"These genera consist mostly of small shells, inhabiting stones, into which they bore holes. S. rugosa is British. Venerupis perforans is found on our coasts in stones. The valves of these shells have no accessory pieces like Pholas.

"Nymphacea is the next family, containing, in the first section,

"In the genus Psammobia we find our Tellina Feroensis. (Plate 4.) The shells of this and the preceding genus resemble the solens in a trifling degree, being a little open at the sides. In form they are near Tellina, but have not the fold on the anterior valve, but an angle on each valve. The ligament is exterior.

"In the second section are—

"There is but one species of Tellinides from the island of Timor. The genus Corbis is fossil, with one exception, Corbis fimbriata, from the Indian Ocean. Lucina is a pretty genus of shells. L. carnaria is frequently found in collections. The interior of the valves is of a deep red colour: the muscular impressions are very distant from each other; one is greatly lengthened out; the valves delicately striated. Capsa is taken, you perceive, from Donax.

"Tellina is found fossil on the borders of the Red Sea, also in the county of York. Of Donax and Mactra the fossil species are few.

"In the third section of this order we find six families. 1st. Conchæ, which are of two kinds, fluviatic, living in fresh-waters; and marine, or living in the sea. Of the first are,

Cyclas, taken from Tellina.
Cyrene, partly from Tellina and Venus.
Galathea, Venus paradoxa, (one species.)

"Cyclas rivicola (Plate 9.) will give you an idea of this genus: it is Tellina cornea of Linnæus. The species are very common in lakes, rivers, and ponds: it abounds in river-sand, from which you may often procure perfect specimens. Lamarck observes that it is rare in France; but appears common in the Thames.

"Cyrene is a foreign genus.

"In the Conchæ marìnæ the genera are very numerous. They are all assembled under the Venus of Linnæus. Lamarck reduced the genus; but it has been yet further divided by later writers.

"Pullastra was the name of a species, and includes V. pullastra, V. papilionacea, V. decussata, V. litterata, V. virginea.

"Astarte includes some British species, V. Scotica, &c.

"Potamophila is a scarce river-shell from Ceylon. Some species have also been brought from Congo by African travellers. The form is triangular, very thick, covered with an olive-green epidermis. Lamarck's two genera have been still further reduced; but I shall refer you to the Museum for their new names. Observe, in Venus there are three cardinal teeth, close together, on each valve, with divergent lateral teeth. V. lamellata is rare and beautiful, from the seas of New Holland. There are many species of Venus in a fossil state. In Cytherea we find four cardinal teeth on the right valve, three of them near together, the fourth quite apart. The left valve has three cardinal teeth. C. Dionè, the thorny Venus, is a pretty shell with spines. You may easily procure it.

"Astarte has some fossil species in the crag and green sand: A. obliquata is one species.

"Venericardia is wholly a fossil genus: one species is found in the crag, V. senilis.

"The family Cardiacea contains

"Isocardia cor is British. (Plate 9.) There is a beautiful species, Isocardia moltkiana from the East Indies, which is much valued by collectors.

"In the family Arcacea we find,

"The hinge of Arca in this arrangement is always straight, furnished with a number of teeth; the ligament is external. The shells are open at one end, for the animal throws out at the aperture a number of threads, by which it fastens itself to the rocks. The species are thirty-seven, and also several fossil.

"The orbicular form of Pectunculus, and its arched hinge, distinguish this genus from the preceding one. They are allied to the Pectens by their form, and their crenulated internal margin.

"The hinge of Nucula is set with little teeth on each side, like a comb. It is pearly within, and sometimes small pearls are found in the shell. Pectunculus costatus is found in London clay.

"Trigoniana is a small family containing Trigonia and Castalia. The first is a fossil genus chiefly. Some species are found in the Portland stone, or oölite beds.

"The next family contains the Nayada, chiefly composed of fluviatic, or fresh-water shells. They are covered with an olive-brown epidermis, which is constantly found eroded, or destroyed at the beaks. The muscular impressions are lateral and much separated; one of them is formed of two or three distinct irregular impressions.

