CHAPTER X.

[Sidenote: NERITACEA. NERITINA.]

"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,

_Navicella._ _Neritìna_ _Nerìta_, Nerite, or hoof-shell. _Nerìta_ _Nerìta_. _Natìca_ _Nerìta_.

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

"_Navicella_ is a foreign genus from the Indian rivers.

[Sidenote: NERITINA. NERITA.]

"_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.

[Sidenote: IANTHINA. SIGARETUS. STOMATELLA.]

"_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,

_Sigaretus_ _Helix_? _Stomatella._ _Stomatia._ _Haliòtis_ _Haliòtis_, sea-ear.

"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.

[Sidenote: HALIOTIS. VERMETUS. SCALARIA.]

"With the genus _Haliòtis_ you are already acquainted. The animal appears to be very elegantly formed, if the plate I have seen of it be correct.

"There is a number of fine specimens in the British Museum.

* * * * *

"The family _Plicacea_ contains only,

_Tornatella_ _Voluta_, chiefly. _Pyramidella._

"All the species have plaits, or folds, on the columella. The shells are marine and foreign in both genera. (Plate 9.)

* * * * *

"Our next family, _Scalariana_, contains the genera

_Vermetus._ _Scalaria_ _Turbo_. _Delphinula_ _Turbo_.

* * * * *

"The single species of the first genus, _Vermetus lumbricalis_, inhabits the sea near to Senegal. The shell is tubular, thin, twisted spirally; it is fixed on marine substances by the end of its thin, pointed spire. The shells are usually found in groups.

"The genus _Scalaria_ is one of the most elegant among univalve shells. The singularity of the numerous ribs renders the shells easily known from all other genera of turreted _Mollusca_. The aperture is nearly round, the whorls gibbous, or inflated with carinated ribs: the colour is usually pink or white. It is very costly. (Plate 9.) These shells are brought from the East Indies.

"The fossil species are very elegant: they are found in the strata above the chalk.

"Two other species are common, _S. communis_ and _S. lamellosa_. The first is a British species, and is called the _false Wentletrap_.

[Sidenote: DELPHINULA. SOLARIUM.]

"The shells of the genus _Delphinula_ are solid, thick, somewhat discoid, often armed with spines, without any apparent columella. The recent species inhabit the Indian Ocean. There are several fossil species of _Delphinula_.

"The _Turbinacea_ family contains many genera, with which you will soon become familiar. I believe you are already acquainted with this shell, which, from the time it was first known to collectors, has always been celebrated for beauty. It is now called _Solarium perspectivum_. (Plate 9.) The large, spiral, crenated umbilicus is its great peculiarity. The French call the shell _Cadran_, _dial_. In its natural state the shell is covered with an epidermis. There are a few fossil species, one in the oölite of our own country. The English name is _staircase trochus_.

* * * * *

[Sidenote: ROTELLA. TROCHUS. TURBO.]

"The genera of _Turbinacea_ are,

_Solarium_ _Trochus_. _Rotella_ _Trochus_--wheel-shell. _Trochus_ _Trochus_--top shell. _Monodonta._ _Turbo_ _Turbo_. _Planaxis_ _Buccinum_. _Phasianella_ _Turbo_. _Turritella_ _Turbo_.

"The genus _Rotella_ contains small, flattened, wheel-shaped shells, common in most collections. They are smooth and polished.

"_Trochus_ is still a large genus. _Trochus marmoràtus_ is a fine species from the Indian Ocean. There are several handsome species on our own shores. _T. magus_ is one of them; it has a large, deep umbilicus, or perforation; the spire is flattened; the whorls are crowned with tubercles. The _Trochi_ of tropical climates are thinner than those of northern latitudes.

"When any of these shells are placed upon their _base_, their axis is always inclined: of course they never stand perfectly upright.

"There are several fossil species.

"The genus _Monodonta_ holds a middle place between _Trochus_ and _Turbo_, differing from the former in the aperture, and from the latter in the columella, which is arched and truncated at the base. They are all marine shells.

[Sidenote: LITORINA. TURBO. PHASIANELLA.]

"From the well-known genus _Turbo_, a new one has been formed, called _Litorina_, which includes all the shells of our own coasts that formerly ranked under _Turbo_. Consequently we find the periwincle has changed its generic name, and from _Turbo_ it is altered into _Litorina_. _T. muricata_ is now of the same new genus, _Litorina muricata_. (Plate 7.)

"_Turbo pica_ is a large pearly shell known as the magpie. The golden-mouthed _Turbo_ is very brilliant; the aperture appears as if gilded, so fine is the yellow tint. It comes from the Molucca Isles. _Turbo smaragdus_, from New Zealand, is a rare and beautiful species of a bright green colour.

"_Phasianella_ is a beautiful genus of shells, formerly very costly. A small but elegant species is found on our own shores, _P. pullus_, _Turbo pullus_ of some authors. The colour is pink.

"_Phasianella bulimoides_, from New Holland, is the largest of the species, and once a very rare shell.

"The term _Turritella_ will give you an idea of the form of our last genus in the family _Turbinacea_. The shells are like _little towers_, with a circular aperture. The older conchologists gave the name of _screw_ to all turreted shells, without attending to the form of the aperture. Hence we find screws among _Turbo_, _Buccinum_, and _Strombus_ (spindle).

[Sidenote: TURRITELLA.]

"_Turritella duplicata_ is a heavy shell, often more than four inches long. It is sulcated and carinated; the colour is yellow-white. _T. bicingulata_ is white, marbled with yellow; the whorls are girded with two ridges. There are several fossil species of this genus in London clay."

"I think," said Lucy, "that three or four genera have been taken out of _Turbo_--_Scalaria_, _Delphinula_, _Litoralia_, and _Turritella_, and that _Litoralia_ is not Lamarck's genus."

"You are correct," replied her father, "and indeed so many alterations are continually taking place in the generic names of shells, that I cannot enter into all the niceties of modern conchologists. However, the generic name of a shell, according to Linnæus or Lamarck, is usually given, therefore I hope you will not be greatly at a loss upon meeting with some apparently unknown genus."