Chapter XI: Fossil Testaceous Mollusca, or Shells (2)

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, occasioned by 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 of _Pholades_ occupying the perforations they originally formed and inhabited. But all the specimens I have observed in the Galt, Greensand, and Oolite were _xylophagous_ (_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 shells _in 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, or nearly 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 of _Nerita_. _Lign. 166, fig. 6_, represents a fragment of stone with two of the casts, which have been broken, and in each, at _a_, a univalve is imbedded. At _b_, 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.

[Sidenote: 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. The _Teredo navalis_, or Ship-worm, which is the most vermiform of 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. The _Teredo_ is 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 by _Teredines_, 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).

Trigonia. _Lign. 127, fig. 1, 2, 4._--These bivalves are related to the _Arcadeæ_ and _Nuculæ_, 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 margaritacea_ and _T. 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 in _Lign. 127_. The casts of most of the species are smooth, as in _fig. 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 of _Trigonia 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 unfit for 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.

[Sidenote: 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, the _Monomyaria_, 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 distribution is 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, or _Unionidæ_, 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 genus _Anodon_: 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 genus _Unio_ appear 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 termed _mussel-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 genus _Unio_; some geologists refer them to _Cardinia_, 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.

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, and _Foss. Tilg. For._ p. 57), and subsequently found in numerous localities of the Wealden.[374]

[374] They are figured in Geol. S. E. p. 250; and in Dr. Fitton's Memoir, Geol. Trans, vol. iv. pl. 21.

* * * * *

_Wealden; Isle of Wight._]

In 1844 I discovered a large species in the Wealden at Brook Point. I have named it _Unio Valdensis_.[375] I have collected and obtained nearly fifty specimens; they present two varieties, the one contracted and narrow, the other broader and deeper; this difference is probably sexual; the wide and deep shells may be the females; for in the living American Uniones the same characters are observed. Some examples are remarkably well preserved; the ligament remaining in a carbonized state, and the body of the mollusk in the condition of molluskite; even a tint of the original tawny reddish colour of the shell is present. The same species has been found in the Wealden strata, near Tunbridge Wells, by Mr. Barlow, C. E.

[375] _Unio Valdensis_ resembles in form the Mexican species, U. Panacöensis (River Panaco), but is probably more nearly allied to an unnamed Australian species of which Mr. G, Sowerby has numerous examples.

I shall reserve my remarks upon the important aid these fossils afforded in the determination of the fluviatile origin of the Wealden, for our _Excursion to Tilgate Forest_.

Cyclas. _Wond._ p. 404. _Ly._ p. 28.--Another genus of fresh-water bivalves is termed _Cyclas_, of which there are ten species in the Wealden formation: and, with the exception of four or five recent forms, which occur in the tertiary fresh-water strata, none others have been found in England.[376] The shells of the genus Cyclas are oval, transverse, equivalved bivalves, with the hinge-teeth very small: the substance of the shell is thin and fragile; the figures in _Wond._ and _Ly._ accurately represent the appearance of the fossil _Cyclades_ of the Wealden, and tertiary strata. Entire layers of two or three species of these shells occur in the argillaceous deposits of the Wealden, generally in a friable state, but from among the masses of crushed shells, perfect specimens may be obtained, and sometimes with the remains of the epidermis and ligament. The hard stone, termed calciferous grit, in the neighbourhood of Hastings, Tilgate Forest, Horsham, and other places in the Weald of Sussex, abounds in casts of the same species, associated with the _Uniones_, previously described. In the cliffs on the southern shores of the Isle of Wight where the Wealden beds emerge, and also in the Isle of Purbeck, these shells are equally abundant. Together with the _Uniones_, they occasionally appear in the limestone, called Sussex Marble; and in the Isle of Purbeck there are beds of limestone wholly composed of bivalves belonging to these two genera, and presenting, in polished slabs, markings formed by sections of the enclosed shells.

[376] _Cyrena_, is a genus so nearly related to Cyclas, that it is difficult to distinguish them, and it will be convenient to retain only the former name.

FOSSIL PTEROPODA.

[Sidenote: PTEROPODA. GASTEROPODA.]

In the Ludlow strata there are found small fragile elongated conical shells without chambers, which are supposed by Professor E. Forbes to be identical with a recent genus of pteropodous mollusca, common in the Mediterranean, called Creseis. They seldom exceed two inches in length.

Of another genus, named Conularia, six Species have been discovered in the Silurian formation.[377]

[377] See Geol. Trans, second series, vol. vi, p. 325.

Fig. 1.--Paludina fluviorum. _Wealden._

2.--Limnæa longiscata. _Tertiary. Isle of Wight._

3.--Cerithium lapidorum. _Tertiary, Grignon._

4.--Fusus contrarius. Crag. Essex.
]

FOSSIL SHELLS OF GASTEROPODA.

