Chapter XI: Fossil Testaceous Mollusca, or Shells (1)

_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 in the 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).

[Sidenote: FOSSIL MOLLUSCA.]

The _Mollusca_, 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. the _Acephala_ and the _Encephala_.

I. The Acephala (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._ The Tunicata (from the elastic _tunic_, or _mantle_, in which they are enclosed) have no shell, and therefore do not come 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] The _Ischadites Königi_ of the Ludlow rock was supposed to resemble _Boltenia_, a pedunculated Ascidian.

_b._ The Brachiopoda (_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._ The Lamellibranchia (_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; the _beard_ of 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 called _monomyaria_, 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.

The _front_ of 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 be simply 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.

[Sidenote: FOSSIL BIVALVE SHELLS.]

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 of siphons.
_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 from _o_ to _p_, its breadth from _l_ to _n_.) ]

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 ligament towards the observer is in the same relative position as the person looking at it; viz. the head in 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 termed _Univalves_. 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 the _Limpet_ and _Chiton_. 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 of this 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.

[Sidenote: 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, or _Buccinum_. 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 the _Melaniæ_ have 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.

The number of living species of mollusca known to naturalists, not including the shell-less genera, exceeds twelve thousand; 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.

[Sidenote: 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 the sands 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.

Fig. 1.--Turritella conoidea; the perfect shell: _nat._

2.--Septarium, with Turritellæ; a polished slab: 1/3 _nat._

3.--A cast of one of the shells, in calcareous spar: _nat._
]

[Sidenote: 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, called _Turritellæ_. _Fig. 1_ is a perfect shell of the same species, extracted from soft clay; and _fig. 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.

[Sidenote: BRIGHTON SHELL-CONGLOMERATE.]

_Dredged off Brighton._

Fig. 1.--An Aggregation of Shells and Corals; the interstices are
filled up with sand, and the mass is consolidated by an
infiltration of carbonate of lime.

2.--Trochus ziziphinus; extricated from the mass with the
following:

3.--Pecten opercularis.

4.--Serpula.

5.--Portion of a Cellepora; magnified.

6.--Sabella.
]

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 by superincumbent 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.

Fig 1.--A mass of Cockle-shells and Whelks, consolidated into a
coarse limestone.

2, 4.--One of the shells, Cardium edule, extracted from the
block.

3.--A slice of the rock, polished, the markings on the surface
being derived from sections of the shells.
]

[Sidenote: MODERN SHELL-LIMESTONE.]

Off the Kentish coast, near the mouth of the Thames, a bank 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 the _Cardium 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 in _Lign. 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 of _Paludina vivipara_, _Ampullaria depressa_, some undetermined species of _Unio_, _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.

_Chalk. Lewes._

Fig. 1.--Rhynchonella plicatilis.

1_a_.--The same species, partly open.

2.--Rhynchonella subplicata.

2_a_.--Front view of the same.

3.--Terebratula semiglobosa; side view.

3_a_.--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.

[Sidenote: 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 the _Fossil Aristocracy_, from the incalculable antiquity of their lineage.

The species are very numerous; more than 300 extinct forms have been determined.[353] Those figured in _Lign. 125_ are 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 of _Terebratulidæ_, 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 resembling _T. fimbria_ of 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 smooth _Terebratulæ_, 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 shown _fig._ 2a, _Lign. 126_.[355] The Rhynchonellæ (as _Lign. 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,--and _T. lenticularis_, both recent and fossil in New Zealand.

Fig. 1 and 2.--Upper and under valve of Terebratula carnea.
_Chalk; Lewes_: _a_, _a_, remains of the calcareous support of
the _brachia_.

2_a_.--Portion of the shell of _Terebratula 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._
]

[Sidenote: SPIRIFER. PENTAMERUS.]

Spirifer (_containing spiral processes_). _Lign. 126._--In the Silurian, Devonian, and Carboniferous limestones there is a profusion of several genera of _Brachiopoda_, whose peculiar 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 ciliated _brachia_, 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 figured _Lign. 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 the _Terebratulæ_ abound in the Lias, Oolite, Chalk, &c., occur in the tertiary formations, and several living species inhabit the seas around Australia and New Zealand. (See _ante_ p. 390.)

