CHAPTER VIII.

[Sidenote: UNIMUSCULOSA. TRIDACNA. MODIOLA.]

Class.--_Conchifera._

Second Order.--_Unimusculosa._

"1st family, _Tridacnacea_.

_Tridacna_ _Chama_. _Hippopus_ _Chama_, (one species.)

"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.

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"_Mytilacea._

_Modiola_ _Mytilus_. _Mytilus_ _Mytilus_. _Pinna_ _Pinna_.

"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_.

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[Sidenote: PINNA. PERNA. AVICULA.]

"_Malleacea._

_Crenatula_ Rare and little known. _Perna_ _Ostrea_. _Malleus_ _Ostrea_, hammer-oyster. _Avicula_ _Mytilus_. _Meleagrina_ _Mytilus_.

"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.

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[Sidenote: MELEAGRINA. LIMA. PECTEN.]

"Family, _Pectenida_: genera--

_Pedum_ Only species, from India, rare. _Lima_ _Ostrea_. _Plagiostoma._ _Pecten_ _Ostrea_, scallop. _Plicatula_ _Spondylus_. _Spondylus_ _Spondylus_, thorny-oyster. _Podopsis._

"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.

[Sidenote: PLICATULA. OSTREA.]

"_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.

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"In the _Ostracea_ are,

_Gryphæa_ _Ostrea_. _Ostrea_ The same. _Vulsella._ _Placuna_ _Anomia_. _Anomia_ The same--Antique lamp.

"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.

[Sidenote: PLACUNA. ANOMIA.]

"_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

_Crania._ _Orbicula_ _Patella_. _Terebratula_ Some from _Anomia_. _Lingula_ _Patella_."

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.

[Sidenote: HIPPONYX MITRATA.]

"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."