The univalve shells, as we have previously explained, are the calcareous cases, or coverings, of a more highly organized class of molluscous animals, than the inhabitants of the bivalves (see p. 366), for they possess a head and mouth with jaws, eyes, and feelers; and while the _Acephala_, with but few exceptions, are incapable of locomotion, the _Encephala_ are almost all of them furnished with organs of progression, and can creep, climb, and swim, or float on the surface of the water. Their shells are for the most part formed of one valve, hence the name of _univalve_; but in some species it is composed of several pieces. The most simple form of shell is that of the hollow cone, of which the _Patella_, or limpet, affords an example; and in the more complicated modifications, the cone is twisted, or convoluted spirally, either in the same plane as in the _Planorbis_ of our rivers, or obliquely, as in by far the greater number of species. The direction of the spire is generally from left to right, the aperture being dextral to the observer when the shell is placed with its apex uppermost, as in _Lign. 133, figs. 1, 2, 3_; but in a few species the spire turns in the opposite manner, and the mouth or aperture is to the left, or sinistral, as in _Lign. 133, fig. 4_. In consequence of the form of the aperture of the shell, the entire or notched condition of its margin, and the presence or absence of a canal or siphon always having relation to the soft parts of the animal, these characters afford data by which the genera and species of the shells may be determined, and information obtained as to the structure and economy of the originals.

The Gasteropoda generally creep by means of a fleshy disk, or foot, which is situated under the belly. Some kinds are terrestrial, others inhabit trees, many live in rivers and streams, others in stagnant and brackish waters; but the greater number are denizens of the sea.

[Sidenote: FOSSIL GASTEROPODA.]

The Common Snail, River Snail, and Periwinkle, are instances of terrestrial, fluviatile, and marine forms. The organs of respiration are situated in the last whorl of the shell; and in some genera the border of the mantle, or integument surrounding the body, is prolonged into a siphon, by which the water is freely admitted, without the head or foot being protruded: in these mollusks the shell has a corresponding channel to receive the siphon, as in the Whelk, or Buccinum, and in the fossil shell _Lign. 133, fig. 4_. The Gasteropoda are generally provided with an operculum, or movable valve, by which the aperture is closed and defended when the animal retreats within its shell. In some species the operculum is a mere horny pellicle; in others it is a solid calcareous plate of considerable relative thickness. These mollusca, as is but too well known of the terrestrial species, consume large quantities of food. Some are herbivorous, and others carnivorous; many prey on living, and others on decaying animal and vegetable substances.[378] As in a fossil state the shells alone remain to afford any clue as to the structure and economy of the originals, characters have been sought for, by which the fluviatile or marine nature, and the carnivorous or herbivorous habits of the living mollusca may be determined. As a general rule, it will be found, that the shells of terrestrial and fresh-water Gasteropoda have the aperture entire, as in the Garden Snail, and in the fossil shell, _Lign. 133, fig. 1_; and that a large proportion of the marine species have the opening notched or channelled, as in the Whelk, and _Lign. 133, figs. 3, 4_; and most of the species with entire apertures are herbivorous. But these inferences must be regarded in a very general sense, and it will require corroborative evidence to establish the marine or fresh-water nature of those fossil shells which do not bear a close analogy to known living species.[379]

[378] "All _Gasteropoda_ commence life under the same form, both of shell and animal, namely, a very simple helicoid shell, and an animal furnished with two ciliated wings or lobes, by which it can swim freely through the fluid in which it is contained. At this stage of existence the animal corresponds to the permanent state of the _Pteropod_, and the form is alike, whether it be afterwards a shelled or a shell-less species."--Prof. E. Forbes, Edin. Philos. Journal, vol. xxxvi. p. 326.

The well known Tiger Cowry (_Cyprcea tigris_) in its earliest stage has a minute helicoid (snail-like) shell.

[379] See _Ly._ p. 30.

The various conditions in which the remains of univalve shells occur in the mineral kingdom have already been so fully explained, that but a few additional remarks on that subject are required (see p. 382).

The Gasteropoda are found to progressively diminish in number with the antiquity of the deposits, and it was once supposed that this type of molluscous organization was not contemporaneous with the ancient Cephalopoda. My discovery of several genera associated with Ammonites in the chalk (see _Foss. South D._ pl. xviii, xix) first tended to invalidate this hypothesis; and the subsequent researches of Dr. Fitton, Professor Phillips, and other geologists have shown that the presence or absence of Gasteropoda in a stratum may generally be ascribed to the circumstance of the deposit having been formed in shallow, or in deep water. Thus when simple univalves largely predominate under circumstances that indicate they were imbedded in their native habitats, it may be safely concluded that the rock is of littoral formation; or, in other words, was deposited in shallow water, near the sea-shore; and, on the contrary, when Nautili, Ammonites, and the shells of other mollusca known to live in deep waters abound in a formation, it may be presumed that the strata were formed in the tranquil depths of the ocean. The number of described species from the British strata is nearly eight hundred; and these are distributed throughout the sedimentary formations, from the Silurian to the newest Tertiary; the latter containing by far the greater proportion.

[Sidenote: FOSSIL FRESH-WATER UNIVALVES.]

Fresh-water Univalves.--The fossil shells of Gasteropoda that are undoubtedly fluviatile, comprise but few genera and species, and are confined to those deposits, which, from the corroborative proofs afforded by other organic remains, are unquestionably of fresh-water origin. Such are the intercalated beds of clay and limestone in the London and Paris basins, the Wealden formation, and certain strata in the Carboniferous system. The most numerous specimens are principally referable to the common fluviatile genera, _Paludina_, _Limnæa_, _Planorbis_, and _Melanopsis_ (see _Ly._ p. 29).

