_inner_, or _columellar_ lip. Posteriorly it commences at the point of
union with the outer lip, where that touches the body whorl, the junction
being generally marked by an angle, and sometimes by a canal. Anteriorly it
terminates where there is generally seen a notch or canal, or sudden angle,
from which the outer lip proceeds. The part which setting out from the body
whorl, and proceeds outwards at a distance from the axis, till it reaches
the anterior canal or notch (or its place in case of absence) is named the
_outer lip_. In many cases the edges are united in such a manner, that it
is difficult to distinguish where the inner lip terminates, and the outer
lip commences: when this is the case, it is usual to describe the margin or
peritrême, as a whole, without distinguishing the parts. The _outer_ lip,
sometimes called the right lip, or _labrum_ of continental writers, is
sometimes acute, not being of thicker substance than the remainder of the
shell. In other cases it is _obtuse_, or thickened and rounded at the edge.
When thickened and turned backwards it is described as _reflected_; when,
on the other hand, it is turned inwards towards the axis, as in the
Cyprædæ, it is _inflected_, or involute. When it is _toothed_, a
distinction must be observed as to whether the dentations are external or
internal. If the teeth are small and numerous, it is _denticulated_; if
larger, it is _dentated_; when expanded into a kind of wing, as in some
species of Strombus and Rostellaria, it is described as _alated_; and a
family in Lamarck's system is named "Alatæ," from this very circumstance.
In some of those which are expanded, the expansion is divided into
separate, attenuated portions, they are then said to be digitated.
_Outer Lips._
[Illustration: Fig. 55, Helix, _reflected_; 56, Cypræa, _involute_,
_denticulated_; 57, Sera, _alated_; 58, Murex, _digitated_; 59,
Rostellaria, _dentated_.]
The _inner_ lip, sometimes named the _columellar_ lip, or "_labium_," is
subject to similar variations as to thickness, dentition, &c. That portion
of it which lies upon the body-whorl is frequently distinguished from that
which intervenes between it and the notch or canal. De Blainville,
restricting the term _lip_ "bord gauche" to the former portion, applies the
term "columella" to the latter; and in some instances this may be the more
convenient method of describing the part in question. The columellar lip is
sometimes _detached_ entirely from the body of the shell, as in Murex
haustellum; in others it is _decumbent_, or lying over the last whorl,
although quite distinct, and in some cases, _thickened_, _callous_, or
_tumid_.
At the lower or anterior part, sometimes called the _columella_, there are
in many instances flattened, laminated folds; these are particularly
conspicuous in the genera Cymba and Melo, where, being obliquely spiral and
laminar, they are extremely elegant, presenting to the eye graduated
repetitions of the line of beauty. In other cases, as in the Turbinellæ,
they are more horizontal and thickened.
In some cases the columella is swelled into a varicose mass; as in Oliva,
Ancillaria, &c.; it is then described as _tumid_ or varicose. It is
sometimes _tortuous_, and sometimes straight, and is susceptible of many
variations, too minute and particular to be described in this part of the
work.
COLUMELLAR LIPS.
[Illustration: Fig. 60, Melo, _obliquely plaited_; 61, Turbinellus,
_horizontally plaited_; 62, Ancillaria, _varicose_, _tortuous_; 63, Natica,
_straight_.]
OPERCULUM.
The aperture of many species of shells remains constantly open; but in a
great number of species it is occasionally closed, whenever the animal is
retracted within the shell, by a calcareous or horny piece called the
operculum. This must be distinguished in the first instance from another
kind of calcareous covering, which in some univalve shells serves to close
the aperture during a certain portion of the year. This piece, named the
_epiphragm_, although hardened and shelly in appearance, is no real part of
the animal or of the shell; being only a secretion temporarily hardened,
for the purpose of defending the animal from external influences during the
_hibernating_ or _torpid_ season, to be dissolved when that season is at an
end. On examining this piece, it will be observed that it is not formed in
regular layers like the rest of the shell; while the true operculum is of a
regularly laminated structure, having a nucleus and receiving obliquely
deposited additions, either in a lateral spiral or concentric direction. It
is attached to the posterior part of the foot on the upper surface; and
when the animal retires within its shell, that part of the foot enters
last, drawing the operculum after it, and thus closing the aperture.
