackish water of estuaries; others live only in fresh-water; and some on the land. Their geographical distribution is alike various: certain groups inhabit deep water only, and are provided with means by which they can maintain themselves near the surface of the ocean, far away from any shore; while others are littoral, that is, live in the shallows along the sea-shores. Many exist in quiet, others in turbulent waters; some are gregarious, like the oyster; while others occur singly, or in groups. The vertical range, that is, the relative depths in which the mollusca live in the sea, is also strictly defined; certain genera being, in a great measure, restricted to moderate depths, others to a few fathoms, and many to the profound abysses of the ocean, which neither the dredge nor the plummet can reach. All these varieties of condition are more or less strongly impressed on the shells, which may be considered as external skeletons;[350]and the accomplished conchologist is enabled, by certain characters, to determine the nature of the animals which inhabited them, and the physical conditions in which they were placed.[351]

[350]In equivalve bivalves the animal lives in an upright position. In inequivalves,i.e.one large and one small valve, the animal lies on its side. The situation of bivalve shells, as oysters, should therefore be noticed, for if they lie on their concave shell, with the flat valve uppermost, it is evident they were overwhelmed in their native bed and in a living state; if they lie indiscriminately on either valve, they were probably dead shells and overwhelmed in that state. If the pallial imprint is notched by a sinus, it shows the presence and size of the tubes of the mantle. Whether there be one or two muscular impressions is of far less importance.[351]For an extended notice of the geographical distribution of testacea, see Prof. Edward Forbes, British Marine Zoology, Part I. p. 141.

[351]For an extended notice of the geographical distribution of testacea, see Prof. Edward Forbes, British Marine Zoology, Part I. p. 141.

The number of living species of mollusca known to naturalists, not including the shell-less genera, exceeds twelvethousand; and almost every day is adding new species, for scarcely a vessel arrives from distant seas without enriching the stores of the conchologist. The numerous genera into which they are divided by systematists, and the constant changes effected in arrangement and nomenclature by every writer on the subject, render it difficult if not impossible to present the reader with any satisfactory epitome of modern conchology.

I must restrict myself to a brief account of some of the most common genera that occur in the British strata; and shall dwell more particularly on those species which prevail in the secondary formations, because they present the most important deviations from the recent types that are familiar to the general observer; by this means, and by reference to figures in standard works, the collector will, I trust, be enabled to identify the fossil shells which may most frequently come under his notice in the course of his geological rambles.

FOSSIL BIVALVE SHELLS; INCLUDING THE BRACHIOPODA AND LAMELLIBRANCHIA.

FOSSIL BIVALVE SHELLS.

Although in the modern Tertiary strata, as the Crag, and in the arenaceous beds of the Eocene formations, shells are generally found in so perfect a state, that no caution or knowledge is requisite for their collection, yet a few preliminary remarks are necessary to point out certain conditions in which the remains of mollusca, or evidence of their existence, occur in the mineral kingdom, and particularly in the older fossiliferous rocks. Shells are found in the strata in the three following states:—

1stly. Shells in which the constituent substance has undergone but little change. Many of the specimens in thesands of the Crag in Norfolk and Suffolk, and in the Eocene beds at Grignon, near Paris, and the Pliocene of Palermo, in Sicily, are as perfect as if collected from the sea-shore, having suffered no loss but that of colour. In some instances, even the varied markings on the surface remain; but in general the shells are bleached, or have a ferruginous stain.

2dly. The form preserved, but the constituent substance mineralized. This state is very common in shells that are imbedded in hard rock, whatever may be the age of the deposit. In calcareous strata the testaceous substance is generally transmuted into calcareous spar, as in most of the specimens from the chalk, oolite, mountain limestone, &c. In sands abounding in silex, the shell is changed into flint, as in the exquisite fossils from the Greensand of Blackdown, Devonshire; in deposits permeated with sulphuret of iron, the shells are often metamorphosed into pyrites, as in the Ammonites in the Lias, Galt, &c.

