Transcriber's note: A few typographical errors have been corrected: they
are listed at the end of the text.
* * * * *
[Illustration]
1. _Spondylus Americanus. (Young.)_
2. _Nautilus pompilius. (Young.)_
_G. B. S. Jun^r. fecit._
* * * * *
A
CONCHOLOGICAL MANUAL.
BY
G. B. SOWERBY, JUN.
ILLUSTRATED BY UPWARDS OF SIX HUNDRED AND SIXTY FIGURES
* * * * *
SECOND EDITION.
CONSIDERABLY ENLARGED AND IMPROVED.
* * * * *
LONDON:
HENRY G. BOHN, YORK STREET, COVENT GARDEN.
MDCCCXLII.
* * * * *
PREFACE TO THE FIRST EDITION.
It may be necessary in introducing this little volume, to state, that it is
strictly conchological, and that it is compiled for the use not only of
those who wish to acquire an elementary acquaintance with the subject, but
also of authors and others, who, desirous of extending their knowledge and
pursuing their researches, require a book of reference, containing a
general outline of what has been done by those who have trodden the same
path before them. It has been thought advisable, for general convenience,
to arrange the principal part of the information in alphabetical order:
adding tables of the systems of Lamarck and De Blainville, to facilitate
the systematic pursuit of the science.
Persons of the class first alluded to, will find great assistance in the
explanation of technical words, their application being further
illustrated, in most cases, by a reference to the figures; and, although
they might have been multiplied, it is trusted that enough are given for
every useful purpose.
The definition of the Classes, Orders, Families, and Genera, in the system
of De Blainville, and a tabular view, are presented for the use of those
who prefer it, or who wish to compare it with that of Lamarck.
In the explanation of the figures, will be found a systematic arrangement
of shells, according to Lamarck, including the names of genera established
or proposed since the publication of his system. The descriptions of
established genera have been rendered as concise and clear as possible. It
is hoped that no essential characters are omitted, and that those living
authors, whose proposed generic distinctions have been passed over in a few
words, will not have to complain of want of justice in the attempt to
interpret their meaning.
In most cases the generic name will be found accompanied by its derivation.
This has been done, in the hope of assisting the memory by associating the
meaning of a term with some peculiarity in the thing described. At the end
of each description of a genus, some general observations occur, pointing
out the principal character which distinguishes it from others, to which it
is nearly allied; and also stating the geographical or geological
distribution and habits of the animal.
The above descriptions and definitions are illustrated by a series of
plates, containing above 500 etchings of nearly as many proposed or
established genera, arranged in Lamarckian order, so as to show at a glance
all the generic forms of each family. And, although from their number, they
could not be very highly finished, it is hoped that they will be found
characteristic.
The compiler cannot replace his pen without acknowledging, with filial
gratitude, the kind assistance of one who has sacrificed much of his time
in bringing his knowledge and experience to bear upon the correctness and
utility of this humble attempt to remove some of the difficulties to which
the commencement of this, as well as of every other study, is exposed.
PREFACE TO THE SECOND EDITION
The favourable reception and rapid sale of the first edition of the
Conchological Manual having rendered a second necessary, the Author takes
this opportunity of explaining the nature of the alterations which have
been made. In doing this, he has to thank his friends for their
suggestions, which, together with his own increased knowledge and
experience, enable him to present a more complete and satisfactory work to
the scientific public.
For the further convenience of those who are studying the rudiments of the
science, an entirely new Introduction is given, in which, commencing with
the structure and gradual developement of the shell, the author has
endeavoured to explain the general principles of Conchology in systematical
order. This Introduction is illustrated by 100 wood-cuts, which will be
found greatly to assist the Student.
The definitions have been rendered more full and complete than before, and
the Author has profited by some manuscript notes communicated by a
scientific friend, to whom he desires to present his humble
acknowledgments. Upwards of four hundred explanations have been given of
words which did not appear in the former edition, three-fourths of which
are of generic and subgeneric names.
A large number of notes, referring to the geographical distribution of the
genera, have been added from the pen of Mr. G. B. Sowerby, Senior.
The plates have been carefully improved; and three, containing upwards of
eighty figures, have been added.
On the whole, it will be found that the amount of matter has been nearly
doubled; all the defects, as far as they have been discovered, have been
removed, and every means used of making the present edition as useful as
possible.
* * * * *
NAMES OF AUTHORS ABBREVIATED.
_Adans._ Adanson. Author of "Voyages du Senegal."
_Bl._ Blainville. Author of "Manuel de Malacologie et de
Conchyliologie," &c.
_Brod._ W. J. Broderip, Esq. Author of various descriptions of Shells
in the Zoological Journal, &c.
_Brongn._ Brongniart. Author of "Memoire sur les terrains du Vicentin,
d'Italie, de France, et d'Allemagne," &c.
_Brug._ Brugière. Author of "Dictionaire des Vers testacés, dans
l'Encyclopédie," &c.
_Cuv._ The late Baron Cuvier. Author of "Regne Animal," &c.
_Defr._ Defrance. Contributor to the "Annales des Sciences Naturelles,"
&c.
_Desh._ Deshayes. Author of "Coquilles fossiles des environs de Paris,"
&c.
_D'Orb._ D'Alcide D'Orbigny.