The opercula of various shells differ in the first place as to their
chemical composition. They are _calcareous_ when formed principally of
calcareous matter, like the rest of the shell, as in Neritina, Nerita, and
some others. They are _corneo-calcareous_, when upon an internal lamina of
horny consistency there is a thickened layer of shelly matter. This is the
case with shells of the genus Turbo and Phasianella, which are on this
account distinguished from those of the genus Trochus; the opercula of the
latter being horny or _corneus_.
The size of the operculum is distinguished by comparison with the rest of
the shell; thus, those of Strombus, Cassis, &c. are small; while those of
Cyclostoma and others are large, filling up the cavity at its outer edge.
The direction in which the successive layers are deposited, forms another
ground of distinction. The disc is formed in some instances of a series of
whorls, the apex or nucleus being more or less central; if these whorls are
numerous, the operculum is described as _multispiral_, as in shells of the
genus Trochus; if few, as in Cyclostoma, it is _paucispiral_. In some
instances the flattened spire consists of but one whorl, it is then
_unispiral_; and when scarcely one turn is completed, it is described as
_subspiral_. When the layers are applied upon each other in such a manner
that the nucleus is central, and the edges of the subsequent layers are
extended beyond each other all around, so as to form rims, the operculum is
described as _concentric_; if the nucleus is lateral, or at one side
without being spiral, it is _lammellated_; and when it forms a terminal
point, enlarging in the form of a finger-nail or claw, it is
_unguiculated_. In the operculum of a Neritina, there is a lateral process,
by means of which it is locked into the columella, the term _articulated_
is then applied. In that of Navicella, there is also a process which
appears to radiate from the nucleus, it has therefore been described as a
_radiated_ operculum.
_Opercula of Spiral Univalves._
[Illustration: Fig. 64, _multispiral_; 65, _paucispiral_; 66, _concentric_;
67, _articulated_; 68, _radiated_; 69, _lammellated_; 70, _unguiculated_.]
BIVALVE SHELLS.
Bivalve shells, named Conchacea by Lamarck, are those which consist of two
principal portions united to and folded upon each other by means of a
hinge. The pieces united compose the shell, while each piece separately is
called a valve. Considering the bivalve shell as a whole, it will be
necessary, in the first instance, to describe the position in which it is
to be observed, in order to give the student a clearly defined notion of
what is intended, when terms expressive of height, depth, length, breadth,
&c. are used, as well as when the anterior and posterior extremities are
spoken of. For this purpose, we must suppose the animal to be living and
creeping along the bed of the sea by means of its foot; where this foot
protrudes, will be the _ventral margin_, and the opposite part the _dorsal
margin_ of the shell. There will then be a valve on each side; and if we
further suppose the animal to be walking forward with its back to the
observer, the _right_ and _left_ valves will correspond with his right and
left sides.
_Measurements._
[Illustration]
The _length_ will be measured from _anterior_ (_a_) to _posterior_ (_p_),
and the lines of growth running in the same direction will consequently be
_longitudinal_ or _concentric_; _transverse_ of some authors. The height
will be from the umbones (_u_), to the _ventral margin_, and lines or bands
in that direction are termed _radiating_; longitudinal, according to some
authors.
The points from which the growth of the shell commences, are called the
_umbones_; these usually turn towards the anterior part of the shell: if
this circumstance fails to point out the anterior, it may in many cases be
distinguished by the muscular impressions of the mantle. If this has a
sinus or winding, it is always near the posterior muscular impression; and
in all cases where there is an external ligament, it is on the posterior
side.
There is sometimes an impression near the front of the umbones, which forms
a semicircle on each valve; the space within this semicircle is called the
_lunule_ (wood-cut, fig. 71 and 72, _l. l. l._); a corresponding
depression, when it exists on the posterior margin near the umbones, is
named the escutcheon.
_Hinge._
[Illustration: Fig. 73, _l t_, lateral teeth; _c t_, cardinal teeth; _c_,
cartilage under the ligament; _l_, ligament; _f_, fulcrum of the ligament.]