3dly, In the state of casts and impressions. Although in loose sand the shells are either empty, or filled with detritus easily removable by washing; in clay, limestone, and sandstone, the cavities are generally occupied by consolidated materials, which had entered when in a soft or fluid state; and frequently the substance of the shell has disappeared, and the stony cast of the interior alone remains. In many instances, the spaces left by the dissolution of the shells are filled with spar, or the casts are closely invested by the surrounding stone, from long-continued superincumbent pressure while the matrix was in a plastic state; and in such cases the casts are often distorted and flattened. But the vacancy is occasionally empty, and on its walls is found an impress of the external surface of the shell, with all the lines and ornaments of the original as sharp as if cast in plaster of Paris.

Lign. 122. Turritellæ, from Bracklesham, Sussex.Tert.Fig.1.—Turritella conoidea; the perfect shell:nat.2.—Septarium, withTurritellæ; a polished slab:1/3nat.3.—A cast of one of the shells, in calcareous spar:nat.

SEPTARIUM WITH SHELLS.

The specimen,Lign.122,fig.2, from the tertiary strata at Bracklesham Bay, Sussex, is a polished slice of indurated argillaceous limestone, from a septarium (nodule divided by fissures), abounding in spiral univalve shells, calledTurritellæ.Fig.1 is a perfect shell of the same species, extracted from soft clay; andfig.3, a cast in calcareous spar, obtained from the septarium. In the polished slab,fig.2, sections of numerous shells are seen. The dark partitions,or septa, are veins of spar, which occupy interstices that have been formed in the clay-nodule by shrinking; and if the specimen be closely examined, the shells will be found split across and displaced by the fissures; thus presenting an interesting illustration of the faults, or dislocations, of the strata, so familiar to the geological observer. In the present instance, the lines on the exterior of the shell do not materially differ from those on the interior, and, consequently, the cast,fig.3, and the shell.fig.1, resemble each other; but in many species there is a striking contrast between the outer and inner surfaces, the external aspect being strongly ornamented, while the internal is smooth; the cast, therefore, in such examples, so little resembles the shell, that an inexperienced collector may readily suppose it belongs to a different species. The bivalve called Trigonia,Lign.127,figs.1, 2, is an instance of this contrast.

The polished slab of the Septarium,Lign.122,fig.2, demonstrates another condition of fossil shells—that of a compact argillaceous limestone—and entire beds of marble are composed of an aggregation of this kind, formed of shells and other animal exuviæ, consolidated by mineral infiltrations. In the older secondary strata this state prevails; and the beautiful markings of many valuable marbles, are merely sections of the enclosed shells. But this process is not restricted to the deposits of ancient date; at the present moment the same operation is silently but constantly going on in our seas, and an examination of the specimen,Lign.123, will afford an exemplification of the manner in which these shelly limestones are produced.

BRIGHTON SHELL-CONGLOMERATE.

Lign. 123. Shell-Conglomerate; now forming in the British Channel.Dredged off Brighton.Fig.1.—An Aggregation of Shells and Corals; the interstices arefilled up with sand, and the mass is consolidated by aninfiltration of carbonate of lime.2.—Trochus ziziphinus; extricated from the mass with the following:3.—Pecten opercularis.4.—Serpula.5.—Portion of aCellepora; magnified.6.—Sabella.

Lign. 123. Shell-Conglomerate; now forming in the British Channel.Dredged off Brighton.

We have here a solid mass of stone, composed of several recent species of shells, corals, &c. It is a fragment of a large block, dredged up from the British Channel, off Brighton. Similar masses have been obtained at different soundings along this part of the Sussex coast; and in some specimens numerous other species of recent shells, as oysters, mussels, whelks, &c. enter into the composition of the consolidated rock. The shelly and coralline limestones and sandstones, so abundant in the ancient strata of England have been formed in a similar manner; and when the modern conglomerate of Brighton shall have been permeated with crystalline matter, and subjected to great pressure bysuperincumbent deposits, through countless centuries, and at length be elevated above the waters, it will constitute beds of shell-marble, in some mountain range, and become an interesting, perhaps the only memento, of the races of mollusca and polypiaria of the present seas, when all record and traces of Great Britain and its inhabitants shall be destroyed.

