CHAPTER II.

[Sidenote: INHABITANTS OF SHELLS.]

"Conchology," said Mr. Elliot, on resuming the conversation with his children, "is that branch of natural history which comprehends the study of testaceous animals, or animals with _shell-coverings_, and includes those of the seas, the rivers, and the land.

"All shells are formed of carbonate of lime. This you may easily prove by applying a little acid to a shell, and you will find that an effervescence takes place.

"The animals that inhabit shells are bloodless, without bones, but provided with a heart, lungs, and mouth, together with other organs needful to their conformation.

"Testaceous animals have the power of enlarging their habitations; they can also repair any injury that may occur to them.

"Many kinds of shell-fish are made use of by man, and form a valuable article of food, such as oysters, cockles, muscles, scallops. The whelk is also used, and a species of murex.

[Sidenote: FOSSIL SHELLS.]

"A species of cowry is in use for money among some people of Africa; and pearls, so much valued as articles of ornament, are obtained from the oyster and mussel genera.

"Within a few years, conchology has become a study of considerable importance, from its close connexion with geology. Students in the latter science must be well acquainted with fossil-shells, because they form so large a portion of organic remains. Species of recent shells, or those still existing, are also often found in a fossil state, while many fossil genera are now totally unknown in our earth and waters.

"Thus you perceive that while you are obtaining knowledge in one science, you are preparing yourselves for making advances in another, most interesting and wonderful. You, Charles, who are likely to become a traveller, will perhaps in future years find the advantage of my present brief lessons.

"I shall first endeavour to make you acquainted with the system of Linnæus; it is easily learned, and you should be familiar with it, as it is still adopted by some writers on conchology.

"But in order to understand my instructions, you must have a clear idea of the terms that I use in describing a shell; now, therefore, give me your attention while I explain some of those terms to you.

[Sidenote: TERMS FOR PARTS OF MULTIVALVES.]

"To begin with the first division, _Multivalves_. There is a group of _Lepades_, it is the species called goose-barnacle, of which so many strange and silly tales have been told in former times. (Plate 2, _Lepas anatifera_.) This species is furnished with a kind of stem, like a bladder, and is called the _peduncle_, (_c_) and is fastened to other bodies. The _feelers_ (_d_) are feathery projections, which the animal keeps in continual motion, for the purpose of catching its food. Here is a group of another kind; (Plate 2, _Lepas tintinnabulum_;) these are without a peduncle, and are called _sessile_. The _base_ (_a_) is that part of the shell by which it is fixed to other bodies: (_a_) the _operculum_ is formed of four small valves on the summit. (_b_).

* * * * *

"In the shells of the second division, _Bivalves_, we shall find a greater number of parts. _Valves_ are the different pieces that compose a shell. When both the valves are alike in form, the shell is called _equivalve_: when the valves are different in the same shell, it is called _inequivalve_. _Mya_, _Solen_, _Tellina_, are equivalves: _Ostrea_, _Anomia_, _Pinna_, &c. are inequivalves.

[Sidenote: TERMS--BIVALVES.]

"The _hinge_ is formed by the teeth of one valve inserting themselves between those of the other valve, in some genera; in others, by the teeth fitting into the _cavities_ of the other valve (Plate 3., _a_.) When the teeth are placed in the centre of the hinge they are called _cardinal teeth_. _Lateral teeth_ are situated on the sides of the valves, and are generally long and flat, sometimes hollow. Some hinges are straight, others curved. Here is the hinge of _Arca_, furnished with many small teeth. (Plate 3, _b_.)

"The _ligament_ is a membrane that connects the valves, and keeps the hinge in its proper place: it is always situated near the beaks. The ligament is very perceptible in the cockle, in _Pecten_, or scallop, in _Tellina_, &c.

"The _beaks_ are the most pointed parts of the bivalve shell (Plate 3, _c_.); when the valves are closed, the line where they meet is called the _seam_. (Plate 3, _d_.)

"The _anterior slope_ is that part of the shell where the ligament is placed, and is also called the _area_. (Plate 3, _e_.) The _posterior slope_, or _areola_, is the other side of the beaks. (_f._)

"The _lunula_ is a crescent-like depression on either the area or areola. The edge of the valve is called the _margin_; it is often finely _crenulated_, or toothed. The interior of the valve is called the _cavity_. (_g._) In the valves of this ark-shell here are two broad marks, shining and glossy. (_g._) In those of the oyster and muscle that I now show you, there is but one. These marks are _muscular_ impressions; they are the parts where the muscles of the animal have been affixed, and are termed _cicatrix_.

[Sidenote: TERMS--UNIVALVES.]