The _hinge_ of the shell is on the _dorsal_ margin, and is composed of the
various apparatus by which the two valves act upon each other in opening
and shutting. It consists of a _ligament_, which is placed on the dorsal
margin, just at the back of the umbones, and unites the two valves
together; the _cartilage_ or thick gristly elastic substance, sometimes
found close to the _ligament_, to which it then forms an inner coating, and
sometimes received into a pit within the shell. It serves the purpose of
keeping the shell open when not forcibly closed by the adductor muscles. An
inner layer of shelly matter upon which are placed teeth, and pits to
receive them on the two valves reciprocally. Each of these it will be
necessary to treat of more at large; observing, at the same time, that in
some species of Bivalves these parts may be wholly or partially wanting.
Thus we meet with some shells, such as the Muscle, without teeth; and there
is the group containing Pholas, &c. the hinge of which is destitute of
teeth and ligament, the two valves being kept together by loose cartilages,
and by the contracted space in which they are confined.
_Ligament and Ligamentary Cartilage of the Hinge._
These two distinct substances have been described by many writers as
though, composing the same mass, they were of one substance; but the
difference may very easily be explained. The _true ligament_ is external,
being fixed on the edge of one valve behind the umbones, and passing over
in an arch to the corresponding edge of the other, very correctly retaining
the name of _ligament_, because it serves the purpose of binding the two
together. The thick, elastic substance, which Mr. Gray names the
_cartilage_, is sometimes found in connexion with the ligament, so as to
form one mass with it, although it is always separable and placed within
it: it is sometimes placed quite within the shell, and separated from the
ligament, in a pit or hollow formed for its reception in the hinge lamina,
near the centre. It is found in both valves, and being elastic, the portion
in one valve presses against that in the other, so as to keep the valves
apart, unless voluntarily closed by the adductor muscles of the animal. The
ligament is sometimes spread over an external area, as in Arca, while the
cartilage is placed in several grooves of the same area, beneath the outer
covering.
_Hinge lamina, Teeth and Fulcrum of the Ligament._
In a great variety of cases, there is a thickening of the substance of the
shell within, under the dorsal margin; this is named the hinge lamina. It
is sometimes merely callous; but in many cases it has raised _teeth_ in
both valves, those in one valve entering into corresponding cavities in the
other. Those which are placed immediately below the umbones, and seem to
take their rise from beneath them, are called _cardinal teeth_; those at a
distance from the umbones, which are seen to lie along the upper margin of
the shell are named _lateral teeth_.
When the cardinal teeth terminate in a double point, which is not
unfrequently the case, they are said to be _bifid_. The lateral teeth, in
various species, are distinguished as terminating _near_ to, or at a
_distance_ from the umbones. In the Nuculæ and Arcæ there is a row of teeth
placed across the hinge lamina. In which case, the lateral cannot be
distinguished from the cardinal teeth.
_Muscular Impression._
[Illustration: Fig. 74. _a, anterior; p, posterior; m i, muscular
impressions._]
Lamarck divides the Bivalve shells into two general orders; the first is
named "Dimyaria," having two adductor muscles; and the second,
"Monomyaria," having but one. These adductor muscles are used for the
purpose of drawing the valves together, being composed of contractile
fibrous gristle, fastened firmly to the inner surface of each valve. The
place where they are thus fixed may be seen when the animal is removed, by
depressed areas, which are generally pretty well defined, and are named
_muscular impressions_. Where there is but one adductor muscle, there will
be but one of these impressions near the centre of each valve, but in the
Dimyaria, where there are two, the impressions are seen, one on the
anterior, and one on the posterior of each valve, just below the _hinge_
lamina. They are sometimes _complex_, that is composed of several portions
in a group; but in general, they are simple and well defined.
They are also described as large or small, in proportion to the size of the
shell; regular or irregular in form. The animal is attached to the inner
surface by the fibrous portions of the mantle, which creates a linear
impression or _cicatrix_, commonly described as the _palleal impression_,
or muscular impression of the mantle. It runs near the ventral margin from
one muscular impression to the other, sometimes in a smooth _continuous_
line or band, and sometimes in an interrupted series of small impressions.
