A Conchological Manual, George Brettingham Sowerby [little readers .TXT] 📗

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