"_Ears_ are two processes on each side of the beak; the _Pecten_, or scallop, is an example. (Plate 3, _i_.)

"_Sinus_, in _bivalve_ shells, is a small hollow in the hinge.

"_Byssus_, or beard, is an appendage composed of silky threads, by which the muscle and _Pinna_ fasten themselves to the rocks. (Plate 3, _f_.)

"_Cordiform_ is a term applied to heart-shaped shells.

"A _cartilage_ is the same as a ligament. When the valves of a shell are very nearly flat, they are said to be _compressed_: when a valve has teeth, it is said to be _dentated_.

"When the valves of a shell do not shut close, they are said to be _gaping_. (Plate 4, _Mya_.)

"A _muscle_ is a fleshy, pliant organ, by which the animal is attached to its shell. I have already pointed out to you the impressions of those muscles within bivalve shells.

"A _suture_ is a toothed joint, in bivalves.

"A shell with ears is said to be _auricled_.

* * * * *

"The third division, _Univalve_ shells, have also their several parts. The first section has a regular spire. Here are two shells of this section, _Voluta_ and _Buccinum_, both sawed asunder, in order to show the interior structure of the shell. (Plate 3.) The aperture, or opening, being turned _towards you_, the front of a univalve is seen; reverse it, and you see the back.

[Sidenote: TERMS--UNIVALVES.]

"The top, or highest part, is the _apex_; (_a_) the lowest part is the _base_ of the shell. (_b._)

"The _spire_ (_c_) is formed of wreaths, or whorls, (_ddd_) which terminate in the apex: the lowest whorl is the body of the shell. (_e._)

"The _aperture_, opening or _mouth_, (_f_) as it is sometimes called, is on the right-hand when the front of the shell is turned towards you. The aperture is an important distinction in univalves: some genera have a circular opening, as the _Turbo_, or periwincle; some longitudinal, as the cowry; others semi-lunar, as the _Helix_, or snail genus. (For examples of these apertures see Plates 1 and 7.)

"The _beak_ is the lengthened process (_g_) at the lower part of the shell.

"The _canal_, or _gutter_, runs through the beak. (Both these parts are perceptible in _Murex_ and _Strombus_, Plate 7.)

"_Sutures_ are spiral lines which separate the whorls; they are sometimes crenated, or notched, sometimes sulcated, or furrowed.

"The _columella_, or _pillar_, extends through the centre of the shell withinside. The _Buccinum_ and _Voluta_ both show the columella. (Plate 3.)

[Sidenote: TERMS--UNIVALVES.]

"The _pillar-lip_ of the aperture, or columella margin, is on the left-hand side of the shell; the _outer_ lip on the right-hand. Occasionally this order is reversed, but not commonly.

"The _operculum_, or lid, in univalves is that part which fits exactly into the aperture, and incloses the animal; it serves as a door to the shell. (Plate 3.) The operculum is either horny, like that of the periwincle, or of a harder substance, like the shell itself.

"The _umbilicus_ is a circular hole in the body of the shell. This perforation produces a very curious effect when it is very large. When the umbilicus is wanting, the shell is called _imperforate_.

"If the spire is truncated, it is _decollated_; if it is surrounded with spires, it is _coronated_, or crowned.

* * * * *

"Second section, without a regular spire. The _teeth_ in univalve shells, as the cowry, are ridges upon the aperture. (Plate 1.) In _Voluta_ they are regular folds or plaits upon the columella.

"A _fissure_ is a notch or slit, as in _Patella fissura_. (Plate 3.)

"Some shells of this section are internally lipped, as _Patella equestris_; (_Calyptræa_ of other authors, Plate 3;) some are chambered, as the slipper-limpet; some are cap-shaped, having the apex much curved--these are the cap-limpets.

[Sidenote: TERMS--UNIVALVES.]

"The _vertex_ in _Patella_ is as the superincumbent part of the shell.

"The _epidermis_ is the outward skin that covers the surface of some shells.

"_Fauces_ are narrow entrances, as at each end of the cowry.

"_Tubercles_ are protuberances, or knobs, on the surfaces of shells.

"_Striæ_ are raised or flat lines upon the surfaces of shells. When the surface is marked with lines longitudinally and transversely, it is _decussated_.

"_Sulci_ are furrows.

"_Fornicated_ signifies arched, greatly excavated.

"_Umbo_ is the swelling part near the beak of bivalve shells; the same as _boss_.

"_Longitudinal_, running nearly the whole length of the shell in univalves.

"_Concentric_, having the same centre.

"_Convolute_, when the exterior whorls spirally involve the interior.

"_Varices_, longitudinal, gibbous sutures formed in the shell, at certain distances on the whorls.

"_Carinated_, having the form of the keel of a boat."