Rhynchonella, _Fischer_. The "_plaited_" _Terebratulæ_ differ 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 distinct _family_, Rhynchonellidæ, which will include _Pentamerus_. The shell is not punctate; the arms are spiral, supported only at their origins by shelly processes; the larger valve is _beaked_ acutely, 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 in _Terebratula_. 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 name _Pentamerus_ (_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, and Producta form a third family, with horizontal spiral arms, unsupported by shelly processes. _Davidsonia_ is a Leptæna _attached_ by the ventral valve, and the only genus in this family which is fixed by _the 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.

[Sidenote: 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. The _Lingulæ_ aære 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. Although _Hippurites_ have 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 fixed by 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 the _only fossil_ figured 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_.

[Sidenote: FOSSIL OYSTERS.]

Ostrea, _Lign. 120._--The Oyster is well known to possess no power of locomotion; it is attached to rocks, pebbles, and other bodies, and forms extensive beds, consisting of numerous individuals, of all sizes. There are many fossil species; the British strata yield between forty and fifty. In some localities. Oysters are found in thick beds, of great extent, apparently on the spots they occupied when living. One of the most interesting localities I am acquainted with, is Sundridge Park, near Bromley, in Kent, where a hard conglomerate, entirely made up of oyster-shells, and the shingle that formed their native bed, is quarried. This stone is much employed for ornamental rock-work, and several walls in and near Bromley are constructed of it: these display the fossils, some with the valves closed, others open, others detached, and the whole grouped as if artificially imbedded to expose the characters of the shells. These oyster-beds belong to the tertiary strata of the London basin; they extend to Plumstead, and other places in the vicinity; and in some localities, the oysters are associated with other bivalves, called _Pectunculi_. In the tertiary clays near Woolwich and Bexley, fossil oyster-shells abound. In the neighbourhood of Reading, in Berkshire, an extensive layer of fossil oysters occupies the same geological position, namely, the lowermost sands and clays of the London basin. Wherever the strata around London are perforated to a sufficient depth, this oyster-bed is reached. Very recently an Artesian well was bored at Hanwell, in Middlesex, and at the depth of two hundred and eighty feet this stratum of sand with oyster-shells was found. At Headley, near Reigate, in Surrey, there is a similar deposit. These oysters very closely resemble the edible species.

The White Chalk contains several species of Ostrea, but I believe no beds of these shells have been found in it; on the contrary, the shells are diffused promiscuously through the strata. I have collected a few groups of from thirty to forty shells, evidently the young or fry of the species (_O. semiplana_) figured _Lign. 120_. This specimen is an interesting example of the petrifactive process which the mollusca have occasionally undergone; the soft parts of the oyster are transmuted into flint, and the shell is changed into carbonate of lime, having a crystalline structure. Both valves were perfect when discovered, but I chiselled off the greater part of one shell to expose the silicified body of the animal.

A small oyster, called _Ostrea vesicularis_, is a characteristic shell of the chalk; one valve is convex, the other flat; it is abundant in the Chalk of Norfolk, and also in the Firestone of some localities: it is figured _Ly._ p. 212. Another small species, having the margin plicated (_O. plicata_), is also frequent in the Chalk. A large shell, with the margins deeply indented by angular folds, resembling the recent cockscomb oyster, is abundant in the Chalk Marl and Firestone; particularly near Dover, and around Selbourne in Hampshire, where it attracted the notice of White, by its resemblance to the living "Cockscomb Oyster" of the West Indies; it is named _Ostrea carinata_, and figured _Ly._ p. 212, _fig._ 204. One other species may be noticed, the Ostrea deltoidea, which has been found in every locality of the Kimmeridge Clay in England and France. It is a very flat species, and of a triangular form; the specific name is derived from a supposed resemblance to the Greek letter Δ, _delta_. I believe that in England no shells of this genus have been observed in strata older than the Lias.

Fig. 1.--Trigonia clavellata. Oxford Cloy, _near Weymouth_.
2.--Trigonia gibbosa; a limestone cast. _Isle of Portland._
3.--Cidaris Blumenbachii. _Oolite._ _Calne, Wilts._
4.--Trigonia costata. _Oolite._ _Highworth, Wilts._
5.--Spine of the Cidaris Blumenbachii.
6.--Gryphya incurva. Lias. _Cheltenham_.
7.--Ammonites Walcotii. Lias, _near Bath_.

]

[Sidenote: GRYPHYÆA.]

