FOSSIL TESTACEOUS MOLLUSCA, OR SHELLS.
[Illustration: Lign. 120. Fossil Oyster, from the Chalk.
_Kemptown, Brighton._]
On Fossil Mollusca.--Numerous as are the fossil remains of the various
types of animal organization which have already passed under review,
they are far exceeded in number and variety by those of the beings
whose mineralized relics we now propose to investigate. Although every
one is familiar with the external appearance of the shells cast up by
the waves on the shores of our island, and of those which, from their
varied colours and elegant forms, are preserved 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.]
[Illustration: Lign. 121. Illustration of Fossil Bivalve Shells; _nat._
Petricola Patagonica. _D'Orb._
Interior of right valve, and the same valve with the animal as seen on
the removal of the left valve.
_a._ Labial Palpi.
_b._ Mantle.
_c._ Margin of shell.
_d._ Branchiæ.
_e._ Anal siphon.
_f._ Branchial siphon.
_g._ Foot.
_h._ Retractor muscle 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.
[Illustration: Lign. 122. Turritellæ, from Bracklesham, Sussex. _Tert._
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.]
[Illustration: Lign. 123. Shell-Conglomerate; now forming in the
British Channel.
_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.
[Illustration: Lign. 124. Shell-Limestone; from the mouth of the Thames.
Fig 1.--A mass of Cockle-shells and Whelks, consolidated into a
coarse limestone.
2, 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.
[Illustration: Lign. 125. Terebratula and Rhynchonella; _nat._
_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.
[Illustration: Lign. 126. Terebratula and Spirifer.
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.
[Illustration: 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 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.
[Illustration: 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 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).
[Illustration: Lign. 129 Inoceramus Cuvieri. _Chalk. Lewes._
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.
[Illustration: 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.
]
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.]
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.
* * * * *
[Illustration: Lign. 131. Unio Valdensis. (1/3 _nat._)
_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_.
[Illustration: Lign. 132. Cyclas and Melanopsis. _Wealden; Sussex._]
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.
[Illustration: Lign. 133. Fossil Shells of Gasteropoda.
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.