Paludina. _Lign. 133, fig. 1._ (_Wond._ p. 401, _Ly._ p. 29.)--This common river shell is of a conoidal form, and the whorls of the spire, and the aperture, are rounded. Eleven British species are known. In the tertiary fresh-water beds of Headon Hill, at Alum Bay, Paludinæ with the shells perfect, and of a dull white colour, are abundant; and also in the limestone at Shalcombe, in the Isle of Wight, in the state of casts. In both these localities the Paludinæ are associated with other fresh-water shells. But the grand deposit of shells of this genus is the Wealden formation; throughout which there are extensive beds of marble, coarse limestone, and clays, almost wholly composed of Paludinæ, and minute fresh-water Crustaceans, of the genus _Cypris_, which will be described in a subsequent chapter. The compact paludina-limestone of Sussex, called Petworth or Sussex marble, is principally made up of one species, the _P. fluviorum_, _Lign. 133, fig. 1_, and is an aggregation of Paludinæ, held together by crystallized carbonate of lime; the cavities of the shells, and their interstices, being often filled with white calcareous spar. A polished slab, displaying sections of the enclosed shells, is figured in _Wond._ p. 402. Upon examining slices of this marble with the microscope, the cavities of the shells are found to contain myriads of the cases of Cyprides.[380] The Wealden limestone of the Isle of Purbeck, _Lign. 134_, known as Purbeck marble, is, in like manner, composed of Paludinæ, but of a much smaller species. Both these marbles were in great repute with the architects of the middle ages, and there are but few of our cathedrals and ancient churches which do not still contain examples, either in their columns, monuments, or pavements, of one or both varieties. The polished marble columns of Chichester Cathedral, and those of the Temple Church, in London, are of Purbeck marble; in other words, they are composed of the petrified shells of snails, that lived and died in a river, flowing through a country inhabited by the Iguanodon and other colossal reptiles, all of which have long since become extinct. With the exception of the _mussel-band_ limestone of the Carboniferous system, previously described, these are the only British fresh-water marbles[381] There are four species of Paludina in the Wealden, and four in the Tertiary strata of Hants.

[380] For a particular account of this marble, see Geol. S. E. pp. 182-187.

[381] The collector may obtain specimens, and polished slabs of these limestones, of Mr. Martin, mason, Lewes, Sussex.

Polished Slab of Purbeck Marble.]

Limnæa. _Lign. 133, fig. 2._--Several species of these fresh-water mollusks inhabit our lakes and ponds, and may be known by their pointed spire, elongated oval body, and delicate thin shell: on the inner lip of the aperture there is an oblique fold. Fossil shells of this genus are found with Paludinæ in the fresh-water tertiary deposits. Headon Hill and other localities in the Isle of Wight abound in these shells; and in the limestone of Calbourn beautiful casts are very numerous. The Paris basin yields several species; and there are six species in the Isle of Wight Tertiary; I have not observed any decided examples in the Wealden. In the sands and clays the shells are well preserved; in the limestones the casts only remain. Shells of another genus of fresh-water spiral univalves, termed _Bulimus_ (_Ly._ p. 30), are found associated with the above. A large species (_B. ellipticus_, _Min. Conch._ tab. 337), occurs in the limestone at Binstead, near Ryde, and at Calbourn; I have collected specimens two inches long from the former locality; they are generally in the state of casts, with a white friable coating of the shell.[382]

[382] A very large species of _Limnæa_ from Bavaria (labelled _L. maxima_) is in the British Museum. It is a cast six and a half inches long, and is placed with the recent shells. Prof. E. Forbes has discovered a Limneïd (_Physa_) in the Purbeck strata.

Planorbis. _Ly._ p. 29. _Wond._ p. 400.--The shells of this genus are also numerous in our rivers and lakes, and may be distinguished by their discoidal form, the shell being coiled up in a nearly vertical plane. There are about twenty living species; and sixteen are enumerated as fossil in the British tertiary; five occur in the Isle of Wight basin, in the localities of the fresh-water genera already mentioned; Headon Hill, in particular, yields shells of this genus in great abundance and perfection.

Melanopsis. _Ly._ p. 29.--These are spiral univalves, the appearance of which will be better understood by the figures, than by any description. I allude to this genus because a small species is very numerous, with the other fresh-water shells, at Headon Hill; and two or more species are found in the argillaceous strata of the Wealden (see _Geol. S. E._ p. 249, and _Lign. 132_).

[Sidenote: FOSSIL MARINE UNIVALVES.]

Marine Univalves.--Of the fossil marine Gasteropoda there are no less than eighty genera in the strata of the British Islands, and the species amount to several hundreds. To distinguish the species and genera, reference must, of course, be made to works expressly devoted to fossil conchology, as Sowerby's _Mineral Conchology_, and _Genera of Fossil Shells_; or to the works of French authors, particularly those of Lamarck, edited by M. Deshayes, and of M. Blainville. The _Penny Cyclopedia_ contains admirable notices of fossil shells, under the respective heads of the classes, orders, and genera, of the recent Mollusca.

Buccinum, of which the common Whelk is an example.--Fusus, _Lign. 133, fig. 4_. _Wond._ p. 244.--Pleurotoma, _Ly._ p. 31. _Wond._ p. 244.--Cerithium, _Lign. 133, fig. 3_. _Wond._ p. 244.--Ancilla, _Wond._ p. 244. _Ly._ p. 31.--Voluta, _Ly._ p. 202, _fig._ 180.--Murex, _Ly._ p. 164.--Rostellaria, _Ly._ p. 201.--To the eight genera here enumerated a very large number of the marine simple univalve shells belong; and they are principally found in Tertiary strata.