_Drap._ Draparnaud. Author of "Histoire Naturelle des Mollusques
terrestres et fluviatiles de la France," &c.
_Fer._ De Ferussac. Author of "Histoire Naturelle des Mollusques
terrestres et fluviatiles," &c.
_Flem._ Fleming.
_Gmel._ Gmelin. Author of an edition of Linnæus's "Systema Naturæ," &c.
_Guild._ Rev. Lansdown Guilding.
_Hübn._ Hübner.
_Humph._ The late George Humphrey.
_Lam._ Lamarck. Author of "Animaux sans Vertebres," &c.
_Lin._ Linnæus. Author of "Systema Naturæ," &c.
_Mont._ Montague. Author of "Testacea Britannica," &c.
_Montf._ Montfort. Author of "Histoire Naturelle des Mollusques," &c.
_Müll._ Müller. Author of "Vermium terrestrium et fluviatilum,"
"Zoologiæ Danicæ," &c.
_Ranz._ Ranzani. Author of "Considerations sur les Balanes," &c.
_Schum._ Schumacher.
_Sow._ Sowerby. The late James. Author of "Mineral Conchology," &c.
George Brettingham, Senr., "Genera of Shells," "Species Conchyliorum,"
&c. G. B. Jun. "Conchological Manual," "Conchological Illustrations,"
"Thesaurus Conchyliorum," Descriptions of New Shells in the Zoological
Proceedings, &c.
_Sw._ Swainson. Author of "Zoological Illustrations," "Exotic
Conchology," "Lardner's Cabinet Cyclopedia," &c.
_Turt._ Turton. Author of "British Shells."
* * * * *
INTRODUCTION.
The Science of Conchology affords a very delightful and instructive
amusement for the leisure hours of those who, retiring occasionally from
the gaieties of fashionable life, seek pleasure in the quiet contemplation
of some of the smaller, but not less wonderful operations of creative
wisdom. And, although the study of shells would be more complete, and rank
higher in the scale of philosophical pursuits, were it always accompanied
by that of the animal inhabiting them, it nevertheless presents means of
intellectual gratification, to many who cannot follow it beyond the cabinet
and the boudoir. These may examine with admiration and mental improvement,
the beautiful colouring and architecture of these wonders of the deep, they
may exercise their taste and judgment in the selection and arrangement of
specimens, and their discrimination in detecting and appreciating the
distinctions upon which the arrangement is founded.
It is but little that can be known of the subject without forming a
collection of greater or less extent; for, as it would be uninstructive
merely to delight the eye with the bright colours and elegant form of
shells, without possessing correct information respecting them, so it would
be insipid and useless to learn technicalities without being acquainted by
personal observation with the subjects to which they are applied. The first
endeavour should, therefore, be to obtain a few shells as examples of the
larger divisions, and, when these are understood, to proceed with the
smaller groups, until a collection be formed to represent as many generic
forms as possible. It may be as well here to advise those who are forming a
collection to be very particular in every practicable instance to have the
shells properly named at the time of purchasing; as it will save much
trouble, and materially assist in the attainment of the desired object. To
this end, recourse should be had to those naturalist tradesmen, who unite
the attainment and diffusion of real scientific knowledge with their
commercial pursuits.
Supposing, however, that the person who desires to learn the science,
possesses a small parcel of unarranged and unnamed shells, without any
previous acquaintance with the subject, the following introductory
explanations, are drawn up with the view of enabling him, without further
assistance, to obtain a general insight into its principles, equal to that
of those who have studied it long and laboriously. To effect this, he must
read them, carefully comparing the descriptions with the figures referred
to, and with the specimens which he may have at command.
After describing the nature of the science and defining its objects, we
shall proceed to explain the structure of those objects, and the manner of
their growth. We shall then enter somewhat minutely into the principles of
classification, the distinctions upon which they are founded, and some of
the technical terms used to express them. After which we shall pass through
the arrangement of Lamarck, defining the general divisions adopted under
the terms of "_Classes_, _Orders_, and _Families_," as far as they are
capable of definition. The subdivision of the latter into _genera_ will
only be entered into so far as to enumerate the principal of them, the more
minute descriptions being reserved for the alphabetical part of the work.
Let none be discouraged by the number of generic distinctions proposed and
adopted in modern times; for if well defined, they will be found to
facilitate rather than encumber the science. The knowledge of species must
be the foundation of every system, and the greater their number, the more
necessary it becomes to subdivide them; if, for instance, all the species
now known were to have been included in the 50 genera of Linnæus, a single
genus would have contained many hundreds of incongruous species, in which
case it would be much more difficult to remember them, than if they were to
be divided into a far greater number of genera. Every well marked division,
however arbitrary its limits, tends to simplify the subject, and to
facilitate the researches of the student.
NATURE OF THE SCIENCE.
Conchology is the study of shells, viewed and described as to what they are
either in themselves, or in relation to the soft, inarticulate animals
which produce them, and of which they form a part. These animals are called
MOLLUSCA, and perhaps the best general description of them will be found in
De Blainville's "Manuel de Malacologie et de Conchyliologie." The following
is a translation, "Animal in pairs, the body and its appendages soft,
inarticulate (not jointed), enveloped in a muscular skin, commonly called
the mantle, which is extremely variable in form, and has developed either
within or upon it a calcareous portion, consisting of one or several
pieces, commonly called a SHELL."
