Chapter 23

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, calledPectunculi. 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 thecontrary, 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) figuredLign.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, calledOstrea 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 figuredLy.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 ofWhite, by its resemblance to the living "Cockscomb Oyster" of the West Indies; it is namedOstrea carinata, and figuredLy.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.

Lign. 127. Shells and Echinite from the Oolite and 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 theCidaris Blumenbachii.6.—Gryphya incurva.Lias.Cheltenham.7.—Ammonites Walcotii.Lias,near Bath.

GRYPHYÆA.

Gryphya.Lign.127,fig.6.—The shells to which the termGryphæa, orGryphites, 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 beennoticed 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 calledmarnes 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 nameExogyrawas 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.

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 figuredMin. Conch.tab. 78, and inGeol. 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.

Lign. 128. Spondylus spinosus.In Chalk-flint. Lewes.

In the cretaceous strata of North America, Dr. Morton has discovered a Spondylus (S. dumosus) very nearly related toS. 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 variusof 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]See Penny Cyclop. Art,Spondylidæ.

Plagiostoma, Llhwyd, 1699. This genus, adopted by Mr. Sowerby in the Mineral Conchology, is scarcely distinguishable fromLimaof Bruguiere (1791). Most of the recent species are ornamented with small asperities, from which the namelima(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 (seeFoss. 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 namedPecten 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).

Lign. 129 Inoceramus Cuvieri.Chalk. Lewes.Fig.1.—Beak and hinge of anInoceramus.a.The hinge line.2.—Two valves ofI. Cuvieri, displaced,and both showing the external surface.

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 cretaceousand 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 (seeLign.129,fig.1a.), and by the fibrous structure of their constituent substance, which closely resembles that of the recentPinna;[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 inLign.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 figuredFoss. South D.pl. xxvii. and inMin. 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 nameCatillus.[362]Pernaand 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.

[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 nameCatillus.

[362]Pernaand 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 ofInoceramus sulcatus, andI. concentricus(Wond.p. 330, fig. 1 and 3). In most examples the shell is inthe 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.

Lign. 130. Flint, with fragments of Inoceramus.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.

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 theNereis. The shells are often cellular from this cause, and the cavities are found either hollow, or filled with chalk, or, as in the exampleLign.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 inLign.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 ofClionitesfor 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 theMonomyaria, 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.

TheCardium,Venus, and Mussel shells, are familiar examples of theDimyaria. The conglomerates, now forming in the British Channel, from accumulations of the recent species of Cockle (C. edule), have been previously noticed; seeLign.124,p. 386. In the strata of Englandthere are upwards of thirty species: the Crag contains several, particularly a large and delicate shell, theCardium Parkinsoni(Min. Conch.tab. 49). Others are peculiar to the London clay, as theCardium 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: theCardium striatum, (Murch. Sil. Syst.tab. 6,fig.2,) found in the Aymestry limestone.

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 theCalcaire 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, calledPectunculi(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, resemblingthe commonArca.[366](SeeMin. 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, isP. 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 theN. bivirgataof the Galt of Folkstone, andF. Cobboldiæof the Norwich Crag.

The species ofNuculawith the posterior side produced into a long beak have been separated under the nameLeda; they have a pallial sinus, indicating a siphon to the mantle;—

PINNA. MYTILUS. MODIOLA.

Pinna.—The common largePinna, of the Mediterranean, is well known, and differs so entirely from other shells, as tobe 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 theCalcaire grossierof Grignon. Shells of this genus are very rare in the White Chalk, most of the supposedPinnæbeing imperfect examples ofInocerami; 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 resemblesPinna conica(of Röemer), and is related toP. lanceolataof 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 termedModiola, 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 striatedModiola(which may be namedM. 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 termedLithodomus. 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), figuredFoss. 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.

FOSSIL PHOLADES.

Pholas.Lign.166,fig.5, 6.—The common boring bivalve called Pholas, must have attracted the attention of every stroller by the sea-shore, from the numerous perforations in blocks of chalk, and other limestones, occasionedby its operations. Some species burrow in wood, and often commit serious ravages in piles and other submarine works constructed of timber. In the earlier ages of our planet we find evidence of the existence of the same kind of living instruments for the disintegration of floating wood, and the reduction of masses of rock into detritus. But no traces of these shells have been found in strata below the Oolite. One species occurs in the Coral Rag, another in the Kimmeridge Clay; two in the Galt and Greensand; and three or four in the tertiary deposits. In the Crag, blocks of stone are occasionally found with the shells ofPholadesoccupying the perforations they originally formed and inhabited. But all the specimens I have observed in the Galt, Greensand, and Oolite werexylophagous(wood-eating) species. In the Shanklin Sand, masses of fossil wood, literally honey-combed by the perforations of Pholades, are frequent; but the shells themselves are rare. Mr. Sowerby has figured a beautiful specimen of silicified wood, from Sandgate, with numerous shells of this genus (Pholas priscus.Min. Conch.tab. 581).Lign.166,fig.5, represents a fragment of fossil wood, with three shellsin situ;a, a shell seen longitudinally; and below, the rounded anterior extremities of two other shells are exposed.