The animals of these shells are characterized by their respiratory organs, which are formed of one or two pectiniform gills, with a tube or siphon more or less elongated, for the free admission of sea-water to the branchial apparatus. This organization is indicated in the shell, either by a notch, or by a prolonged tubular canal. All the species are, with scarcely any exceptions, inhabitants of the sea, and carnivorous.

I have selected for illustration of the genus _Fusus_, a celebrated shell of the Crag, known among collectors as the "_Esssex reversed Whelk_," _Lign. 133, fig. 4_; the spire is twisted in the opposite direction to the usual mode, and the mouth is consequently to the left of the observer; the same species occurs with the spire in the common direction. The shells of the genus _Pleurotoma_ are distinguished by an incision, or notch, in the side of the right or outer lip; and those of _Cerithium_, by the form of the mouth, see _Lign. 133, fig. 3_. The latter is a very numerous genus, and more than two hundred fossil species are enumerated; it contains many elegant forms. The Tertiary strata at Grignon are particularly rich in these fossils; the shells are of a pearly whiteness, and as perfect as when recent. Some Cerithia are of considerable size; the _C. giganteum_ is from ten to fourteen inches in length. The genus Potamides comprehends shells closely resembling the Cerithia in form, but which are inhabitants of fresh-water.[383] This is an instance of the difficulty which sometimes exists of arriving at certain conclusions as to the habits of the mollusks, from their testaceous coverings alone.

[383] Mr. Woodward informs me that they can only be distinguished when _fossil_, by the absence of _varices_, or "periodic mouths." The recent species are known to be inhabitants of fresh-water, by their dark epidermis, corroded points, and horny multi-spiral _opercula_.

The Plastic Clay beds at Castle Hill, Newhaven, and in the vicinity of Woolwich, abound in two species of shells, which were originally described by Mr. Sowerby, as _Cerithia_ (viz. _C. funatum_ and _C. melanoides_),[384] but are now referred to the fresh-water genus, Melania; by some conchologists to _Potamides_. At Castle Hill they are accompanied by fresh-water bivalves, and leaves of dicotyledonous plants.

[384] Foss. South Downs, tab. xvii. figs. 3, 4.

Of the genus _Rostellaria_, there is a remarkable species in the London Clay, called _R. macroptera_, from the large wing-like expansion of its outer lip, in adult specimens; see _Ly._ p. 201. An elegant Rostellaria is found in the Galt, at Folkstone,[385] (_Foss. South D._ tab. xix. figs. 12, 14,) and other localities; and also in the Chalk Marl.

[385] "This shell belongs to the recent genus, _Aporrhaïs_, and is related to Cerithium, not to Strombus."--_Mr. Woodward._

* * * * *

Casts of a large ventricose, globular univalve, called Dolium,[NV] have been found in the Chalk Marl, at Clayton, near Hurstpierpoint, in Sussex. This species is distinguished by its transverse tuberculated bands; it is a very rare production of the lower chalk of Sussex (_Min. Conch._ tab. 326). Turbinated shells related to _Trochus_, and belonging to several genera, occur in the Cretaceous deposits. As is the case generally with the univalves of this formation, but slight traces of the shells remain; the thin internal nacreous lining is sometimes found adhering to the cast.

[386] This Chalk fossil is not a _Dolium_: it is probably related to _Ringinella incrassata_ (Geol. Suss. t. xix. _fig._ 3), one of the _Tornatellidæ_, a family largely developed in the chalk.

In the Chalk of Touraine, species of the genera _Conus_ (_Lign. 135, fig. 1_) and _Solarium_ (_Lign. 135, fig. 2_) are found with the shells preserved. The specimens figured, _Lign. 135_, are selected to familiarize the student with the difference so commonly observable, between the outer surface of the casts, and that of the shells: in both these fossils the shells are marked externally with lines and tubercles; but the casts present only the smooth surface of the interior of the shell in which they were moulded.

Fig. 1.--Conus tuberculatus, with part of the shell remaining
attached to the cast.

2.--Solarium ornatum, with the shell.

2_a_.--Specimen of the same species, deprived of the shell.
]

In the most ancient fossiliferous formations, the Carboniferous; Devonian, and Silurian, many species and genera of Gasteropoda have been discovered. Professor Phillips enumerates more than ninety in the mountain limestone of Yorkshire (_Phil. York._), belonging to the genera _Turbo_, _Pleurotomaria_, _Natica_, _Euomphalus_, _Loxonema_, _Macrocheilus_, _Platyceras_, and _Metoptoma_. Thirty-four species from the Silurian rocks are figured and described in _Murch. Sil. Syst._ p. 706.

The _Natica_, _Lign. 136, fig. 3_, sometimes attains thrice the size represented, and has been found in many localities in England and Ireland.

Pleurotomaria. _Lign. 136, fig. 4._--This is an extinct genus, distinguished from Trochus by a fissure on the right lip, the position of which is indicated by the band along the back of the whorl in _Lign. 136_; several species occur in the Mountain Limestone; the markings of the original shell are sometimes preserved, as in the example delineated This genus is common in the Oolite; a splendid species, with the shell entire, is found in the Kimmeridge Clay, at Hartwell; limestone casts of the same species are abundant in the Portland stone at Swindon, in Wiltshire.