Lign. 124. Shell-Limestone; from the mouth of the Thames.Fig.1.—A mass of Cockle-shells and Whelks, consolidated into a coarse limestone.2.—One of the shells,Cardium edule, extracted from the block.3.—A slice of the rock, polished, the markings on the surfacebeing derived from sections of the shells.

MODERN SHELL-LIMESTONE.

Off the Kentish coast, near the mouth of the Thames, abank of consolidated shells, chiefly of one species, is in the progress of formation, from which blocks may be obtained of great firmness and solidity (Lign.124); these, when cut and polished (fig.3), display a variety of markings, produced by the sections of the shells. Extensive shoals of loose shells, composed almost wholly of theCardium edule, exist in several localities, near the embouchure of the Thames; and these are continually shifting with the changes of the wind and tide; it is only in a few places that consolidated blocks occur, like that of which a fragment is figured inLign.124. These examples of shelly limestones and sandstones now in progress of formation will familiarize the student with the nature and origin of those ancient deposits of a similar character, which contain extinct species and genera of mollusca.

"The vast deposits of fluviatile shells which exist in Florida, at Picolata, Volusia, and Enterprize are of great geological interest. The two latter places present bluffs and hills of from forty to fifty feet in height, extending half a mile or more from the river, that are composed of scarcely anything but well-preserved shells ofPaludina vivipara,Ampullaria depressa, some undetermined species ofUnio,Helix septemvolvis,Melania, and a few others. There is but a scanty mixture of earth, and the shells are clean, and look as if they had been washed ashore after the death of their inhabitants. In some places the beds are sandy, and are hardening into a calcareous shelly sandstone. In one such bed the superficial stratum furnished a few bones of turtles and undetermined fragments, the bones of some large vertebrate animal. This is, I believe, the locality where Count Pourtalés collected human bones in a recent sandstone.... No microscopical forms were detected in these beds after the most careful search."[352]

[352]Dr. J, W. Bailey, in Smithsonian Contributions, vol. ii. Article viii. p. 23.

Lign. 125. Terebratula and Rhynchonella;nat.Chalk. Lewes.Fig.1.—Rhynchonella plicatilis.1a.—The same species, partly open.2.—Rhynchonella subplicata.2a.—Front view of the same.3.—Terebratula semiglobosa; side view.3a.—The same species, seen from above.4.—Terebratula subrotunda.

Fossil Shells of the Brachiopodous Mollusca.—These are bivalve shells, of which nearly five hundred species are found in the British strata. They occur in incredible numbers in the ancient rocks, to which several genera are restricted; while some continue through all the formations,and inhabit the present seas; but the existing genera are few.

TEREBRATULA.

Terebratula(bored, alluding to the perforated beak),Lign.125.—The common species of this genus must be familiar to all who have ever looked into a quarry of Chalk, or of Shanklin sand, in the south-east of England. They have been humorously called theFossil Aristocracy, from the incalculable antiquity of their lineage.

The species are very numerous; more than 300 extinct forms have been determined.[353]Those figured inLign.125are from the White Chalk, and are beautifully preserved; even vestiges of the colour occasionally remain. In a living state, the animal is fixed to foreign bodies by a byssus, or peduncle, which passes through the opening in the beak, or arched extremity, of the shells,[354]The most interesting circumstance relating to these mollusca, is the respiratory apparatus, which consists of two long ciliated tubes, spirally coiled, united at their base, and supported by slender calcareous processes, which are often preserved in the fossils. Thus, in specimens from the soft chalk, the calcareous earth may be removed from the interior of the shell, and the appendages exposed, as in the examples,Lign.126,figs.1, 2; and in the shells that are empty, these processes occasionally remain distinct, or are coated by a thin pellicle of calcareous spar, or pyrites.

[353]See Catalogue ofTerebratulidæ, published for the British Museum.[354]In the British Museum (Eastern Zoological Gallery, case table A) there are between thirty and forty recent terebratulæ (T. australis, Quoy, a plaited species, much resemblingT. fimbriaof the Inf. Oolite, Cheltenham) attached with their byssi to a block of stone, from Port Jackson, where it was found by Mr. Jukes just below low-water.

[353]See Catalogue ofTerebratulidæ, published for the British Museum.