"Unio has two teeth on each valve; one is cardinal, the other lengthened out. The ligament is exterior—the shell pearly. Unio pictorum is common in rivers. The shell is used to hold small masses of gold or silver for artists, under the name of shell-gold.

"Anodon is also to be met with in our rivers.

"A. anatina is eaten by ducks and crows. The latter, when the shell proves too hard to penetrate, mount with it into the air, and letting it fall, pick out the fish from the broken shell.

"Chamacea has only three genera.

Etheria, a rare genus, from the Indies and Madagascar.

"Diceras is a fossil genus—only two species known according to Lamarck.

"Linnæus had assembled in his genus, Chama, shells with equal and with unequal valves, shells fixed to other marine bodies, with those that are free; some with one, others with two muscular impressions. In the present genus, Chama, the shells are irregular, thick, scaly, or spinous. The hinge has one thick tooth, often notched: the beaks are bent inwards. They are found in the Indian, American, and Mediterranean seas. There are several fossil species.

"The first order, Bimusculosa, is finished. In our next lesson we shall proceed to the families and genera contained in the second, Unimusculosa."

CHAPTER VIII.

Class.—Conchifera.

Second Order.—Unimusculosa.

"1st family, Tridacnacea.

"In the first genus we find the great Tridacna gigas, the largest and heaviest shell yet known. It sometimes weighs five hundred pounds. The hinge has two teeth, the lunula is open, the valves equal, the ligament exterior.

"Mytilacea.

"The greater part of these genera attach themselves to marine substances by a byssus. The Modiola genus are rarely found fixed. The ligament internal, lodged in a marginal gutter. Beaks nearly lateral; hinge without teeth. The genus Pinna is unaltered. Small crustaceous bodies, resembling the crab, are sometimes found in the shells of the Pinna.

"Malleacea.

"The first genus is found in the seas of warm climates. The shells are thin and foliated. The hinge of Perna differs widely from that of the oyster. It is linear, formed of sulcated teeth. There is a sinus under the extremity of the hinge, for the passage of the byssus. Compare P. isognomon with the common oyster, and you will find few points of resemblance between them. (Plate 5.) Perna ephippium is also a curious species, very pearly within. The hammers are rugged and singular in form. They are all foreign, from the oriental seas.

"Avicula, or Swallow, so called from the resemblance of the shells to a bird flying, was considered as a single species by Linnæus. Lamarck makes eighteen species in his new genera. Meleagrina has two species. The pearl-bearing muscle, as it is called, is found in the Persian Gulf, the Gulf of Mexico, &c. The interior of the shell is coated with thick pearl, and within it are formed those globular substances known by the name of pearls.

"Family, Pectenida: genera—

"The genus Lima is longitudinal, auricled, or eared; hinge without teeth, with a hollow receiving the ligament. These are very pretty shells, generally white, almost transparent, resembling the Pecten. Lima comes from the American seas, and is a species easily obtained. There are also several fossil species.

"Plagiostoma is wholly a fossil genus, of which several species are found in this country, in lias, &c.

"The pectens are so easily known that I need only mention some fine species, such as P. pallium, a splendid shell, from the Indian seas: P. pleuronectes is a finely polished, smooth species from the Indian Ocean.

"The genus is divided into sections, viz. ears equal, ears unequal. You may find some common species on our own shores, and you may procure fossil species: they are numerous.

"Plicatula is a genus taken from Spondylus. Spondylus gæderopus, from the Mediterranean, is a common shell in collections.

"Podopsis is a fossil genus.

"Ostracea.

"The oysters and pectens differ so widely that they do not even rank in the same family.

"In the Ostracea are,

"There is but one recent species of the first genus; but many fossil.

"The oyster is said to possess the most limited faculties of all shelly tribes. Immovable upon the rock or marine substance to which it is fastened, it receives no other nourishment than what the waves contribute, and indicates no other sign of life than opening and closing the valve of the shell. This genus still retains a great number of species: one section has the margin of the shell either simple, or waved, the other folded. O. edulis, common oyster, belongs to the first division.