Fig. 1.--Euomphalus pentangulatus; Upper surface.

2.--Polished section of the same species.

3.--Natica plicistria. _Yorkshire. Mt. L._

4.--Pleurotomaria flammigera. (_Phil. York._) _Mt. L._
]

There are two species of this genus (formerly named _Cirrus_ by Mr. Sowerby) which are of frequent occurrence in the White Chalk of England, in the state of casts, and are figured in my _Foss. South D._ tab. xviii., under the names of _Cirrus perspectivus_, and _Trochus linearis_. The Chalk Marl of Sussex yields in some localities (Hamsey, Middleham, Clayton) fine casts of Pleurotomaria, which appear to be distinct from those of the upper cretaceous strata.

[Sidenote: EUOMPHALUS.]

Euomphalus.[387] _Lign. 136, figs. 1, 2._--The shells of this extinct genus are deeply umbilicated, discoidal, spiral univalves, having the innermost whorls of the shell divided by imperforated partitions. The internal structure of these shells will serve to prepare the student for those more complicated forms of the testaceous apparatus presented by the Cephalopoda, which will form the subject of the next chapter. There are several recent univalves the animals of which retreat in the progress of growth from the apex of the spire, and the vacated portion is shut off by a shelly plate. In some genera a series of concave septa are thus formed; but in others (as _Magilus_) the deserted cavity is filled by a compact accretion of calcareous matter, and a solid elongated shell is produced. The _Euomphalus_, of which there are many species in the Silurian, Devonian, and Carboniferous strata, belongs to the former group. As the animal increased in size, it deserted the smaller and innermost portion of the spire, and a nacreous partition was secreted by the posterior part of the mantle, the interspace remaining hollow; as this process was repeated at different periods, several cells were successively formed. This chambered structure is shown in the specimen _Lign. 136, fig. 2_, in which the cells are filled with spar, but the outer cavity is occupied by limestone like that in which the shell was imbedded; a proof that no communication existed between the chamber occupied by the body of the animal, and the space from which it had withdrawn. The calcareous spar, as in the vegetable remains previously described (p. 71), has percolated the substance of the fossil, and crystallized in the innermost cells. We shall again have occasion to refer to this interesting fact, when investigating the chambered cells of the Cephalopoda. It may be necessary to remark, that it does not appear that the vacant interspaces in the Euomphalus served the special purpose of the air-chambers of the Nautilus and Ammonite.

[387] So named by Mr. Sowerby, in allusion to the deeply umbilicated character of the disk.

Murchisonia angulata.

_Devonian; Eifel_.]

Murchisonia. _Lign. 137._ An elongated spiral shell, having the outer lip deeply notched, as in the Pleurotomaria (_a, Lign. 137_). There are upwards of 50 species of this genus, which are characteristic of the palæozoic rocks. They occur in the Permian, Devonian, and Lower Silurian deposits; the specimen figured is from the Devonian, or Old Red of the Eifel.

Chiton. Valves of Chitons have been found in the Magnesian limestone, near Sunderland, by Prof. King, (_Permian Fossils_, _Pal. Soc._ p. 202, pl. xvi.), and in the Silurian rocks of Ireland, by Mr. Salter, _Geol. Journal_, vol. iii. p. 48.

[Sidenote: SPHÆRULITES.]

Sphærulites.[388] _Lign. 138._--No vestiges of a shell of this genus had been noticed in the English strata, until my discovery of some fragments in the Lewes Chalk in 1820; from the lamellated structure of these fossils, I mistook them for corals, until specimens were obtained sufficiently perfect to show the form of the originals; these were described in the _Geol. S. E._ (p. 130), under the name of Hippurites. But these fossils are more nearly related to the _Sphærulites_, which differ from the shells of the former genus in having only one internal longitudinal ridge, and in the external surface being roughened by irregularly raised plates, as in _Lign. 138, fig. 1_, which is a specimen from the Pyrenees, collected by M. Alex. Brongniart; the operculum is seen at _a_.

[388] This genus has been referred by some conchologists to the Bivalves, and by others to the Univalves.

Fig. 1.--Sphærulite, with its _operculum_, _a_.

2.--Sphærulites Mortoni (G. A. M.), _from Lewes_: 1/2 _nat._

2_a_.--Cellular structure of _fig._ 2, in a transverse section: ×

2_b_.--Structure, as seen in a vertical section: ×
]

The species found in the Sussex Chalk, _Lign. 138, fig. 2_, is characterized by the longitudinal striæ on the outer surface. In some examples there is an external longitudinal furrow, and a corresponding internal ridge.[389]

[389] The specific name is in honour of Dr. George Morton, of Philadelphia, author of the "Synopsis of the Cretaceous Group of the United States."

The Sphærulites sometimes occur in groups in the Sussex chalk; I had a large water-worn mass, consisting of five or six individuals, anchylosed together. Some beautiful specimens collected by the late Mr. Dixon from the Chalk, near Worthing, are now in the British Museum.[390] The structure of the Sphærulite is accurately delineated in _Lign. 138, figs. 2a 2b_. The cavities of these shells are occasionally filled with flint, but in general with chalk, which may be entirely cleared away, as in _fig. 2_. The Hippurites of the limestone of the Pyrenees are frequently occupied by calcareous spar, and the substance of the shells is occasionally transmuted into the same mineral.

[390] Petrifactions, p. 468.

* * * * *

[Sidenote: MOLLUSKITE.]