Gryphya. _Lign. 127, _fig._ 6._--The shells to which the term _Gryphæa_, or _Gryphites_, is applied, are related to the Oyster, but distinguished by the deep concave under valve, and its curved summit, or beak, and the almost flat, or opercular upper shell. The Gryphites are of a finer laminated structure than the oysters, and the ligament of the hinge is inserted in an elongated curved groove. There are about thirty British fossil species, none of which have been noticed below the Lias, in which formation one very remarkable species is so abundant as to be considered characteristic of the Liassic deposits. It is so faithfully represented, _Lign. 127, _fig._ 6_, that description is unnecessary. In the upper argillaceous beds of the Oolite and Kimmeridge Clay, a very small gryphite, (_G. virgula_, _Ly._ p. 260_) is so abundant, that it constitutes entire layers. The low cliffs on the west of Boulogne harbour, like those near Weymouth, are composed of this clay, and myriads of the gryphites are scattered on the shore, with other shells of the same deposits; these shelly beds are called _marnes à gryphées_, by the French geologists. A very large gryphite, _Gryphæa sinuata_, (_Min. Conch._ tab. 336,) is found in the Shanklin sand of the Isle of Wight, and of Kent and Sussex. At low water, in the sand along the shore under Dunnose Cliff, near Shanklin Chine, numerous specimens are always obtainable.[358]

[358] The name _Exogyra_ was applied to the Chama-shaped species of Gryphæa by the late Mr. Sowerby, and other writers; but subsequent authors have included these shells in the present genus.

[Sidenote: SPONDYLUS. PLAGIOSTOMA.]

Spondylus. _Lign. 128._--A species of this genus is so frequent in the Chalk, that it ranks with certain Terebratulæ, as characteristic of that formation. One valve is covered with long slender spines, which, in the usual examples, are destroyed by the mode of extracting them. The specimen figured shows the appearance of a shell partly cleared; the remainder of the chalk might be removed by a penknife (taking care to leave the longest spines supported by brackets of chalk), and it would then resemble the beautiful fossils figured _Min. Conch._ tab. 78, and in _Geol. S. E._ p. 125. Between the beaks there is a triangular aperture in the spinous valve, which some naturalists, with much probability, suppose was once filled up with shell, as in the recent species.

In the cretaceous strata of North America, Dr. Morton has discovered a Spondylus (_S. dumosus_) very nearly related to _S. spinosus_; but it differs in its general form, and has both valves beset with strong spines. I have the fragment of a large bivalve from the Kentish Rag (Mr. Bensted's quarry), which has the peculiar structure of the Water-clam (_Spondylus varius_ of Mr. Broderip); namely, hollow interspaces formed by shelly layers or partitions, which were secreted by the posterior part of the mantle, or investing integument of the animal, as it gradually receded from that part of the shell. In the recent Water-clam the cells are full of fluid.[359]

[359] S ee Penny Cyclop. Art, _Spondylidæ_.

Plagiostoma, Llhwyd, 1699. This genus, adopted by Mr. Sowerby in the Mineral Conchology, is scarcely distinguishable from _Lima_ of Bruguiere (1791). Most of the recent species are ornamented with small asperities, from which the name _lima_ (file) is derived; they are symmetrical shells attached by a byssus.

Several smooth species of this genus are found in the Chalk,[360] Oolite, and Lias. A very large species (_P. giganteum_), sometimes ten inches in diameter, abounds in the Lias (_Ly._ p. 274). It is somewhat depressed in form, with the surface slightly striated; each valve has a pointed beak, with two lateral expansions, or ears, as they are termed by conchologists.

[360] See Foss. South Downs,, plate xxvi.

Plicatula, is another genus of this family, of which there are three British fossil species. A delicate shell, with slender depressed spines (_P. inflata_. _Foss. South D._ pl. xxvi.), occurs in the Chalk Marl. The recent species are natives of the seas of warm climates.

Pecten.--The common scallop-shell will serve as a type of this genus. The animals of these shells, unlike the oysters, have the power of locomotion, and when in the water, may be seen moving with rapidity, and flapping their shells to and fro with great activity. Numerous species are found fossil. In the Pliocene, and other marine tertiary deposits, Pectens abound; in the White Chalk there are several elegant forms (see _Foss. South D._ plate xxv.); many kinds in the Oolite and Lias; and several in the Devonian strata.