"O. folium is of the second; a curious species, from the Indian and American seas: the shell is fixed to wood and to the roots of trees on the sea-shores.

"Vulsella is a foreign genus, from the Indian and other seas.

"Placuna does not adhere to any marine substance. The valves are flat, thin, and transparent; the very small space between them shows that the animal must be extremely flattened: there are two singular ribs at the hinge in the form of a V.

"P. placenta, Chinese window-glass, is so transparent when young, that it serves instead of that material in China.

"Anomia. The shells of this genus are fixed, like the oyster, to marine bodies. They live and perish on the spot where they are at first produced. I have noticed the muscle by which they attach themselves. Lamarck informs us that a hard, small operculum is to be seen at the extremity of this muscle, and fills up the hole in the flat valve when the muscle is contracted. (Plate 5.)

"The family Rudista contains only a few genera, which will be quite uninteresting to you at present.

"The next, Brachiopoda, has

Lucy could not forbear interrupting her father upon hearing the name of Patella. "How can that genus be mixed with the Conchifera?" she inquired.

"The shell is bivalve," he replied; "raised upon a fleshy peduncle, and fixed to marine substances; the hinge is without teeth, having the form of a duck's beak; the colour a greenish tint. It is found near the Molucca isles.

"Yet more remarkable is the Hipponyx mitrata, a common shell, known as Patella mitrata, long supposed to be a univalve, the upper valve only being known. A French naturalist discovered the lower valve, and both have one muscular impression in the form of a horse-shoe.

"I think that it will be best to pause a little before we enter upon the study of the twelfth class, Mollusca, which contains most of the univalves of Linnæus."

CHAPTER IX.

Twelfth Class—Mollusca.

"As I have observed that you have been very diligent in studying Lamarck since our last lesson," said Mr. Elliot, "I propose to make you acquainted with the variations in the univalve genera.

"The animals of the Mollusca are soft, without joints, generally possessing a head, eyes, and tentacula, or feelers. They have also a fleshy membrane, called a foot, which they use for climbing. The orders, excepting the first, are named from the position of this foot. They are five in number. The first order contains very few genera. One genus, named Cleodora, contains a species brought from Africa. The shell is curious, transparent, and shaped like the head of a halberd.

"The animals of the second order, Gasteropoda, have a muscular foot, or disk, on which they rest. The families are seven. The first, Tritonia, I shall pass over.

"Phyllidiana includes the genera—

"The Chiton moves like the Patella, upon a disk, or foot.

"The body of Patella is entirely covered by the shell. You may have many opportunities of examining the British species. P. pellucida is very transparent, with blue lines.

"The family Semi-phyllidiana contains

"The Umbella shell is flat and white, and is sometimes four inches in diameter. It is common in the Isle of France: there is another from the Mediterranean.

"Calyptracea is a larger family: it has many genera taken from Patella:

Parmophorus is found in the seas of New Holland and New Zealand. The margin of the next genus is distinguished by a slit: the shell is conic. (Plate 3.) Fissurella has the top of the shell perforated; it is called the key-hole limpet, from the shape of the aperture. Pileopsis is obliquely conic. It was with this division that the curious Hipponyx ranked.

"Calyptræa is very thin and brittle, with an internal lip. (See Frontispiece.)

"Crepidula has the shell partly covered, or arched: it looks like a little slipper.

"Ancylus spina-rosa is a pretty species from the south of France: the genus is fluviatic. A. lacustris and fluviatilis are both natives of our fresh-waters.

"The next family, Bullæana, has

"Acera and the following genus have each but one species, Bulla carnosa and B. aperta of Linnæus. The original genus Bulla was composed of an assemblage of shells of various characters, having little resemblance except in their gibbous form. Bulla naucum is an example of the genus of Lamarck's system; so is B. lignaria. (See Frontispiece.) B. ampulla and B. striata are common in collections.

"Aplysiana is a small family, containing

"One species of Aplysia is found on the Devonshire coast: the name Sea-hare marks the singularity of the two tentacula, which resemble the ears of the hare. The body is folded up in a loose skin, or mantle: upon the middle of the back it carries a circular shield, thin, transparent, and yellowish, in which it resembles the slug. These animals swim with ease.