Molluskite; or the carbonized remains of the soft parts of mollusca.--Before proceeding to the consideration of that numerous and important division of the mollusca the _Cephalopoda_, I will offer a few remarks on a carbonaceous substance resulting from the gelatinous matter of which the soft bodies of these animals are composed, and for which I have proposed the name of _molluskite_, to indicate its nature and origin.

Fig. 1.--Coprolite of a fish (_Macropoma_). _Chalk, Lewes._

2.--Coprolite of a fish (_Squalus_). _Chalk marl. Ramsey._

3.--Molluskite of a _Rostellaria_. (_Mr. Bensted._) _Kentish Rag,
Maidstone._
]

This substance is of a dark brown or black colour, and occurs either in shapeless masses, which are irregularly distributed among the shells and other organic remains, in sandstone, limestone, &c., or as casts of shells, or occupying their cavities, as in the specimen _Lign. 139, fig. 3_, which is a vertical section of a spiral univalve (_Rostellaria_), filled with the soft parts of the animal, converted into molluskite. Upon analysis this substance is found to contain a large proportion of animal carbon.[391] The rocks of firestone at Southbourne, on the Sussex coast, are mottled with brown molluskite and hard amorphous concretions, consisting of carbon and phosphate of lime, mixed with sand and other extraneous matter. Casts of shells, of the genera Venus, _Arca_, &c., entirely composed of the same kind of materials, are also abundant in those rocks. The lowermost bed of Galt, at its line of junction with the Greensand beneath, at Folkstone, and in many other localities, is largely composed of similar matter, resembling in appearance the fossils called Coprolites, hereafter described. The outer chamber of the Ammonites and other shells, so abundant in the Galt, are often filled with this substance. But the most interesting deposit of molluskite is in the Kentish Rag of Mr. Bensted's quarry, near Maidstone. This phenomenon had not escaped the notice of that intelligent and accurate observer, who liberally placed at my disposal numerous shells, particularly of _Trigoniæ_ and _Terebratulæ_, which were filled with molluskite, and large slabs of the sandstone, full of concretionary and amorphous masses of the same. The latter, Mr. Bensted suggested, may have been derived from the soft bodies of the dead Mollusks, which, having become disengaged from their shells and aggregated together, had floated in the sea, until they became enveloped in the sand and mud, which have gradually consolidated into the arenaceous stone termed Kentish Rag, In illustration of this opinion, Mr. Bensted directed my attention to the following remarkable fact, related in the _American Journal of Science_:--In the year 1836, a fatal epidemic prevailed among the shell-fish of the Muskingum River, in the state of Ohio. It commenced in April, and continued until June, destroying millions of the mollusca that inhabited the beds of the tributary streams, and the river. As the animals died, the valves of the shells opened, and, decomposition commencing, the muscular adhesions gave way, and the fleshy portions rose to the surface of the water, leaving the shells in the bed of the river. As masses of the dead bodies floated down the current, the headlands of islands, piles of drifted wood, and the shores of the river, in many places, were covered with them; and the air in the vicinity was tainted with the putrid effluvium exhaling from these accumulations of decomposing animal matter. The cause of the epidemic was unknown.

[391] Some of this _molluskite_ has, at my request, been analyzed by Mr. Rigg, who obliged me with the following remarks:--"After removing the lime by means of hydrochloric acid from ten grains of this substance, there remained 1.2 grain of dark powder, which gave, by analysis with oxide of copper, .16 of a cubic inch of carbonic acid, and a small portion of nitrogen. On subjecting to the same kind of analysis two grains of the darker body, without previously acting upon it by any acid, .054 of a cubic inch of carbonic acid was obtained; so that from these results there is no doubt but the darker portion of the molluskite contains about .35 per cent, of its weight of carbon in an organized state."

"Now nearly the whole of the shells in the beds of Kentish Rag," Mr. Bensted remarks, "have their shells open, as if they were dead before their envelopment in the deposit. And, from the large quantity of water-worn fragments of wood perforated by Pholades imbedded with them, it seems probable that this stratum had originally been a sand-bank covered with drifted wood and shells, thus presenting a very analogous condition to the phenomenon above described." The gelatinous bodies of the _Trigoniæ_, _Ostreæ_, _Rostellariæ_, _Terebratulæ_, &c., detached from their shells, may have been intermingled with the drifted wood in a sand-bank; while, in some instances, the animal matter would remain in the shells, be converted into molluskite, and retain the form of the original, as in the spiral univalve, represented in section, _Lign. 139, fig. 3_.

* * * * *

A microscopical examination of the Maidstone molluskite detects, with a low power, innumerable portions of the nacreous laminæ of shells, intermingled with the carbonaceous matter, many siliceous spicula of Sponges, minute spines of Echinoderms, and fragments of Corals; these extraneous bodies probably became entangled among the floating animal matter. A large proportion of the shelly laminæ, examined with a high power, displays the peculiar structure of the _Terebratulæ_ (see _Lign. 126, fig. 2a_), of which several species are abundant in the Kentish Rag.