Masses of wood perforated by Pholades, from which all traces of the shells have disappeared, have given rise to some curious fossil remains, which are often very enigmatical to the young collector. In the Kentish Rag, as for example, in Mr. Bensted's quarry, near Maidstone, large blocks of stone are found, covered with groups of subcylindrical mammillary projections, which are obtuse or rounded at the apex. In some examples the interstices between these bodies are free; in others they are occupied by a reddish brown, friable substance, presenting obscure indications of ligneous structure: and rarely, distinct woody fibres may be observed, the direction of which is transverse, ornearly at right angles, to the mammillated projections. These blocks are, in truth, the stony casts of cavities formed by Pholades, in masses of wood, both the vegetable structure, and the shells, having perished.

In the White Chalk specimens of this kind are occasionally found.

A remarkable fact, relating to some of the specimens from the Iguanodon quarry, remains to be mentioned. Upon breaking off the projections, to ascertain if any traces of the shells of the Pholades remained, we discovered in several, near the apex, a univalve shell, a species ofNerita.Lign.166,fig.6, represents a fragment of stone with two of the casts, which have been broken, and in each, ata, a univalve is imbedded. Atb, the ligneous structure of the original wood is visible. The only hypothesis that will account for the appearance of these univalves in their present position, is that of supposing that the Nerites crawled into the cavities made in the mass of timber, after the shells of the Pholades had been removed; and that the wood became imbedded in a sand-bank, and the univalves enclosed in the cavities; the ligneous structure in a great measure perished, and the stony casts of the perforations of the borers, with the imprisoned univalves, remained. The Nerites, as shown in the example figured, do not occupy any particular position in the tubes; one has the apex towards the end of the cavity, and the other lies in a transverse direction.[368]

[368]In a fragment of a perforated column, from Puzzuoli, in my possession, by favour of Sir Woodbine Parish, there were numerous living univalves in the cavities made and previously inhabited by the lithodomi.

TEREDO.

Teredo.Ly.p. 24.—It will be convenient to notice in this place another genus of boring shells, whose fossil remains are far more abundant than those of the Pholas. TheTeredo navalis, or Ship-worm, which is the most vermiformof all the mollusca, forms tortuous cylindrical hollows in wood; and in some climates commits the most extensive injuries to ships, the piles of harbours, bridges, and other submarine works formed of timber. A reference to the illustration given by Sir C. Lyell will render detailed description unnecessary. TheTeredois furnished at one extremity with testaceous valves, by which it bores its way into the wood, while from the surface of its soft body a calcareous matter is secreted, which lines with a shelly covering the hollows or channels formed by the animal in its progress. The fossil species differ from the recent in the valves being united to the calcareous tube. Wood perforated byTeredines, and occupied by their shelly tubes, occurs in almost every locality of the London Clay. Those specimens in which the wood is petrified, and the cavities of the tubes are filled with calcareous spar of various colours, furnish beautiful sections, when cut and polished (Pict. Atlas, pl. viii.fig.8, 9). When the canal in the Regent's Park was being formed, large blocks of perforated calcareous wood were discovered, having the ligneous structure well preserved, and the tubes of the Teredines occupied by yellow, grey, and brown spar, forming specimens of great beauty and interest. Wood, with Teredines, or some analogous boring mollusks, occurs sparingly in the chalk of this country; but in the cretaceous strata at Maestricht, large masses are frequently found.[369]Fossil wood may occasionally be observed with perforations that have been made by other kinds of boring shells; but the preceding remarks will suffice to convey an idea of the nature and origin of such appearances.[370]

[369]In the British Museum there is a mass of silicified wood from the Upper Greensand of Blackdown, perforated by a Teredo, whose valves remain in the burrows.[370]Other genera of boring shells also occur fossil, as Fistulana, Gastrochæna (Min. Conch.tab. 526), Saxicava (Min. Conch.tab. 466).

[369]In the British Museum there is a mass of silicified wood from the Upper Greensand of Blackdown, perforated by a Teredo, whose valves remain in the burrows.

[370]Other genera of boring shells also occur fossil, as Fistulana, Gastrochæna (Min. Conch.tab. 526), Saxicava (Min. Conch.tab. 466).