[354]In the British Museum (Eastern Zoological Gallery, case table A) there are between thirty and forty recent terebratulæ (T. australis, Quoy, a plaited species, much resemblingT. fimbriaof the Inf. Oolite, Cheltenham) attached with their byssi to a block of stone, from Port Jackson, where it was found by Mr. Jukes just below low-water.

In the smoothTerebratulæ, the laminations of the shell are full of minute perforations, which may be seen by a lens of moderate power; the appearance of this structure,when highly magnified, is shownfig.2a,Lign.126.[355]The Rhynchonellæ (asLign.125,figs.1, 2,) do not possess this organization.

[355]An interesting Memoir on the Microscopal Examination of Shells has recently been communicated to the Royal Society by Dr. Carpenter.

Several species of Terebratula are found both living and fossil,e.g.Terebratula vitrea, living in the Mediterranean, fossil in Sicily,—T. caput-serpentis, recent in the British seas, fossil in the Crag,—andT. lenticularis, both recent and fossil in New Zealand.

Lign. 126. Terebratula and Spirifer.Fig.1 and 2.—Upper and under valve ofTerebratula carnea.Chalk; Lewes:a,a, remains of the calcareous support of thebrachia.2a.—Portion of the shell ofTerebratula carnea, magnified to exhibit the perforations.3.—Spirifer trigonalis, with part of the upper valve removed, to show one of the spiral processes. (Min. Conch.)Mountain Limestone.

SPIRIFER. PENTAMERUS.

Spirifer(containing spiral processes).Lign.126.—In the Silurian, Devonian, and Carboniferous limestones there is a profusion of several genera ofBrachiopoda, whosepeculiar forms render them easily recognisable. Among these, the Spirifers are the most interesting, on account of their spiral calcareous processes, which in the recent state supported the ciliatedbrachia, being often preserved. A specimen, in which part of the upper valve of the shell has been removed, and one of the spires exposed, is figuredLign.126,fig.3. (Wond.pp. 735, 736).[356]

[356]See a Memoir on the Anatomy of the Brachiopoda, by Professor Owen. Zoological Trans, vol. i. p. 145,et seq.

All these genera are extinct; they prevail in the oldest fossiliferous rocks, and gradually disappear as we ascend to the newer formations; the last trace of their existence is in the Lias, in which one species has been found. But theTerebratulæabound in the Lias, Oolite, Chalk, &c., occur in the tertiary formations, and several living species inhabit the seas around Australia and New Zealand. (Seeantep. 390.)

Rhynchonella,Fischer. The "plaited"Terebratulædiffer from the typical species (e.g.T. australis, caput-serpentis, vitrea, &c.) more than even the Spirifers differ, and must be regarded as forming a distinctfamily,Rhynchonellidæ, which will includePentamerus. The shell is not punctate; the arms are spiral, supported only at their origins by shelly processes; the larger valve isbeakedacutely, and has a notch within the beak through which the pedicle passes; sometimes the notch is converted into a foramen, by two little plates, (deltidium,) as inTerebratula. The form of the Rhynchonellæ is tetrahedral.Lign.125.

Pentamerus,Ly.p. 352.—With the Spirifers, and other Brachiopoda of the Silurian System, some bivalves which, in their general figure, resemble certain species of Terebratulæ, frequently occur. These shells differ in their internal structure from all other genera, in having a septum, or plate, by which their cavity is divided into four chambers; and in one valve the septum itself contains a cell, thus making five chambers, whence the namePentamerus(five-celled).The casts of these shells often have fissures, produced by the decomposition of the septa; and occasionally these cavities are occupied by calcareous spar. Specimens of this kind commonly split into two parts, in one of which two, and in the other three, chambers may be detected; the fifth chamber is the canal of the peduncle. Four species are known, and all belong to the Silurian rocks.

Orthis,Leptæna, andProductaform a third family, with horizontal spiral arms, unsupported by shelly processes.Davidsoniais a Leptænaattachedby the ventral valve, and the only genus in this family which is fixed bythe shell itself.

Calceola.a genus of Brachiopoda; the shell of an inverted pyramidal form, the upper valve nearly flat; found in the Devonian strata of the Eifel, and in Devonshire.