The dark masses and veins so common in the Sussex and Purbeck marbles are produced by molluskite. If at the period of their envelopment the shells were empty, they became filled either with grey marl and limestone, or with white calcareous spar; but if they enclosed the bodies of the Mollusks, the soft mass was changed into carbonaceous matter; and in polished sections of the marble, the _molluskite_ appears either in black or dark brown spots, or fills up the cavities of the shells. The dark blotches and veins observable in the fine pillars of Purbeck marble in the Temple Church, London, are produced by molluskite; and the most beautiful slabs of Sussex marble owe their appearance to the contrast produced by this black substance in contact with white calcareous spar.[392]

[392] See a "Memoir on the Carbonized Remains of Mollusca," by the author. Read before the Geological Society of London, February, 1843; and published in the American Journal of Science.

Carbon, resulting from animal remains, is of frequent occurrence in many strata; and the fetid emanations from certain limestones, upon being broken or rubbed, are attributable to the evolution of sulphuretted hydrogen, from the animal matter which they contain.

* * * * *

[Sidenote: GEOLOGICAL DISTRIBUTION OF SHELLS.]

Geological Distribution of the Bivalve and Univalve Mollusca.--If the more rare and splendid organic remains may be regarded as the "Medals of Creation," the fossil testaceous mollusca, from their durability, numbers and variety, may be considered as the current coin of Geology. Occurring in the most ancient fossiliferous strata in small numbers, and of peculiar types,--becoming more abundant and varied in the secondary formations,--and increasing prodigiously, both numerically and specifically, in the tertiary, these relics are of inestimable value in the identification of a stratum in distant regions, and in the determination of the relative age of a series of deposits. To the solution of the former problem the sagacity of the late Dr. William Smith first suggested their applicability;[393] while the idea, so happily conceived, and so philosophically candied out, by Sir C. Lyell, of arranging that heretofore chaotic mass of deposits, termed the Tertiary, into groups, by the relative number of recent and extinct species of shells, demonstrated the important aid to be derived from this class of organic remains, in the determination of some of the most difficult questions in geological science.

[393] See an interesting memoir of Dr, Smith, from the pen of his distinguished nephew, Professor Phillips.

Many useful tables have been constructed by Professor Phillips,[394] Sir C. Lyell, M. Deshayes, M. D'Orbigny, Prof. E. Forbes, and other eminent observers, to illustrate the geological distribution, in the several formations, of the genera and species of fossil shells hitherto described. To the English student, Mr. Morris's "Catalogue of British Fossils," of which an enlarged edition is in the press, will be the most valuable for reference. In the works which we have especially recommended for reference (_ante_, p. 10), figures are given of some of the characteristic shells from each formation, as follow; commencing with the most ancient deposits.

[394] A Treatise on Geology; and Art. _Geology_, Encyclopæd. Metropolitana.

Silurian System. _Ly._ p. 350.

Orthis orbicularis; _Ly._ fig. 409.
------ grandis;--fig. 427.
Terebratula navicula;--fig. 410.
------------ Wilsoni;--fig. 413.
Pentamerus Knightii;--fig, 411.
---------- lævis;--fig. 426.
Atrypa reticularis; fig. 414. _Wond._ p. 786.
Lingula Lewisii; _Ly._ fig. 412.
Strophomena depressa;--fig. 421.

Devonian System. _Ly._ p. 342.

Calceola sandalina; _Ly._ fig. 403.
Stringocephalus Burtini;--fig. 404.
Megalodon cucullatus;--fig. 405.

"The Silurian System," by Sir R. I. Murchison, a splendid work on the rocks and fossils of the above formations, contains numerous figures of the shells peculiar to each group of strata; and many other species are delineated in the Memoir on the Devonian deposits of Devonshire and Cornwall, by Sedgwick and Murchison, _Geol. Trans. New Series_, vol. v. plates lii-lvii. A Memoir on the Palæozoic Rocks of Germany and Belgium, by the same distinguished geologists, is also accompanied by many figures of fossil shells belonging to the same geological epochs. _Geol. Trans. New Series_, vol. vi.

See also Prof. M'Coy's "Silurian Fossils of Ireland," and his Description of the British Palæozoic Fossils in the Woodwardian Museum at Cambridge, in Prof. Sedgwick's "Synopsis of the Classification of the British Palæozoic Rocks," of which two Parts are already published.

Carboniferous System. _Ly._ 308. _Wond._ p. 736.

Producta punctata; _Wond._ p. 736.
-------- Martini; _Ly._ fig. 390.
Pleurotomaria flammigera; _Lign._ 136, fig. 4.
Euomphalus pentangulatus; _Lign._ 136, fig. 1.
Natica plicistria; _Lign._ 136, fig. 3.
Spirifera trigonalis; _Wond._ p. 736.
--------- triangularis; _Wond._ p. 736.
--------- glabra; _Ly._ p. 389.
Serpula carbonaria; _Ly._ fig. 375.
Avicula papyracea;--fig. 378.

For the shells of the Mountain Limestone, reference should be made to the second vol. of Prof. Phillips's "Geology of Yorkshire;" to Prof. M'Coy's "Carboniferous Limestone Fossils of Ireland;" and to Prof, de Konick's "Anim. Foss. Belg." The fossils of other portions of the Carboniferous System are illustrated in Phillips's "Palæozoic Fossils of Devon;"[395] and in Prestwich's Memoir on Coalbrook Dale (_Geol. Trans._).

[395] To prevent confusion, it may be necessary to state that Professor Phillips, in the work referred to, terms the Silurian strata the "_lower palæozoic_" and the mountain limestone, the "_upper palæozoic_"

Magnesian Limestone and Trias. _Ly._ p. 301.