A large Mediterranean species (_Pecten Jacobæus_, _Ly._ p. 152) occurs in the Pliocene strata of Palermo, in every stage of growth, and as perfect as if recent. The Chalk and Shanklin sand contain a small inequivalved Pecten, the lower valve of which is convex, and pentangular, the upper flat, and both strongly ribbed, or pectinated; it is named _Pecten quinquecostatus_ (_Foss. South D._ pl. xxvi. _Ly._ p. 212); and in the cretaceous strata of North America a variety of this species is found.

In the Chalk Marl a large and beautiful Pecten (_P. Beaveri. Min. Conch._ tab. 158) is very common, and I have obtained from Hamsey and Southerham examples in the most perfect state of preservation; it is a characteristic shell of the Chalk Marl of England (_Foss. South D._ plate XXV. fig. 11).

Fig. 1.--Beak and hinge of an Inoceramus.
_a._ The hinge line.

2.--Two valves of I. Cuvieri, displaced, and both
showing the external surface.
]

[Sidenote: INOCERAMUS.]

Inoceramus. _Lign. 129._--This name, which refers to the fibrous structure of the shell, has been given to a fossil genus, of which there are about thirty species in the cretaceous and oolitic formations; and very recently four or five species have been discovered in the Silurian strata of Ireland.[361] These shells are chiefly characterized by their hinge (see _Lign. 129, fig. 1a._), and by the fibrous structure of their constituent substance, which closely resembles that of the recent _Pinna_;[362] and under the microscope is found, like that shell, to consist of prismatic cells, filled with carbonate of lime.[363] The species vary in size from an inch to three or four feet in diameter. The shell, in consequence of the vertical arrangement of the fibres, readily breaks to pieces, and it is often extremely difficult to extricate a specimen with the hinge and beaks tolerably entire. That they were equally brittle when recent is evident from the numerous fragments diffused through the chalk and flint, and occasionally imbedded in pyrites.[364] The form of the hinge is shown in _Lign. 129, fig. 1_: in the lower specimen two valves of the same individual are seen displaced, one lying over the other. The usual chalk species are figured _Foss. South D._ pl. xxvii. and in _Min. Conch._

[361] The term Inoceramus is restricted by the French geologists to the beaked and laminated species of the Galt; and the chalk Inocerami are arranged under the name _Catillus_.

[362] _Perna_ and all the Aviculidæ have the same structure, Inoceramus scarcely differs from Perna.

[363] Dr. Carpenter on the Microscopical Structure of Shells. To detect this structure, the shell should be immersed in diluted hydrochloric acid, and when partially dissolved, the cells will be apparent.

[364] It was many years before I succeeded in obtaining a specimen with the hinge perfect; and M. Brongniart, unable to obtain one from the chalk of France, gave the figure of this genus from my Foss. South D. pl. xxvii. in the Géog. Min. Env. de Paris.

In the Galt, or Folkstone-marl, two small species of this genus are to be found in every locality I have visited. They were first figured and described by Mr. Parkinson, under the name of _Inoceramus sulcatus_, and _I. concentricus_ (_Wond._ p. 330, fig. 1 and 3). In most examples the shell is in the state of a white, friable earth, and readily decomposes, leaving patches of iridescent nacre on the casts; but I have seen examples which prove that the originals were of a fibrous structure, like the Inocerami of the Chalk.

_Chalk. Lewes._

_a._ Marks the section of a fragment of shell, with numerous cavities,
occasioned by the depredations of Cliona Conybearei.

_b._ Portion of shell partially decomposed, and exposing siliceous,
globular bodies, connected by filaments, which are flint casts of
the hollows left by the Cliona.
]

The shells of the Inocerami, like those of the oyster, and other living mollusca, were exposed to the attacks of some parasite, and perhaps of some Annelid, as the _Nereis_. The shells are often cellular from this cause, and the cavities are found either hollow, or filled with chalk, or, as in the example _Lign. 130_, with flint. In the latter case, upon the decomposition of the shell, the siliceous casts remain in relief on the surface of the flint, as in _Lign. 130, b_. Such specimens are common in the broken flints of the South Downs, and in the shingle on the sea-shore of chalk districts; and their origin would be difficult to understand without this explanation.[365]

[365] The Rev. W. Conybeare first ascertained the origin of these fossils, and figured and described them in an elegant Memoir, published in Geol. Trans, vol. ii. first series. Mr. Morris proposes the name of _Clionites_ for the fossil bodies derived from the depredations of the Cliona on the Inocerami and other shells. See Annals Nat. Hist. 1851, and my Pictorial Atlas of Organic Remains.