The term Mollusca was formerly restricted to those soft animals which were
destitute of shells, although possessing in other particulars, the
characters described above, and it was used in order to distinguish them
from the TESTACEA, which were covered or internally supported by calcareous
parts. In the system of Linnæus, the soft portions are first arranged under
the general designation of "Vermes Mollusca," and described without regard
to the presence, absence, or character of the shells; and then the shells
are separately characterized under the appellation of "Vermes Testacea,"
without any further notice of the animal, than an indication of the genus
to which it belongs; thus the animal of Cypræa is said to be a Limax, and
that of Tellina a Tethys.
The nearest approach to correctness, and the most philosophical method of
study will be found in the modern system, adopted by Lamarck and his
followers, of observing these animals as a whole, and arranging them
according to the assemblage of characters which they present; of course
taking into consideration the existence or non-existence, form and
structure of the shell, on the same principle, which, in arranging the
vertebrated animals would lead us to study the hair, hoof, nails, claws,
&c. as well as the other parts.
At the same time, it must be admitted that there are many private
collectors of Shells who would find it a difficult, if not impossible task
to study minutely and successfully the soft parts of the Mollusca. Ladies,
for instance, could not be expected to handle with pleasure and
perseverance, these fleshy substances, which in order to be preserved from
putrefaction, must be kept in spirits; and yet such persons may, with
improvement and advantage to their own minds, enjoy the interesting and
scientific amusement of studying and arranging the clean and beautiful
natural objects which are so easily preserved, and so exquisitely curious
in their structure. Let it also be remembered, that if shells had not been
rendered commercially valuable, by the zeal and emulation manifested by
_mere_ Conchologists for the possession of rare specimens, few travelling
merchants and sea captains would have thought them worthy of a corner in
their cabins. In this case, few specimens being brought to the country, the
more Philosophical Naturalist would have been left without the means of
obtaining materials to work upon, or of attracting public attention to his
favourite pursuit.
On account of these and other considerations, it has been thought advisable
that the present undertaking should bear a purely conchological character.
The peculiarities of the shells alone being detailed for the assistance of
those who collect and study them, while at the same time, in deciding upon
their affinities and places, in the arrangement, it will be necessary to
take advantage of the conclusion to which those have arrived, who have
studied the animal in all its parts. And the conviction must be expressed,
that if ever a complete Natural System shall be formed it will result from
the labours of the last mentioned class of naturalists.
DEFINITION OF A SHELL.
Before entering minutely into the description of shells, it will be
necessary to distinguish from the true testaceous Mollusca two kinds of
animals which have formerly been associated with them. Of these, the first
is the class of CRUSTACEA, consisting of crabs, crayfish, &c. These differ
from shell-fish, not only in structure and chemical composition, but also
in the fact that the animal has jointed limbs, and that the substance of
the flesh is inseparable from the hard external covering, which invests
each particular joint as with a sheath; whereas the Molluscous animal is
but partially attached to its shell, from which it possesses the power of
partly withdrawing and returning. The second class is that to which the
sea-urchin, or Echinus, belongs, of which there are many genera and
species. The testaceous covering of Echini is composed of a number of small
pieces, placed edge to edge, forming a more or less globular external
covering to the flesh, which is supported in the centre by a number of
bones leaning upon each other in a pyramidal form. The _test_ is of a
fibrous texture, guarded on the outside with moveable spines, which turn on
ball and socket joints.
A true shell is composed of one or more calcareous pieces, commonly called
valves, each piece formed by a series of layers, applied obliquely upon
each other, in such a manner that each new layer begins within, and
terminates a little in advance of the one before it.
STRUCTURE AND GROWTH.
We shall now endeavour to describe the manner in which the growth of each
separate valve, or each regularly formed shell, proceeds from the nucleus.
Before the young animal has left the egg, if it be an _oviparous_ species,
or the body of the parent if _viviparous_, the nucleus of the shell is
generally formed, and specimens are sometimes preserved in which the young
shell is seen within the egg, as in the cut, fig. 1, 2; or adhering to the
inner surface of the full-grown shell by the dried mucus of the animal, as
seen in fig. 3.
[Illustration: 1. Egg of a Bulinus. 2. The same broken, shewing the young
shell. 3. The young of a Paludina, as seen in the aperture of the shell.]
In both cases, the nucleus is generally of a more horny and transparent
composition than the parts subsequently produced. As soon as the animal is
hatched, or, in other words, leaves the egg or body of the parent, of
course it begins to increase in size, and to require a corresponding
enlargement in the shell. To effect this, a small quantity of mucus
substance, secreted by the mantle of the animal, is deposited on the edge
of the aperture. When this is dry and become sufficiently hard, it is lined
by a more calcareous secretion; and these together form a new layer, which
is followed by others in succession; each new layer being larger than the
one that preceded it until the whole being complete, the full-grown animal
is invested with a shell commensurate with its own proportions. Thus from
the apex or nucleus the formation proceeds, as it were, downwards, taking
the shape of the part which secretes it, on which it is in a manner
moulded.