"Dolabella resembles the Aplysia in some degree; the genus is foreign, and one species is known to inhabit the bays of the Isle of France, where it covers itself with a portion of mud."

"I cannot understand why animals related to the slugs should find a place here," said Charles.

"Have not slugs the characteristics of the Mollusca class?" asked his father. "And are you quite sure that they are without a shell?

"Our next family, the Limacina, has

"Onchidium is a genus from the shores of the Indian seas. The animals have a shield: they live near the sea, and some are known to swim, often coming to the surface to breathe the air.

"Parmacella was found by an English traveller in Mesopotamia. It has a shell covered by a shield. But you have not answered my question respecting the Limax, or slug."

"I do not recollect," replied Charles; "yet how often we see slugs!"

"If I may be allowed to answer," said Lucy, "I think that the slug has what I now understand to be a shield. I have often watched the animal contract itself, and seen a broad, flat piece upon the back, which I thought was a kind of shelter for it."

"The Limax, or slug," continued Mr. Elliot, "is, in fact, provided with a coriaceous escutcheon, or shield, beneath which the animal partly conceals itself. The Limax agrestis, or spinning-slug, has the power of suspending itself by a kind of thread, formed of the viscid substance that covers the body.

"Testacellus is a very interesting genus, lately found in England: the animal has a resemblance to the common slug: it carries the shield on the hinder part of the body.

"Testacellus scutellum feeds on earth-worms, and can so much lengthen the body that it follows them under-ground.

"Our next order will show great alterations in the very large genus Helix. I shall name to you those of Lamarck.

"The third order of Mollusca, Trachelipoda, begins with a well-known genus, the snail, Helix. The term signifies that the foot is situated under the neck, or anterior part of the body. The families in this order are numerous: they are divided into two sections; the first includes those that breathe only in the air; the second those that can exist only in the water, and are furnished with a syphon.

"First section: family Colimacea; genera numerous; animals live upon land only; tentacula generally four; during winter they enclose themselves in their shells, with a false operculum.

"What a number of new genera!" said Lucy. "I see the forms of the shells vary very much; and how beautiful these little transparent shells are!"

"They will find a place shortly. Here is a well-known species, H. aspersa, in most of its varieties; H. pomatia, the apple-snail, now naturalized in the county of Surrey; H. ericetorum, white with brown bands, very frequent on chalky soils; H. citrina, transparent, pale yellow, sometimes with one dark band; H. muralis, from the walls of Rome; H. bidentalis, from Teneriffe; and the little Helix hispida, which you may search for in your own garden; it is small, dark brown, and rough.

"Carocolla has the shells more flattened than Helix.

"Anostoma depressa is a rare and curious shell.

"Helicina is a West Indian genus. You saw them just now. We shall find Helix in two other families.

"Pupa is a curious genus. The shells resemble a chrysalis. A few minute species are found in Britain. P. muscorum I have found buried among damp moss. The larger species are natives of tropical regions. These shells are often found decollated.

"Clausilia papillaris is a pretty shell. (Plate 9.)

"Clausilia rugosa is found in some parts of Britain, under old hedges, at the foot of old trees, and similar places. It is a tapering shell, with the aperture reversed, or left-handed, and bi-dentated: the colour red-brown. It is to be found in the vicinity of Dorking, in Surrey.

"Bulimus is a large genus. A common small species is the Gaudaloupe Bulimus.

"The largest land-shells are found in the genus Achatina. The greater number are African.

"Succinea contains a few species. One of them, S. amphibia, is common near fresh-water.

"Auricula has some resemblance to a Voluta. The aperture is longitudinal: the columella has one or more folds.

"The forms of the species in Cyclostoma are variable; but the aperture is circular, and the margin revolute, or rolled back. C. elegans is often to be found on hedge-banks or chalk soils. It is a pretty shell, sometimes white, often tinted with purple.