Avicula. _Lyell_, p. 274.--Above fifty species of this genus of shells have been found in the British strata; their general character will be readily understood by reference to the pearl-oyster, (_Avicula margaritifera_,) which is so largely imported for the manufacture of mother-of-pearl ornaments. A remarkable species is found in the Lias, called, from the great disproportion in the size of the shells, _Avicula inæquivalvis_, (_Lyell_, p. 274.) The recent species are inhabitants of warm climates.

Our limits will not admit of further notice of the _Monomyaria_, and we proceed to the second division of the plated-gilled mollusca.

Dimyaria: _Bivalve Shells, with two muscular imprints_.

The conchifera, or bivalve shells, of this group, found fossil, are more than double in number those of the preceding; nearly eight hundred species are known in the rocks of Great Britain, of which by far the greater number is marine. But we must restrict our notice of this division to a few genera, that more space may be devoted to that important class, the Cephalopodous Mollusca.

The _Cardium_, _Venus_, and Mussel shells, are familiar examples of the _Dimyaria_. The conglomerates, now forming in the British Channel, from accumulations of the recent species of Cockle (_C. edule_), have been previously noticed; see _Lign. 124_, p. 386. In the strata of England there are upwards of thirty species: the Crag contains several, particularly a large and delicate shell, the _Cardium Parkinsoni_ (_Min. Conch._ tab. 49). Others are peculiar to the London clay, as the _Cardium semigranulatum_, a beautiful shell, having the surface smooth, except on the posterior side, which is covered with strong ridges, beset with minute granules; it is found in many localities (_Min. Conch._ tab. 144). Among the silicified shells of the Shanklin sand of Devonshire, an elegant Cardium, _C. Hillanum_, (_Min. Conch._ tab. 14,) occurs. But one species is known in the formations below the Lias: the _Cardium striatum_, (_Murch. Sil. Syst._ tab. 6, _fig._ 2,) found in the Aymestry limestone.

[Sidenote: VENERICARDIA. PECTUNCULUS.]

Venericardia. _Ly._ p. 199.--These shells are abundant in the tertiary strata; one large species, _V. planicosta_, (_Ly._ p. 199, _fig._ 171,) is found in immense quantities in the clay and sand at Bracklesham Bay, in Sussex, from the young to the adult state; some examples are very large, and perfect. In the sand at Grignon, near Paris, the same shell is abundant, possessing the usual white and delicate aspect of the fossils of that celebrated locality of the _Calcaire grossier_. Only one species has been noticed in the British secondary strata.

Pectunculus. _Wond._ p. 244, _fig._ 8.--In the London clay at Bracklesham Bay, Highgate, Hordwell Cliff, and in the arenaceous limestone of Bognor rocks, an immense number of the bivalve shells, called _Pectunculi_ (_little pectens_), occur. Some of the French marine tertiary strata also abound in the same, and other species of this genus. In the above-mentioned Sussex localities, these shells are so numerous, as to be the most frequent fossils that come under the notice of the collector. They are readily known from their associates by their rounded equivalve shells, and the single arched row of teeth along the hinge, resembling the common _Arca_.[366] (See _Min. Conch._ tab. 27). At Plumstead, near Woolwich, a smaller species is found; and also occasionally with the oysters at Bromley.

[366] The species so abundant at Bognor, is _P. brevirostris_, _Min. Conch._ tab. 472. I have seen a block of the limestone, in which, spread over an area of a foot square, there were upwards of fifty specimens lying in relief.

Nucula.--Several species of a small elegant bivalve, related to the preceding, but distinguished by having two rows of teeth on the hinge, diverging from an interspace between the beaks, are found in the Crag and other tertiary deposits (_Min. Conch._ tab. 180, 192). Two species occur in the Galt (_Foss. South D._ pl. xix. _fig._ 5, 6, 9), at Ringmer, Folkstone, Bletchingley, &c., sometimes with the shell perfect, but generally in the state of casts composed of indurated clay, and having impressions of the muscles and of the two rows of hinge-teeth. The shell of one species is marked with fine transverse grooves, or striæ (_N. pectinata_); the other is of a flattened ovate form, and the surface smooth (_N. ovata_).