The nucleus, or first formed portion, may for technical purposes be
considered, mathematically, as the apex of a spiral cone. And here it must
be observed, that whether the shell consist of one or several pieces, each
piece has a separate nucleus, and the process of formation is separately
repeated with each. The word cone is used for convenience, and its meaning
extended so as to include all those structures which commencing at a point
enlarge downwards.
[Illustration: 4. Imaginary cone. _a._ Apex. _b._ Base. _l._ Lines of
growth.]
From the apex, the next layer is deposited on its edge, and advancing
beyond it necessarily adds to its extent. Thus, suppose for the sake of
illustration, the part marked _a_ in the diagram, fig. 4, to represent a
nucleus, the cross lines (_l_) will shew the consecutive layers, which
enlarge their circle as they add to their numbers. This disposition of
shelly matter into layers is marked externally by concentric striæ, or
_lines of growth_, while on the inside the edges of the laminæ are
consolidated into a kind of enamel. If a perpendicular section of a solid
portion of a shell were magnified, it would present, in many instances, an
appearance resembling the diagram, fig. 5; _a_ may be taken to represent
the horny part of the layers which form the outer coating, named
"_Periostaca_," or "_Epidermis_;" the undulating line _b_, is formed by the
edges of the calcareous layers, and causes the striæ, or lines of growth,
which are often distinguishable on the surface of the shell; the space _c_
is the middle part of those layers, and at _d_ they are consolidated into
the enamel which lines the interior.
[Illustration: 5. Supposed section of a part of a solid shell.]
In some species the layers are irregularly grouped together, and their
edges overlap each other, so that they are easily separable, and advancing
beyond each other, give a leafy appearance to the external surface. This
structure is termed _foliaceous_. A very familiar instance of this may be
observed in the common oyster. If a specimen of this shell be broken, the
substance will be seen to exhibit a degree of looseness, and a magnifying
glass will enable the student to trace distinctly the laminæ of which it is
composed. The accompanying representation of a magnified section (fig. 6)
will shew at _a_, the external surface, with the foliations or leaves; at
_b_, the parcels of layers which form them; and at _c_, the pearly
structure produced by their consolidation, and by the subsequently
deposited enamel which covers their external surface.
[Illustration: 6. Section of an oyster shell enlarged.]
CLASSIFICATION.
The classification of shells, that is, their systematic arrangement into
_classes_, _orders_, _families_, _genera_ and _species_, cannot be made to
depend entirely upon the characters observable in them, viewed by
themselves; for this reason, that many similarly formed shells form the
habitations of animals perfectly distinct, and that many molluscous animals
are found to agree with each other in every respect but in the form of
their testaceous support. There are, however, many important distinctions
to be observed in the shells themselves, leading to the establishment of
many of those very divisions, which would afterwards be confirmed by an
examination of the soft parts. It is necessary to attend, as far as means
and opportunity will allow, to _all_ the points of difference, both in the
shell and in the animal, in order to form, and in some instances even to
appreciate, a generic or larger distinction. It will therefore be our
endeavour to explain the general principles upon which those distinctions
are formed, and the manner in which they are applied and expressed in
detail by scientific writers.
NUMBER OF PIECES, OR INDEPENDENTLY FORMED PARTS.
The first, most simple and obvious division of shells, is that which
results from the number of separate pieces composing them. Hence the
distinction implied by the terms UNIVALVE, or consisting of a single piece;
BIVALVE, or composed of two pieces; and MULTIVALVE, or composed of more
than two. For an example of _univalve_, take a common whelk; for a
_bivalve_, take a muscle or a scallop; and for a _multivalve_, the
barnacle, or balanus, found adhering to the common oyster.
But although this arrangement may appear at first sight perfectly easy and
plain, some explanation will be necessary in order to guard the student
against understanding the above expressions in their strictest sense,
without qualification. Thus the univalves are said to consist of a single
piece, or spiral cone; but it would be more correct to speak of this piece
as forming either the whole or the principal part of the shell: for there
is in many instances, a much smaller flattened piece attached to the foot
of the animal, which being drawn in when it retires, closes the aperture as
with a kind of door, to which in fact the word valve might be very properly
applied; it is called however the OPERCULUM, and the little horny plate,
frequently drawn out by means of a pin from the aperture of a periwinkle,
will present a familiar example.
[Illustration: Accessary valves of a Pholas.]
The same may be said respecting the bivalves; for besides the principal
portions or valves of which the shell is composed, there are in many
species, one or two smaller separate portions, named "_accessary plates_"
by some authors. They are fixed by means of cartilages, on the back of the
hinge.--The engraving, fig. 7, represents the accessary valves of a species
of Pholas, which was on this account arranged by Linnæus with the
Multivalves. Nearly allied to the Pholades is a set of shells to which De
Blainville has given the name "_Tubicolæ_," or inhabitants of tubes. In
this case, the bivalve shell is connected with a testaceous tube or pipe,
to which it is attached either by one or by both valves, or in which it
lies attached only by the cartilages of the animal. In the genus
Aspergillum, the two small valves are soldered into the sides of the tube
in such a manner as to constitute a part of it. One of these shells, called
the Water-spout, might be taken up by a person not aware of its real
nature, and regarded as a pipe or tube prettily fringed, and nothing more;
but upon a closer examination, he would find the two valves, the points of
which are visible from the outside of the tube.
