CHAPTER IV.

Second Division.--_Bivalves_: 14 genera.

"I fear," said Charles, "that this new division will be rather difficult, for my father tells me that we must pay particular attention to the _hinges_ of bivalve shells."

"Then _apply_ yourself to the study of hinges, Charles, and your difficulties will chiefly disappear," answered Mr. Elliot.

[Sidenote: MYA.]

"The hinge of _Mya_, the first on the list, is easily known. The generic characters are, shell gaping at one end, hinge mostly with one thick spreading tooth, not inserted into the opposite valve. The _Mya_ race burrow in the sand. Here is _Mya arenaria_, a large thick shell, frequent on the shores of Kent: the large tooth is sufficiently plain in _this_ species. _Mya truncata_ (Plate 4) is as common, and the curious membranous case, which you will find attached to one end of the shell, is a guide to the species. Both these species are without colour, and have little to attract in their outward appearance. The genus, however, according to Linnæus, varies exceedingly, and contains forty-one species.

[Sidenote: SOLEN. TELLINA.]

"_Solen._ Shell bivalve, open at both ends, tooth of the hinge subulate, or awl-shaped, reflex, often double.

"In this genus, the great length, in comparison with the breadth of the shells in many of the species, is remarkable: some are exceedingly brittle. Our example, _Solen siliqua_ (Plate 1) is a British species. The hinge is not in the centre of the shell, but nearer to one end of it. Some are shaped like the handle of a knife or a razor, others are bent resembling the blade of a scimitar. The _Solen_ lives in the sands of the sea-shore, often burying itself two feet deep, and retaining its shell in a vertical position: thirty-five species.

"The genus _Tellìna_ is remarkable for the beauty of the shells, and, according to the arrangement of Linnæus, contains ninety-seven species. The exterior is sometimes marked with radiations: the surface of some shells is very finely polished, while in others it is covered with striæ and undulations. The species that you have placed before me, Lucy, is _Tellìna Feroensis_; the shell is finely striated, and has also radiations. (Plate 4.)

"The generic characters are chiefly these: shell compressed towards the anterior slope, teeth of the hinge mostly three, the lateral ones smooth, in one valve. Two or three small species are common on our coasts. I should also observe that there is a convex fold on one valve and a concave fold upon the other. Many of the _Tellìna_ genus are found buried in the sea-sands.

[Sidenote: CARDIUM. MACTRA.]

"_Cardium._ Generic character: shell equivalve, convex, ribbed, striated, or grooved, the margin toothed: hinge with two teeth near the beak, and a lateral one on each side: fifty-four species.

"Observe how the beaks of this common _Cardium_, cockle, turn inwards, and the bosses project. Another striking character is the ribs, that are generally longitudinal, and not concentric or transverse, as in _Tellìna_, and, as you will see, in _Venus_. _C. aculeatum_ has small spines on the valves; _C. costatum_, the ribbed cockle, is one of the finest species of this genus, and _C. cardissa_ is a beautiful shell. (Plate 4.) The common cockle is _Cardium edule_.

_Mactra._ Generic character: shell bivalve, unequal sided, middle tooth of the hinge complicated, with a small hollow on each side, and lateral side-teeth: thirty-seven species.

"The shells of this genus are usually thin, brittle, and remarkably light. _Mactra stultorum_ is a common species. (Plate 4.)

[Sidenote: DONAX. VENUS.]

"_Donax._ Margin of the shell often crenulate, the anterior slope very obtuse; hinge with two cardinal teeth, and one lateral tooth.

"The most striking characteristic of _Donax_ is the broad, thick extremity of one end, gradually lessening towards the other. A rich purple tint is very frequent in these shells. _Donax denticulatus_ and _trunculus_ are common British examples. (Plate 4.) You must remark the ligament of _Donax_, which is exterior.

"Our next genus ranks highest for beauty among the bivalves, and takes its name from the goddess _Venus_. The species amount to one hundred and sixteen in the Linnæan system, but other authors have formed several new genera.

"Shell bivalve, having the lips incumbent on the anterior margin; hinge with three teeth, all approximate, the lateral ones diverging at the lip.

"I am afraid," said Lucy, "that we shall find this genus very difficult: I wish you would tell us the new genera that have been formed out of it."

"Learn first to know the general appearance of _Venus_, and remark especially the _teeth_. You may also bear in mind that the _beaks_ are almost always turned _to_ the posterior slope, and _from_ the ligament. The area and areola are also very conspicuous: the area is generally large, and differently coloured to the disk. _Venus Paphia_ is pretty. (Plate 4.) The spinous species, _V. Diòne_, is more beautiful, and is the only shell of the genus that has spines. The brown Venus, _V. chionè_, is very smooth and polished; both species are frequent in collections. The British shells of this genus are neither numerous nor very beautiful.