The most beautiful species of Nucula are the _N. bivirgata_ of the Galt of Folkstone, and _F. Cobboldiæ_ of the Norwich Crag.

The species of _Nucula_ with the posterior side produced into a long beak have been separated under the name _Leda_; they have a pallial sinus, indicating a siphon to the mantle;--

_e.g._ _Nucula ovum_ Alum Shale.
_ -- claviformis_ Lias.
_ -- attenuata_ Coal Shale.
_ -- arctica_ Norwich Crag.

[Sidenote: PINNA. MYTILUS. MODIOLA.]

Pinna.--The common large _Pinna_, of the Mediterranean, is well known, and differs so entirely from other shells, as to be readily distinguished. There are about fifteen or sixteen British fossil species. The earliest appearance of this genus is in the Carboniferous Limestone of Derbyshire (Phil. York. tab. 6), in which there are two species. The Lias contains one species; the Oolite eight; the Cretaceous formation four; and the London clay two. One of the tertiary species, _Pinna affinis_ (_Min. Conch._ tab. 313), occurs in considerable numbers in the Bognor rocks, associated with Pectunculi; it varies in length from one to six or seven inches. A beautiful and delicate species is found in the _Calcaire grossier_ of Grignon. Shells of this genus are very rare in the White Chalk, most of the supposed _Pinnæ_ being imperfect examples of _Inocerami_; but I have seen specimens from Norfolk (collected by the late Mr. Woodward), and one from Sussex, in the cabinet of the Marquess of Northampton.[367]

[367] Dr. Lee has recently discovered in the Kimmeridge Clay on his estate at Hartwell, Bucks, a species of Pinna not previously observed in England. Professor Forbes informs me that it resembles _Pinna conica_ (of Röemer), and is related to _P. lanceolata_ of Sowerby, but appears to be distinct from both.

Mytilus, or Mussel.--There have been found about twenty species of this well-known genus of marine shells in the British strata. They are sparingly distributed through the several formations, from the Silurian to the newer Tertiary. One species (_Mytilus Lyellii_, _Wond._ p. 405, fig. 2) occurs in the Wealden, associated with fresh-water shells.

Of the genus termed Modiola, which comprises those mussels that have a rounded anterior termination, nearly forty British species have been discovered; ranging through the fossiliferous strata, from the Silurian to the Crag. A beautiful species (_Modiola elegans._ _Min. Conch._ tab. 9), with the shell generally retaining its pearly coat, is found in the London Clay, and in the limestone of Bognor.

An undescribed striated _Modiola_ (which may be named _M. striata_, since the striæ are peculiar), occurs in the Kimmeridge Clay, at Hartwell.

Those species of Modiola, which excavate hollows in stones, and inhabit them, are arranged in a genus termed _Lithodomus_. The occurrence of these shells in the remaining erect pillars of the Temple of Jupiter Serapis (_Wond._ p. 106), at Puzzuoli, has afforded important and unequivocal evidence of the physical mutations which that part of Italy has undergone. Two species of Lithodomi have been found, by Mr, Lonsdale, in the Oolite.

Pholadomya. _Ly._ p. 272, _fig._ 290.--This genus of shells (established by Mr. James De C. Sowerby in the Min. Conch. 1826), comprises about twenty British fossils, all of which, with but two exceptions, occur in the Lias and Oolite. They are equivalved shells, with the posterior end short, and rounded, and the anterior elongated and gaping. The surface is generally marked with ribs, or alternate elevations and depressions, diverging obliquely from the beaks to the margin. In the clay at Osmington and Radipole, near Weymouth, a large species (_P. æqualis_, _Min. Conch._ tab. 546) is abundant. The Oolite of Brora, in Scotland, contains several species. The only species found in our Chalk, is the beautiful shell (_P. decussatum_), figured _Foss. South D._ tab. XXV. _fig._ 3, and first discovered by me, in 1820, in a bed of Chalk Marl, which at that time was exposed at low-water, at the base of the cliff at Brighton, near the present entrance to the Chain-pier. The same species has since been found at Clayton, Hamsey, Southbourne, and other localities of the Marl.

[Sidenote: FOSSIL PHOLADES.]