Near the point of union with the posterior muscular impression, there is
sometimes a more or less considerable winding inwards towards the centre of
the shell, and back again towards the point of union. This is named the
_sinus_, and is distinguished as being _angular_ or _rounded_, large or
small, according to the species. When it enters towards the centre of the
shell in a tongue-shaped outline it is said to be _ligulate_. Where it
exists it affords a certain index to the posterior side of the shell; as it
is the region through which the excretory tubes pass.
_Umbones._
These are the prominent points of the dorsal edge, where the growth of the
shell commenced, and are called beaks, by some English writers. In some
instances they are close to each other; in others they are rendered distant
from each other by the intervention of areas in the hinge, as in Spondyli,
&c. In Pectunculus they are _straight_; in Venus _curved_ towards the
anterior margin; in Isocardia, _spiral_; in Chama, _decumbent_; in Diceras,
_free_. In shells subject to external corrosion, the process commences at
the umbones.
[Illustration: Fig. 75, _distant_; 76, _straight_; 77, _curved_; 78,
_spiral_; 79, _decumbent_; 80, _free_; 81, _close_.]
GENERAL CHARACTER OF BIVALVES.
When the _breadth_ is spoken of, the distance between the most convex parts
of both valves, when closed, is intended; but when an expression implying
_thickness_ is used, it refers to the substance of each valve: it is
important to bear this in mind, as many persons have been misled by
descriptions in which the distinction has not been attended to. Glycimeris
(fig. 67 in the plates) is a _thick_ shell, but Anatina (fig. 69 in the
plates) is a _broad_ one.
_Regularity._
A great number of Bivalves are extremely regular in their form. These are
generally locomotive, and consequently free from those obstructions in
growth occurring to stationary shells, which being confined in a particular
position, or to a particular spot, modify their shape according to the
substance with which they come in contact, and thus become irregular. This
is generally the case with shells which are attached to submarine
substances, such as Spondyli, Oysters, &c.; and the degree of irregularity
will depend upon the extent of surface involved in the attachment. In the
case of fixed shells, the attached valve is usually termed the under valve,
and the other which moves freely upon the hinge, is termed the upper valve.
_Form and Proportions._
Bivalves are said to be _equivalve_ when the two valves correspond in
extent, breadth, and thickness; and of course _inequivalve_ in the contrary
case. They are _equilateral_ when a line drawn from the umbones to the
ventral margin would divide the shell into two nearly equal parts; and of
course _inequilateral_ in the opposite case, which occurs in the great
majority of instances.
A Bivalve is said to be _compressed_, when the distance is small from the
most prominent part of one valve to that of the other. It is _cylindrical_
when lengthened, and more or less rounded in its breadth, as in Lithodomus
(fig. 161 in the plates). It is _cordiform_ when the shape presents a
resemblance to an imaginary heart, as in Cardium cardissa (fig. 122 in the
plates), and in the Isocardia (fig. 126 in the plates). It is _linguiform_
when it resembles a tongue in shape, as in Vulsella (fig. 185 in the
plates); _rostrated_ when it protrudes at either extremity, and terminates
in a kind of point, as in Sanguinolaria Diphos (fig. 99 in the plates);
_truncated_ when it ends in a square or angle, as if cut off; an example of
which may be seen in Solen (fig. 60 in the plates).
Other Bivalves are distinguished as being _auriculated_, having processes
flattened and expanded on either side of the umbones, as in Pecten (cut,
fig. 82). When there is one of these on each side of the umbones, it is
_bi-auriculated_; when only on one side, it is _uni-auriculated_. When the
expansion is very broad, as in Unio alatus (fig. 142 in the plates), and in
the Hammer Oyster (cut, fig. 83), the term _alated_ is used.
[Illustration: Fig. 82, _auriculated_; 83, _auriculated_, _alated_.]
With regard to these alated species of _Uniones_, it is necessary to
observe that they are also "_adnate_," as it is termed; the two valves
being joined to each other by the dorsal edge of the expanded parts, and
united so completely in substance with each other, that they cannot be
separated without being broken. Many other terms are used to express
difference in Bivalves, but being generally applicable to Univalves and
Multivalves, as well as to them, they will be found explained at large in
the alphabetical part of the work.
MULTIVALVE SHELLS.
These are of three different kinds; first, the "_dorsal_," as they are
termed by Linnæus, because they form a ridge in the back of the animal.