[Sidenote: SPONDYLUS.]

"_Spondylus._ Valves unequal, rough; hinge with two recurved teeth, with a hollow between them; shell sometimes eared. (See Plates 3 and 5.)

"I think," said Lucy, "that the English name, _thorny-oyster_, is not very suitable: it is more like a scallop; but it differs from both in having two strong teeth in the hinge, and I observed this morning that neither the oyster nor the scallop have any hinge."

"So that was the object you had in view," said Charles, "when you were so quietly handling those shells in the kitchen: I confess I could hardly help laughing; and now my father will say that _you_ are '_Eyes_,' and I the '_No Eyes_,' of 'Evenings at Home.'"

"Perhaps I might have made the observation," replied Mr. Elliot; "but you have reproved yourself, which is far better.

"_Spondylus_ can scarcely be mistaken from any other bivalve shell. The species _gæderopus_ is remarkable for its projecting beak; the surface is rough, with either tubercles or spines. Some authors reckon only four species, others thirteen. The _Spondyli_ are frequently found attached to rocks at some depth in the ocean. The animal is eaten on the coasts of the Mediterranean. We have no British _Spondylus_.

[Sidenote: CHAMA. ARCA. OSTREA.]

"_Chama._ Shell thick; hinge with a thick tooth, sometimes crenate, obliquely inserted into a corresponding channel. (Plate 5.) The shells of this genus vary greatly, which you will perceive upon comparing _C. gigas_ and _C. cor_. (See Plate 9, _Isocardia cor_.) The _Chama_ genus is usually ribbed, foliated, or scaly. _C. Lazarus_ is a beautiful species: _C. cor_ is a British species, and the only one. The whole number is twenty-five.

"Here is _Noah's-ark_, an example of the genus _Arca_, and is found on our own coasts. The long hinge beset with sharp teeth, inserted into each other, renders the genus sufficiently marked; but in some species the hinge is curved. The form varies exceedingly. The number of species is forty-five. (Plate 5.)

"_Ostrea._ In this well-known genus we lose sight of the _toothed_ hinge. Take that _Pecten_, or scallop, which belongs to one division of _Ostrea_ in this system, and tell me what holds the valves together. Charles is silent; what says Lucy?"

"Here are the remains of the same kind of substance which we saw in _Donax_ and in _Venus_. I think it is called the _ligament_."

"Very well remembered," continued her father. "The generic character of this very large portion of bivalves is, shell bivalve, usually with unequal valves: hinge without teeth, having a hollow cavity or sinus, and sometimes grooved. Here is a young common oyster, and, according to the rule of our present system, this shell, _Ostrea isognomon_, is of the same genus. (Plate 5.) The number of species is eighty-four, of which thirteen are British. The old shells of common oysters are often covered with _Serpula_, _Lepas_, and _Anomia_, and some kinds of corallines.

[Sidenote: ANOMIA. MYTILUS.]

"The next genus, _Anomia_, is remarkable for the thin, delicate, and almost transparent appearance of the shells. The valves are unequal, and frequently perforated near the apex; hinge toothless; in the flat valve, two bony rays.

"_Anomia ephippium_ has a large perforation, through which the animal passes a ligament, and attaches itself to other substances. These shells are often to be found on oysters. (Plate 5.) Species thirty-two.

"_Mytilus._ The principal characters are, shell bivalve, rough, often affixed by a thick byssus, or beard; hinge without teeth, with a hollow line extending lengthways. (Plate 3.)

"The common muscle, _Mytilus edulis_, must be well-known to you, and also the fine polish that the shells will take when cleared of the rough exterior by artificial means, _Mytilus barbatus_ is not unfrequent on our shores; the colour is brown, and the shell is shaggy. Number of species, forty-nine.

[Sidenote: PINNA.]

"Our last bivalve genus is _Pinna_. The generic characters--shell bivalve, brittle, erect, gaping at one end, throwing out a byssus; hinge without teeth. (Plate 6.)

"The _Pinna_ race are found plentifully in the Mediterranean, the Indian, American, and Atlantic oceans: the British seas afford three species. The genus is noted for producing a fine byssus, that is manufactured in Italy into various articles, as gloves. The animal is sometimes used as an article of food.

"An ancient writer asserts that the _Pinna_ is attended by a crab, that finds a habitation in its shell, and repays the favour by giving notice, by a gentle nip, when a fish comes within reach; the _Pinna_ opens the valves of the shell, and secures the prey, which serves for the food of both. Now, Charles, you know the whole sense of the quotation--

"'The anchor'd pinna and his cancer friend.'"