Crania,Ly.fig.205. These are small brachiopodous shells, attached to other bodies; very frequently to the Echinites of the chalk. The free valve is commonly wanting, but I have found specimens dispersed in the rock. In many of the quarries in Kent and Sussex, the helmet Echinites bear groups of these shells.Ly. fig.13.

Orbicula.This genus resembles Crania in form, the upper valve being like a limpet, whilst the attached valve is flat; it differs, however, from Crania in being horny and flexible, and is fixed to rocks on the bed of the sea, by a muscular pedicle passing out through a small fissure.

Species of Orbicula are found in strata of all ages, from the Lower Silurian to the Tertiary, and several are now living in tropical seas.

Obolus.Eichwald.In the Lower Silurian (Obolite grit) of Sweden and Russia, is a Lingula, with a hinge and a notch for the pedicle; it has not hitherto been found in Britain.

LINGULA. HIPPURITES.

Lingula.Ly.p. 353,fig.412.—The Brachiopoda referred to this genus have a long peduncle, and their respiratory apparatus has no calcareous support; the recent species burrow in the sand, being usually inhabitants of shallow waters. TheLingulæaære readily distinguished from the Terebratulæ by their imperforate, equivalved shells. One species is found in the Aymestry limestone, and several have been collected from the Mountain limestone, Oolite, and Shanklin sand.

With reference to the species of Brachiopoda, particularly of the Terebratulæ, which inhabit the depths of the ocean, Professor Owen observes, that "both the respiration and nutrition of animals, which exist beneath a pressure of from sixty to ninety fathoms of sea-water, are subjects suggestive of interesting reflections, and lead us to contemplate with less surprise the great strength and complexity of some of the minutest parts of the frame of these diminutive creatures. In the unbroken stillness which pervades those abysses, the existence of these animals must depend on their power of exciting a perpetual current around them, in order to dissipate the water laden with their effete particles, and to bring within the reach of their prehensile organs the animalcules adapted for their sustenance."

Hippurites.This genus belongs to a group of fossil shells whose characters are somewhat problematical, some conchologists referring them to the ordinary bivalves, and others to the Brachiopoda. AlthoughHippuriteshave not been discovered in the British strata, I am induced to notice them in this place, in consequence of their great abundance in the Cretaceous deposits of the South of France, and in the Oolite of the Pyrenees; and also to illustrate the nature of a nearly related genus,Sphærulites, of which one or more species occur in the Sussex Chalk.

The Hippurite is of an elongated conical form, and fixedby its base; it has internally a deep lateral channel, formed by two obtuse longitudinal ridges. The base is sometimes partitioned off by transverse septa, forming cells or cavities, as in the Euomphalus. The aperture, or opening, is closed by an operculum, or upper valve. The substance of the shell is cellular, and very thick, and when fractured much resembles that of the lamelliferous corals: the laminæ are sometimes separated into cells, or cavities, like the Spondyli. These shells often attain considerable magnitude, and in certain districts of the Pyrenees, where they abound, are called "petrified horns" by the inhabitants. It is remarkable, that, while in the Chalk of the South of France, Spain, Portugal, and Greece shells of this genus so prevail, as to be considered the characteristic fossils of the formation, in the North of France they are very rare, and in England have not hitherto been discovered.[357]

[357]As marking the rapid progress of Palæontology in this country, it may be noticed that theonly fossilfigured in the first edition of the Enclycopædia Britannica, in illustration of the article, "Petrifaction," is one of these supposed petrified horns, described by the Abbé Fortis.

Fossil Shells of the Lamellibranchia.—These are bivalve shells, the animals of which differ from the preceding class, as we have already stated, in performing respiration by means of lamellated gills. The valves are united by a strong substance, termed the ligament, which, by its elasticity, admits of the shells being opened to a considerable extent; and they are closed by powerful, short, thick muscles, called adductors. The shells of some of the genera, as the Oyster and Scallop, have but one muscle, (monomyaria); others, as the Cockle, or Cardium, and Venus, have two, (dimyaria); and by these characters the class is arranged in two groups.

Monomyaria:Bivalve Shells, with one muscular impression.

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