They are composed of eight pieces, or separate valves, placed in a
longitudinal series, being joined to each other by inserted lamina, and
named _Articulata_ by De Blainville, on that account. The genus Chiton is
the only example of this kind of Multivalves.
[Illustration: Fig. 84, 85, Chiton. _a_, anterior; _p_, posterior; _d_,
dorsal ridge; _l l_, lateral areas of the valves; _c c_, central areas; _i
i_, inserted lamina; _m_, margin.]
The second kind, M. De Blainville terms the _lateral_ bivalves, the pieces
being placed in pairs on each side of the animal; these compose the
"Pedunculated Cirripedes."
[Illustration: Fig. 86, Anatina.]
They differ considerably in the number and arrangement of the valves; the
small ones, which are found near the peduncle in some species, are
sometimes termed accessary valves; those which form the edge through which
the bunch of Cilia protrude, are termed _ventral_, and those on the
opposite side _dorsal_. The extremity joining the peduncle is the basal, or
anterior; and the upper extremity is the apsiral, or posterior. The
peduncle is the medium of attachment to submarine substances, to which this
well known tribe of shells adhere.
The third kind are termed _coronular_ by De Blainville, and compose the
order Sessile Cirripedes of Lamarck; they consist of a number of valves
placed against each other side by side in a circle, supported on a plate,
or tube, or cup, and closed by an operculum composed of two or more valves.
The _basal support_ is sometimes thick and flat, sometimes forming an
elongated tube, and sometimes hollowed out into a cup. In other species it
is altogether wanting. The operculum always consists of more than one
piece, generally of two pairs: they are either articulated to each other by
serrated edges, and placed against each other conically, as in Balanus, or
they lie flat in two pairs against each other. Through the ventral pair the
_cirrhi_ protrude.
The _parietal_ valves, composing the principal part of the shell, vary in
number, form and position. The _anterior_ valves are placed on the same
side with the cirrhi; the _posterior_, those on the opposite side; and
those which remain between on each side are the lateral valves. In many
cases, particularly in Balanus, each valve is separated into the
_prominent_ and _depressed_ areas, and the inserted lamina. In some
instances, the parietal portion is formed by a single rounded piece.
[Illustration]
In the accompanying cut (87), the prominent areas are distinguished by the
letters _pr_, and the depressed areas by _r_; the posterior valves of the
operculum are marked _p. o._, and the anterior _a. o._ The basal valve
(fig. 88) belongs to a Balanus. Fig. 89 is an Acasta, the cup-shaped base
of which is represented at fig. 90.
In the foregoing explanations we have omitted many of those general terms
which, relating to external characters, are applicable to shells in almost
every division of the system. It may be as well, however, to enumerate a
few of them in this place, although they are explained under their
respective letters in the alphabetical part of the work.
When bars or ribs, or large striæ are crossed by others radiating from the
umbones, shells are said to be _cancellated_, as represented in cut, fig.
91. When there is a series of nodules or spines on the upper part of the
whorls, they are _coronated_, as shewn in cut, fig. 92. When a series of
projecting parts overlay each other, in the manner of tiles, as in the cut,
fig. 93, the word _imbricated_ is applied. When marked by a regular series
of ridges, radiating from the apex, they are _pectinated_; the species of
Chiton, a single valve of which is represented in cut, fig. 94, has
received the specific name of _pectinatus_, in consequence of this
character. Shells are said to be _plicated_ when characterized by angular
bendings or foldings in their surface, as shewn in cut, fig. 95. A strong
instance of this is seen in the Ostræa Crista-Galli. When the margin of any
shell has a series of minute notches, resembling the teeth of a saw, it is
said to be _serrated_; when covered with raised points or spines it is
_aculeated_; and when striated in both directions, it is _decussated_; when
covered with a number of raised rounded points, it is _granulated_; and
having a series of these points placed in a row, near or upon the edge, it
is _denticulated_, as already explained in reference to the outer lips of
Spiral Univalves. When the external surface is rendered uneven by raised
knobs, it is said to be _tuberculated_; and if rendered rough and prickly
by sharp points it is _muricated_, as in the cut, fig. 97. The term
_reticulated_ is applied to fine raised lines, crossing each other, and
resembling fine net-work.