HABITS--_Land, Fresh-water, or Marine Shells._
Another distinction, leading to important results in classification, is
that which is derived from the nature of the element breathed by the
Mollusc. And although this consideration belongs more especially to the
study of the animal itself, yet the habits of the animal materially
influence the structure of the shell.
The TERRESTRIAL or LAND Molluscs live on land, breathe air, and feed on
plants and trees.--Those who find pleasure in horticultural pursuits will
at once call to mind a too familiar example of these Molluscs in the common
garden snail. The Land-shells are all univalves, and constitute a family in
the Lamarckian system under the name "_Colimacea_," or snails,
corresponding with the Linnean genus Helix.--They are generally light in
structure and simple in form.
The AQUATIC, or Fresh-water Molluscs, such as the Planorbis, commonly
called the Fresh-water Snail; the Unio--known by the name of Fresh-water
Muscle, is found in ponds, ditches and rivers. The _epidermis_ of these is
generally of a thick, close-grained character, and they are subject to
corrosion near the umbones. There are but few genera of fresh-water shells
besides the Uniones, among bivalves, and the "Melaniana" among univalves.
Concerning the former it may be observed, that they are all pearly within,
and the colour of the thick horny coating embraces all the varieties of
brownish and yellowish green.
The MARINE, or _sea-shells_, belong to all the classes and orders, and
include by far the greater number of species. They vary in the habits of
the animal, and consequently in the situations in which they are found.
Some are found buried in sand and marine mud, and are named "_Arenicolæ_"
or inhabitants of sand; others in holes of rocks and other hard substances,
then they are named "_Petricolæ_,"--some of these latter form the holes in
which they live by corroding or eating away the stone. A section of these
form the family of "_Lithophagidæ_," or stone-eaters, of Lamarck. Others,
again, take up their parasitical abode in the bodies of animals, and feed
upon their substance; as for instance, the Stylifer, which is found in the
vital part of star-fish, and Coronula, and Tubicinella, found buried in the
skin of the whale.
LOCOMOTION--_Attached, Unattached._
A much more subordinate source of distinction arises from the freedom or
attachment of the shells. Some of them float or walk freely in their
natural element; others are fixed or attached to foreign bodies. Among
those which are attached, there is again a difference as to the mode of
attachment. Some are united to foreign bodies by means of a glutinating
substance, secreted by the animal, and joining part of the surface of the
shell to that of the stone, coral, or other substance. In this way shells
are fixed to each other in groups; this is the case with the Spondyli among
bivalves, and the Serpulæ among univalves. M. de Blainville applies the
term "_Fixæ_" to these shells. Others are kept in a particular place by
means of a _Byssus_ or Tendinous fibrous line or bunch of silky hairs,
acting as a cable, and allowing the Mollusc to ride as it were at anchor.
This Tendon is connected with some part of the animal from which it passes
through an opening or hiatus in the shell, as in the Terebratula and the
Mytilus.
[Illustration]
In the former, represented by the cut, fig. 8, the tendon passes through a
perforation in the upper valve; and in the latter, Mytilus, fig. 9, the
byssus passes out between the valves.
Before proceeding to explain the characters of the different groups,
according to the modern system of classification, it may be desirable to
explain the terms by which the different parts and characters are
described, and to shew the manner in which the shells are measured. For
this purpose we shall treat of the general divisions separately. We begin
with
UNIVALVE SHELLS.
In considering Univalves merely with reference to their mathematical
construction, the first point demanding our attention is, whether they are
symmetrical or non-symmetrical, or, in other words, whether a straight line
drawn through the shell would divide it into two equal parts. The greater
part of univalves are non-symmetrical, being rolled obliquely on the axis;
but many are symmetrical, being rolled horizontally on the axis. The
Nautilus presents an illustration of the latter; the Snail is a familiar
example of the former.
_Symmetrical Univalves._
[Illustration]
In describing these it will be well to commence with the most simple form,
such as the Patella,--taking a conical species as an example. In this it
will be observed that there is no winding or curvature, but a simple
depressed cone, and that the line _a_, _p_, divides it into two equal
parts.
The _anterior_, _a_, (_cut_, fig. 10) is known by the interruption of the
muscular impression which surrounds the central disc (_d._) This
interruption of the muscular impression is in the place where the head of
the animal lies in the shell. The impression itself is caused by the
fibrous muscle which attaches the animal to the shell. The apex (_a_) in
Patella, generally leans towards the anterior (_a_) part of the shell, and
away from the posterior (_p_); and this circumstance has caused some
mistakes, because in Emarginula the apex leans towards the posterior; and
students, instead of examining the muscular impression, which is the only
criterion, have only noticed the direction in which the apex turned, and
concluded that to be the anterior, towards which it inclined. The lines or
ribs running from the base to the apex of the shell, in the direction _r_,
are called _radiating_ lines; and those which encircle the cone in the
direction _c c_, from front to back, are very properly described as
_concentric_. The _length_ is measured from front to back in the line _e_;
the breadth, from side to side, in the line _b_; and the depth from the
apex to the base.