Producta calva; _Ly._ p. 203, fig. 337. } Permian.
Spirifera undulata;--fig. 338. }

Posidonia minuta;--p. 288, fig. 321. } Triassic
Avicula socialis;--fig. 322. }

Prof. King's elaborate Monograph on the Permian Fossils (published by the Palæontographical Society) should be consulted by the student.

Lias. _Ly._ p. 273.

Pleurotomaria Anglica; _Ly._ p. 39.
Avicula inæquivalvis; _Ly._ fig. 302.
Plagiostoma giganteum; _Ly._ fig. 303.
Gryphæa incurva; _Lign._ 127.

Oolite. _Ly._ p. 257.

Gryphæa virgula; _Ly._ fig. 268.
Ostrea deltoidea (Kimmeridge Clay);--fig. 269.
Trigonia gibbosa;--fig. 270.
-------- clavellata; _Lign._ 127.
-------- costata; _Lign._ 127.
Nerinæa Goodhallii; _Ly._ fig. 274.
Diceras arietinum;--fig. 275.
Pleurotomaria;--fig. 299.
Terebratula spinosa;--fig_ 297.
----------- digona;--fig_ 283.
Ostrea Marshii;--fig. 300.
Phasianella Heddingtonensis;--fig. 58.

Many of the characteristic shells of the Oolite and Kimmeridge Clay, are figured in Plates XXII. and XXIII. of Dr. Fitton's Memoir on the Strata below the Chalk; _Geol. Trans. New Series_, vol. iv.

The fossil shells of the Great Oolite are figured and described by Messrs. Morris and Lycett, in the Memoirs of the Palæontographical Society; and valuable Papers on the Brachiopods of the Oolite and Lias, by Mr. Davidson, have been published by the same Society.

Wealden and Purbeck. _Wond._ vol. i. _Geol. S. E._, _Foss. Tilg.
For._, and _Ly._ p. 225.

Melanopsis; _Wond._ pp. 401 and 404.
Cyclas;--p. 404.
Paludina Sussexiensis;--p. 401.
Neritina Fittoni;--p. 401.
Mytilus Lyellii;--p. 405.
Unio antiquus; _Geol. S. E._ p. 250, fig. 1.
---- compressus;--fig. 2.
---- aduncus;--fig. 3.
---- porrectus;--fig. 4.
Valdensis;--_Min. Conch._ pl. 646, and _Lign._ 131.
Corbula alata; _Ly._ p. 229.
Ostrea distorta;--p. 232.

The shells of the Wealden are also figured by Dr. Fitton, _Geol. Trans. New Series_, vol. iv. Pl. XXI.

Chalk Formation.

I.--Shanklin, or Lower Greensand. _Ly._ p. 219.

Dr. Fitton's Memoir, previously quoted, contains numerous figures of the characteristic shells of this division of the Chalk, particularly of the species which abound in the celebrated Whetstone of Devonshire. _Geol. Trans. New Series_, vol. iv. Pl. XIII-XVIII. See also Prof. E. Forbes's Catalogue of Lower Greensand Fossils, in the Quart. Geol. Journal, vol. i.

II.--Galt and Upper Greensand. _Wond._ p. 307; _Ly._ p. 218.

Inoceramus concentricus; _Wond._ p. 330, fig. 1.
---------- sulcatus;--fig. 3.
Terebratula lyra; _Ly._ fig. 219.
Pecten quinquecostatus;--fig. 203.
Ostrea carinata;--fig. 204.

In Plates XI. and XII. of Dr. Fitton's Memoir, there are figures of more than twenty characteristic shells of this division of the Chalk.

III.--White Chalk. _Ly._ p. 211, _Foss. South D._, _Geol. S. E._

Some cretaceous species are delineated in _Lign. 125, 126, 128, 129, 130, 138_; and Sir C. Lyell figures other species; but I must refer the student to the _Foss. South D._, _Geol. S. E._, and Dixon's _Fossils of Sussex_, as accessible works containing numerous figures of the fossil shells of the Chalk. Accurate descriptions and representations of all the British chalk shells, however, are still much required. Mr. Davidson has done much towards the illustration of our Cretaceous Brachiopods; and the shells of the Cretaceous strata of the United States are figured and described in an elegant work by Dr. Morton, of Philadelphia.

Tertiary Formations.

I.--Eocene. _Ly._ p. 174; _Wond._ p. 226.
II.--Miocene. _Ly._ p. 168.
III.--Pliocene. _Ly._ p. 161.

The specimens figured by Sir C. Lyell have been so carefully selected, and are so well engraven, as to present a _coup-d'œil_ of the most characteristic shells of the three grand divisions of the Tertiary Deposits.

I have reserved for especial mention in this place, the work, which will afford the student of British fossil Conchology the most important aid in the identification of specimens, namely, the "_Mineral Conchology of Great Britain_," by the late eminent naturalist, Mr. James Sowerby, and continued by his son, Mr. James De Carle Sowerby; in six volumes 8vo., with several hundred coloured plates. Unfortunately, this work has long been discontinued; and the rapid progress of discovery, and the numerous foreign publications on every department of fossil conchology, almost forbid the hope that it will be resumed by the present proprietor. Although the high price of this work places it beyond the reach of many individuals, it will be found in most libraries of Natural History.

* * * * *

[Sidenote: ON COLLECTING FOSSIL SHELLS.]