"In the family Lymænana, the species are amphibious; inhabiting fresh-water; but rising to the surface to breathe the air. They have but two tentacula.

"As several species are British, you may have the satisfaction of examining them for yourselves.

"The genera are,

"Planorbis is a discoid shell, and one peculiarity of the genus is, that they are all reverse shells. In a discoid shell the spire is depressed; when held up, the whorls turn from right to left, and the aperture is left-handed. The largest species is P. cornu-arietis,[B] which is a native of Brazil. P. corneus is common in ponds and ditches. Empty shells are to be found at the edge of the water. (Plate 9.) If you take the animal to examine, and study its habits, remember that you have no right to injure it, and that you have already promised me that no kind of cruelty shall take place.

"P. vortex is a smaller species. The outer valve is carinated.

"Physa is found in fresh-water upon aquatic plants. They are small shells.

"The animal of the Lymnæa genus has two flat tentacula. L. stagnalis is a very pretty spiral shell, common in ponds.

"L. auricularia is also frequent. It is much smaller than the first species. The last whorl is swelling, and the aperture very wide. They are both thin and brittle.

"The family Melaniana are chiefly foreign. The shells are covered with a dark-coloured epidermis. They are operculated.

"A species of Melanopsis inhabits the river Orontes, in Syria.

"There is yet another family connected with Helix, the Peristomata, containing

"Some of the Valvata genus are found in fresh-water in Britain and other European countries. The shells are small; they are discoid or conoid, and have an operculum. In the shells of this family the margin of the aperture is carried completely round. In Paludina the whorls are convex. They generally inhabit fresh-waters.

"P. vivipara is found in rivers. Quantities of empty shells may be taken from the sand of the Thames.

"Fossil species abound—Petworth marble is full of them."

CHAPTER X.

"The family of the Neritacea," said Mr. Elliot, as he renewed his lessons to Charles and Lucy, "are remarkable in their form. Their left-margin is truncated, without any appearance of a columella. They possess an operculum, and are either marine or fluviatic. The genera are,

"You will recollect that the order Trachelipoda is still continued.

"Navicella is a foreign genus from the Indian rivers.

"Neritìna is a pretty genus of shells, from the European, the East and West Indian rivers. They resemble the Nerìta genus, but are all fresh-water shells; thin, smooth, and variously marked; without any tooth or notch on the right-margin of the aperture.

"N. virginea is common in collections; it is marked with various lines and dots.

"N. fluviatilis is common in our rivers: you may find plenty in river-sand, of red and brown colours, and various sizes.

"N. zebra and N. meleagris are also pretty shells. The little Neritìna viridis, from the West Indian streams, is one of the smallest species, of a pale pellucid green.

"Nerìta is a marine genus. The shells are solid and semi-globose; the left-margin is truncated, the right-margin toothed, or crenulated. This genus is never umbilicated.

"N. polìta is a handsome shell: it is thick, polished, and variously marked; the base of the aperture is yellowish.

"N. peloronta, the bleeding-Nerite, is marked with a crimson spot.

"N. tessellata is sulcated, or furrowed, chequered with black and white.

"Natìca differs from the former genera in these particulars: the shell is umbilicated; the left-margin oblique, not toothed, callous, the callosity sometimes covering the umbilicus. The species are numerous, and several are common in collections. "N. aurantius and N. millepunctata are good shells.

"Ianthina is the last of the snail-like genera. Its beautiful purple colour renders the shell a favourite. They are marine, though so fragile and transparent. The animal floats upon the surface of the sea, by means of a vesicular appendage to the foot, which, it is said, may be inflated or contracted at pleasure. Ianthina shines by night. I. communis is found in abundance in the Atlantic and in the Mediterranean.

"The family Macrostoma contains,

"These genera form a beautiful family, and all bear a resemblance to the human ear.

"Sigaretus is white and pearly; the shell is enveloped in the folds of the mantle belonging to the animal. There are several species, one or two of which were ranked among the Helix race.

"Stomatella is also very pretty; the shells are pearly. S. auricula, from New Holland, has the appearance of a little Haliòtis.

"Stomatia is a small genus.