Let it be observed that patelliform, or limpet-shaped shells are not all
symmetrical; Umbrella, Siphonaria, Ancylus, &c. will form exceptions, of
which we have yet to speak. And the learner may also be reminded that the
Limpets themselves are not _all_ regular in their form: for as they adhere
to rocks and other rough surfaces, and are so little locomotive, in many
instances they partake of the inequalities of the surface, and conform to
its irregularities. This adherence is not effected by any agglutinating
power in the animal, nor by any tendinous process like that described
above; but simply by means of the foot of the animal acting as a sucker.
The next variation in symmetrical univalves is to be observed in the
tubular, curved form, the example of which will be the Dentalium, fig. 12.
[Illustration: Dentalium Elephantinum.]
This has an opening at the anterior termination _a_, called the aperture.
The opening at the posterior end (_p_) is named a fissure, or perforation.
The ribs running along the sides of the shell are _longitudinal_, or
radiating. And the lines round the circumference are _lines of growth_, or
_concentric_--each one having in succession, at earlier stages of growth,
formed the aperture. They are described as concentric, or transverse.
_Symmetrical Convolute Univalves._
The Nautilus, the Spirula, the Scaphite, and the Ammonite are the leading
types of this form; but when we use the term symmetrical, in reference to
these, the word must not be understood in its strictest sense, for no shell
is _perfectly_ symmetrical: but it means that there is no perceptible
difference in the proportion of the two sides; as in the human body, the
right side is larger and more powerful than the left, yet to a degree so
small that it gives no apparent bias to the figure.
CHAMBERED SHELLS.
Many of the shells now under consideration are chambered, that is, the
internal cavity is divided into separate compartments by plates reaching
across it, named _Septa_; and the only connection between the chambers is
formed by the small pipes passing through them, to which the name of Siphon
is attached.
_Septa._
The septa are _simple_ in some species, as in the Nautilus, fig. 13. In
others they are _undulated_, having waved edges, as in some species of
Ammonites; in others they are _angulated_, as in Goniatites, fig. 480 in
the plates; and in the greater number of instances, among the Ammonites,
they are _arborescent_, or branched.
[Illustration: 13. Section of Nautilus.--14. Undulating Septa.--15.
Arborescent Septa.]
In the above section of a Nautilus, fig. 13, diminished in size, showing
the whorls and chambers (_c_), it will be seen that the edges of the septa
(_s_) are formed in one simple curve. In fig. 14, the upper part of an
Ammonite, the undulating line will be seen; and in fig. 15 a specimen is
given of the arborescent septa.
_Siphon._
The Siphon is _dorsal_ when placed near the outside of the whorls;
_central_ when near the middle; and _ventral_ when near the inside of the
whorl, or that part which leans against the last volution. When it passes
uninterruptedly from one chamber to another, it is described as
_continuous_, as in the case of Spirula; when, on the other hand, it only
passes through the septum a little distance, and opens into the chamber, as
in Nautilus, it is _discontinuous_.
_Whorls of Symmetrical Univalves._
They are _disunited_ when they do not touch each other, as in the case of
Spirula (fig. 471 in the plates); but in the contrary case they are said to
be _contiguous_. In some species of Nautilus the whorls overwrap each other
in such a manner that the early whorls are entirely covered by the last,
the edges of which reach to the centre of the disk: the spire is then said
to be _hidden_; as in the Nautilus Pompilius. In Nautilus umbilicatus the
spire is nearly hidden, the whorls not quite covering each other; but in
the greater number of the Ammonites, the largest part of the preceding
whorls is seen. To express the degree in which the whorls overwrap each
other, has caused much difficulty in concise descriptions. Perhaps it would
be well to apply the term _spiral disc_ to so much of the shell as is seen
besides the last whorl, and to describe it as large or small in diameter,
compared with the whole: or to say that the whorls of the spire are half,
or one-third, or one-fourth covered, as the case may be.
_Aperture of Symmetrical Univalves._
In Ammonites Blagdeni and some others the aperture is of an oblong square;
it is then said to be _sub-quadrated_; in Nautilus triangularis it is
_angulated_; in Ammonites Greenoughi it is of an interrupted oval shape,
described as _elliptical_. In the greater number of Orthocerata, it is
rounded or _circular_. The entrance of the last whorl into the aperture of
some rounded species of Nautilus causes it to take a _semi-lunar_ form; if
rounded at the sides it is said to be reniform or kidney-shaped; if pointed
at the sides it is _semi-lunar_; and in some species of Ammonites, it is
five-sided or _quinque-lateral_.
_Measurement of Symmetrical Conical Univalves._
[Illustration]
The _width_ is measured across the aperture, which is the widest part of
the shell. The _length_ (_l_) from the dorsal part (_d_) of the aperture to
the dorsal part of the _whorl_ (_d_) on the opposite part of the shell. The
_ventral_ part of the whorls is that nearest to the axis, and the _dorsal_
that which forms the outline of the figure.
NON-SYMMETRICAL UNIVALVES.