Trigonia.Lign.127,fig.1, 2, 4.—These bivalves are related to theArcadeæandNuculæ, but distinguished by the peculiar character of the hinge; the right valve has two large oblong teeth, which diverge from the umbo, and are strongly furrowed, and fit into two corresponding grooved cavities, in the opposite, or left valve. These shells are very thick and nacreous; they abound in certain strata of the Oolite and lower Cretaceous formation, but have not been observed in any deposits of this country older than the Lias; there are nearly thirty British species. Two living species of Trigonia (Trigonia margaritaceaandT. Jukesii) are known, both inhabitants of the seas of New Holland, where they are associated with Terebratulæ. Some of the argillaceous beds of the Oolite, as the Oxford and Kimmeridge clays, abound in Trigoniæ; Osmington and Radipole, near Weymouth, are celebrated localities for these fossil shells, which are found there in great perfection; and on the French coast, where similar strata appear, the Trigoniæ are equally abundant. Under the cliffs, near Boulogne harbour, the shore is strewn with them. Three common species are figured inLign.127. The casts of most of the species are smooth, as infig.2; and the collector should, therefore, search for impressions of the outer surface, when the shell is absent, as is generally the case in the Portland Oolite and Shanklin Sand, in which Trigoniæ are very numerous. Near Highworth, in Wiltshire, very fine and large examples ofTrigonia costata,fig.4, occur, with the shell preserved. The impressions of the large, oblong, diverging teeth of the hinge, are usually so strongly marked in the casts, as to render it easy to identify the shells of this genus. The quarries of the Portland Oolite at Swindon, Wilts, teem with casts of Trigoniæ, collocated with Ammonites. In the Isle of Portland they are also very numerous, some beds of stone being so friable, from the numerous cavities left by the removal of the substance of the shells, as to be unfitfor paving, or other economical purposes. Very sharp casts may be obtained from this rock by merely breaking the stone to pieces. In the Whetstone of Blackdown, Devon, beautiful silicified Trigoniæ are occasionally found. Tisbury, in Wiltshire, yields very fine specimens, and in some examples, Mr. G. B. Sowerby has detected remains of the ligament.

FOSSIL FRESH-WATER BIVALVES.

The animals of the shells hitherto described are, with scarcely any exception, inhabitants of the sea; and the marine origin of the strata in which they occur, may consequently be inferred, with but little probability of error. I now propose noticing the fossil remains of those bivalves which inhabit rivers, lakes, streams, and pools of fresh water. The marine, or fresh-water, character of fossil shells, is inferred from their resemblance to the recent mollusca, whose habits are known; for the shells alone present no unequivocal marks, by which even the experienced conchologist can pronounce whether an extinct form belonged to a marine or to a fluviatile mollusk, although certain characters may admit of an approximative inference. Thus, for instance, as none of the known living fresh-water bivalves belong to the previous division, theMonomyaria, the presence in a stratum of numerous shells with but one muscular impression, would afford a fair presumption of the marine origin of such, deposit. The remains with which the shells are associated and the mineralogical characters of the strata in which they occur, would, of course, afford important corroborative evidence.[371]

[371]See Sir C. Lyell on the distinction between fresh-water and marine deposits.Ly.p. 27,et seq.

The living fresh-water bivalves comprise but a few genera and species; and those which have been found fossil in the British strata belong to but four or five genera. Their distributionis restricted to strata of undoubted fluviatile origin, and to those local intercalations of fresh-water and land productions in marine deposits, which occur in some of the secondary, and in many of the tertiary formations.

Unio.Ly.p. 28.—The river Mussels, orUnionidæ, have a solid, pearly shell, with two principal and two lateral teeth on the hinge; and their umbones, or bosses, are generally smooth, or longitudinally undulated. Those which have no cardinal teeth are arranged under the genusAnodon: but it is not necessary for our present purpose, to enter into minute conchological distinctions. In number, variety, and beauty, the species which inhabit the large rivers of North America present a striking contrast with the few and homely British fresh-water mussels; nor have we, in a fossil state, any shells of this family at all comparable with those living types.[372]The earliest fossil Mollusca referred to the genusUnioappear in certain layers of clay and argillaceous ironstone belonging to the Carboniferous system of Derbyshire, Coalbrook Dale, &c. (Min. Conch.vol. i. tab. 33). In the former county, these strata are termedmussel-band;[373]and some beds constitute a compact shell-limestone, which admits of being manufactured into vases, &c., and takes a good polish; the sections of the shells in this marble are white, on a dark ground. There is, however, considerable doubt whether any of the Carboniferous shells really belong to the genusUnio; some geologists refer them toCardinia, a group of sea-shells found especially in the Lias.

[372]See American Journal of Science, vol. xlvii. p. 402, "Unionidæ."[373]"A solid stratum of ironstone, which extends from Tupton Moor to Staveley." Martin's Petrificata Derbiensia, pl. xxvii.

[372]See American Journal of Science, vol. xlvii. p. 402, "Unionidæ."

[373]"A solid stratum of ironstone, which extends from Tupton Moor to Staveley." Martin's Petrificata Derbiensia, pl. xxvii.

The earliest undoubted shells of this genus from the British strata, are, I believe, those first discovered by me in the strata of Tilgate Forest, (Foss. South D.p. 45, andFoss.Tilg. For.p. 57), and subsequently found in numerous localities of the Wealden.[374]

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