These are _conical_, _irregular_, _spiral_, or _convolute_. The _conical_
form is when there is no enrolment of the apex. Although the Patellæ were
described as symmetrical, there are several species of Patelliform shells
which are not symmetrical. In Umbrella, for instance, the apex is oblique,
the shells being placed obliquely on the animal. In the genus _Siphonaria_,
there is a groove on one side, where the brachia or gills of the animal
rest. In the genus Ancylus, it will be observed that the apex bends on one
side, and the animal is like the Limnæa, which has a spiral shell. The cup
and saucer Limpets, or Calyptrædæ, present a group which requires to be
described, differently from the symmetrical or true Limpets. Their
structure is very curious, and they vary considerably among themselves,
some of them being simply conical, others nearly flat, or discoidal, and
others more or less spiral. But their principal peculiarity consists in
their having a small internal process or plate variously shaped, commonly
named their _septum_.
_Septa of Limpets._
The septa of Limpets assume a variety of forms, the principal of which will
be seen in the accompanying engravings.
[Illustration]
The form from which the group derives its generic appellation is that of
the cup-shaped or _Cyathiform_ species (fig. 17). In the Crepidulæ, or
Slipper-Limpets, the septum is flat, reaching across the opening, like the
deck of a vessel; it is then described as _transverse_ (fig. 20). In
Calyptræa Equestris, it has two prominent points, and is described as
_bi-furcated_ (fig. 18). In another species, it is a three-sided plate
rather spiral at the apex (fig. 19).
_Measurement of Cup and Saucer Limpets._
[Illustration]
The line marked _a_, _p_, _ll_ indicates the direction in which the shell
is to be measured for _length_. _a_ indicates the _anterior_, _p_ the
_posterior_. The line _d_ (fig. 23), from the apex to the base, is the
_depth_. The line _b_ (fig. 28), is in the direction of the breadth.
_Irregular non-symmetrical Univalves._
Serpuliform shells are irregularly twisted (_tortuous_) hollow tubes, which
were formerly considered to have been secreted by a kind of worm, but now
known to be the shells of true Molluscs, of a kind not very widely
differing from those which have regularly spiral shells. The greater part
of these are attached to foreign bodies, or to each other in groups. Some
are attached by the whole length of the shell, they are then said to be
_decumbent_. Some of these are coiled round like the Spirorbis, the little
white shell seen on the carapace of the Lobster or on leaves of sea-weeds;
they are then said to be discoidal; others again, such as the _Vermetus_,
approach more nearly to the spiral form. The deviation from the regular
spire only taking place after the few first volutions.
SPIRAL NON-SYMMETRICAL UNIVALVES.
As these constitute the largest class, it will be necessary to dwell upon
them in detail. First as to _measurement_.
[Illustration]
The length is measured from the apex, to that part of the aperture _a_
(fig. 24), at the greatest distance from it. The _breadth_ is in the
opposite direction. The _anterior_, or front part of the aperture, is
marked _a_, where the head of the animal protrudes.
_Spire of non-symmetrical Univalves._
[Illustration: Fig. 25, _obtuse_; 26, _acute_; 27, 28, _decollated_; 29,
_concave_; 30, _papillary_; 31, _mammellated_; 32, _discoidal_.]
In counting the whorls of which the spire consists, we commence at the
apex, and reckon downwards to the last, or body whorl. The spire is
described as being long or short in relation to the aperture: in which
case, all that is above the aperture is measured with the spire. Its apex
requires particular notice, as the character of the whole shell frequently
depends upon the particulars observable in this part. It is sometimes
_obtuse_, or blunt; sometimes _acute_, or sharp. In the Cones it is
frequently flat, and in Planorbis it is concave. It is sometimes of a
different structure from the rest of the shell, retaining the horny and
transparent appearance which characterized it when the animal was first
hatched. The Tritons present an instance of this, although it is not always
observable, owing to the tenderness of the substances which causes it to
break or fall away in many specimens. A very remarkable instance also
occurs in Bulinus decollatus (cut, fig. 27, 28), so named, because the
apex, to the depth of several whorls, falls off, and the shell is
_decollated_. In this, and many more instances, among Pupæform land shells,
the occurrence of this circumstance seems to be by no means rare or
accidental, a provision having been made for filling up the opening by a
septum. A _papillary apex_ is one which is swelled at the extremity into a
little rounded nob, or nipple; and a _mammellated_ apex is one which is
rounded out more fully into the shape of a teat.
_Whorls._
The spire is described as consisting of _numerous_ or _few_ whorls, and
sometimes the number of them is particularly stated. A whorl consists of
one turn of the spiral cone. The whorls are described as _flattened_, when
the sides are not bulged out so as to cause the outline of the spire to
deviate considerably from straightness: when the contrary is the case, the
whorls are said to be _ventricose_, and either _rounded_ or _angulated_.
The degree of rapidity with which the whorls become enlarged presents an
important source of distinction. The _suture_, or seam, which separates one
whorl from another is also noticed as being _distinct_ or otherwise;
_canaliculated_, or grooved; or covered by an enamel, which in some
instances is swelled into a ridge or _tumid_.
[Illustration: Fig. 33, _few_; 34, _numerous_; 35, _rounded_, _ventricose_;
36, _angular_, _ventricose_; 37, _flattened_.]
_Suture._
[Illustration: Fig. 38, _canaliculated_; 39, _enamelled_.]
_Varices._
Varices are caused by periodical rests or stoppages in the growth of the
shell, when the edge of the aperture thickens, and renders the shell as
complete as when full grown. Again, after an interval, another check takes
place, and another thickened edge is formed, and so on in succession, until
the animal arrives at maturity, and the shell is full-grown. The thickened
edges successively forming the aperture, remain visible on the outside,
through all the subsequent stages. When these rests take place at frequent
periods, the varices will of course be numerous as in Harpa and Scalaria.
They occur at regular or irregular distances, varying in shape and other
characters. When the varices occur at regular intervals, and form a
connected ridge from whorl to whorl up the spire, they are said to be
_continuous_, as in Ranella; when on the contrary, the varix on one whorl
does not come in contact with that on the other, they are described as
_discontinuous_. In order to distinguish a regular varix from a mere
external ridge, it will be sufficient to notice whether its edge overlaps
the external surface, and whether it resembles the open edge of the
aperture, which true varices do.
[Illustration: Fig. 40, _numerous_; 41, _few, continuous_; 42, _few,
discontinuous_.]
_Aperture._
The aperture or opening of the spiral tube, was formerly described as the
mouth; a term calculated to convey an erroneous impression, when applied to
a part of the shell which has no correspondence with the mouth of the
animal. The word _aperture_ is used by modern writers in a general sense,
including the cavity, its edge, and the canals. The cavity itself is
distinguished in various shells as to its shape, which depends much upon
the degree of modification produced by the last whorl. In some cases, as in
Cyclostoma, where the aperture stands apart from the last whorl, the shape
is round, or nearly so. The Scalaria presents a good example of this. In
others, where the inner edge or lip, wrapping over the body whorl is nearly
straight, the aperture is _semi-lunar_, or half-moon shaped: this is
remarkable in the "_Neritacea_" of Lamarck, named, on that account,
"_hemi-cyclostomata_" by De Blainville. In a great number of instances, the
lower part of the body whorl enters obliquely into the upper part of the
aperture, the result being a _pyriform_, or pear-shaped opening. The
aperture is described as _long_ when it is largest in the direction of the
axis, and _wide_, in the contrary case. The _anterior_ is the part at the
greatest distance from the apex, and the body whorl; the _posterior_, the
part nearest to the apex. Thus some apertures are described as _posteriorly
contracted_ and _anteriorly widened_, or the reverse. A _linear_ aperture
is one contracted in its whole length, as in Cypræa. When the whorls are
angulated, a _trigonal_ aperture is the result, as in many species of
Trochus. Some are _transversely oval_, that is in an opposite direction to
the axis, and others _longitudinally oval_. When the whorls are formed with
two outer angles, a somewhat quadrated aperture is formed. There are other
variations too numerous to mention.
_Apertures._
[Illustration: Fig. 43, Helicina, _semilunar_; 44, Pirena, _pyriform_; 45,
Cypræa, _linear_; 46, Trochus, _trigonal_; 47, Cyclostoma, _rounded_; 48,
Chilina, _posteriorly contracted_; 49, Stomatia, _transversely oval_; 50,
Murex, _longitudinally oval_.]
The entire edge of the aperture described generally, is named the
Peritrême, but this term can only be conveniently applied in cases where,
in some at least of its characters, it is the same all round, so that one
descriptive term is applicable to the whole. As, however, this is of rare
occurrence, it is found convenient in descriptions to separate the rim from
the outer lip. In a great number of instances, this is done naturally, by a
canal, or notch at the anterior or lower extremity, and by the posterior
union of that part which overlays the body whorl with the other portion. At
these two points the outer and inner lips separate from each other: we
therefore describe the
_Canals of the Aperture._
When there is neither notch nor canal, anteriorly or posteriorly,
interrupting the edge of the aperture, it is described as entire. When
there is a notch or sinus at the anterior extremity, it is said to be
_emarginated_. When the edge of this notch is expanded, and drawn out in
the form of a beak, it is said to be _canaliferous_, or to have a _canal_.
When, in addition to this, the lips are thickened and contracted
posteriorly near their junction, and drawn out so as to form a groove, it
is said to be _bi-canaliculated_, or to have two canals. The _anterior
canal_ is said to be long or short, according to the proportion which it
may bear to the rest of the shell. Thus the canal of Ranella ranina (fig.
393 in the plates), may be described as _short_; while that of Murex
haustellum, (fig. 396, pl.) is _long_. When it is wide near the aperture,
and becomes gradually contracted towards its termination, it is said to be
_tapering_, as in Pyrula (fig. 388, pl.); when the termination is sudden,
it is described as _truncated_. If, on placing the shell upon a plane, with
the aperture downwards, the canal is seen to rise upwards, it is
_recurved_. In Buccinum and Nassa it is turned suddenly over the back, and
forms a short, curved elevation; it is then described as _recurved_ and
_varicose_. If the edges meet, so as to form a tube, it is said to be
closed, as in some species of Murex and Typhis. The posterior canal is, in
some cases, _free_, or standing out from the spire, as in some species of
Ranellæ; while in others it is _decumbent_, running up the sides of the
spire, as in Rostellaria (fig. 402, pl.).
_Canals._
[Illustration: Fig. 51, Fasciolaria, _truncated_; 52, Nassa, _recurved_,
_varicose_; 53, Cerithium, _recurved_; 54, Typhis, _closed_.]
_Lips, or edges of the Aperture._