CHAPTER III.

[Sidenote: SYSTEM OF LINNÆUS.]

"As you both assure me," said Mr. Elliot, "that you do not fail to make yourselves familiar with the _nomenclature_, or terms, used in conchology, I shall proceed this morning to the arrangement of Linnæus.

"This system is established upon the _outward_ appearances, or external characters, of the covering bestowed by nature upon the animal, not upon the form of the animal itself.

"The three principal divisions you are already acquainted with, namely, _Multivalves_, shells composed of several parts called valves; _Bivalves_, formed of two parts; and _Univalves_, composed of one part or piece only.

"These divisions contain several _genera_, and the genera usually include many individual shells, but occasionally only one.

"The characters of every genus are permanent, and are therefore to be observed in every one of the species contained in the genus.

"Species are determined by shape, colours, or appearances on the surface of the shell: there are sometimes many varieties of the same species.

[Sidenote: LINNÆAN GENERA.]

"The Linnæan genera are thirty-six. I have copied a list of them for you. In that tray are the specimens mentioned in your list:

ARRANGEMENT OF LINNÆUS.

First Division--_Multivalves_: 3 genera.

* _Chiton_, coat of mail, example, _C. squamosus_.

* _Lepas_, acorn-shell or barnacle, ex. _L. anatifera_ and _tintinnabulum_.

* _Pholas_, stone-piercer, ex. _P. candida_.

Second Division.--_Bivalves_: 14 genera.

* _Mya_, trough-shell, ex. _M. truncàta_.

* _Solen_, razor-sheath, ex. _S. siliqua_.

* _Tellìna_, wedge-shell, ex. _T. Feroensis_.

* _Cardium_, cockle, ex. _C. cardissa_.

* _Mactra_, kneading-trough, ex. _M. stultòrum_.

* _Donax_, wedge-shell, ex. _D. trunculus_.

* _Venus_, Venus, ex. _V. Paphia_.

_Spondylus_, thorny-oyster, ex. _S. gæderopus_.

_Chama_, clamp-shell, ex. _C. gigas_.

* _Arca_, ark-shell, ex. _A. Noæ_.

* _Ostræa_, oyster, ex. _O. isognomon_.

* _Anomia_. antique lamp, ex. _A. ephippium_.

* _Mytilus_, muscle, ex. _M. edulis_.

* _Pinna_, wing-shell, ex. _P. pectinàta_.

[Sidenote: LINNÆAN GENERA.]

Third Division: 2 sections.--1st. _Univalves_ with a regular spire: 14 genera.

_Argonauta_, paper-sailor, ex. _A. argo_.

_Nautilus_, sailor, ex. _N. pompilius_. Conus, cone, ex. _C. Hebræus_.

* _Cypræa_, cowry, ex. _C. monèta_.

* _Bulla_, dipper, ex. _B. naucum_.

* _Voluta_, wreath, ex. _V. utriculus_.

* _Buccinum_, whelk, ex. _B. reticulatum_.

* _Strombus_, screw, ex. _S. pes-pelicàni_.

* _Murex_, rock-shell, ex. _M. ramòsus_.

* _Trochus_, top-shell, ex. _T. bifaciàtus_.

* _Turbo_, wreath, ex. _T. muricàtus_.

* _Helix_, snail, ex. _H. nemoràlis_.

* _Nerìta_, nerite, ex. _N. striàta_.

* _Haliòtis_, ear-shell, ex. _H. tuberculàta_.

* * * * *

2nd Section, without a regular spire: 5 genera.

* _Patèlla_, limpet, ex. _P. vulgàta_.

* _Dentàlium_, tooth-shell, ex. _D. elephantìnum_.

* _Sérpula_, worm-shell, ex. _S. triquétra_.

_Terèdo_, ship-worm, ex. _T. navàlis_.

* _Sabella_, Sabella, ex. _S. Belgica_.

Total number of genera in the arrangement of Linnæus, thirty-six.

The genera marked with an asterisk, contain species found in Britain.

[Sidenote: CHITON. LEPAS.]

"Multivalves may be divided into two kinds, the _pedunculated_, or those fixed to other bodies, as rocks, stones, planks, &c.; or _free_, as the _Chiton_ and _Pholas_.

"Our first genus is _Chiton_. The shell is easily known. The fixed character is, many valves placed over each other along the back. I have already noticed the membrane which connects the valves, which is also a permanent character. It is elastic: the sides are either scaly, as in _C. squamòsus_, (Plate 1,) and hairy, or spinous. The species are determined by the margins. Some of the Chiton genus are common upon our own coasts; they are frequently found among seaweed and stones, rolled up like a ball. _C. fasciculàris_ and _C. lævis_ are British; there are some other British species. The animal adheres to rocks, like the _Patella_, or limpet. The number of species forty.

"Second genus, _Lepas_. Shell multivalve, fixed at the base; valves erect, or _upright_.

"Observe how much the situation of the valves differs in _Chiton_ and _Lepas_. It is scarcely possible to mistake the one for the other. The feathery tentacula, or feelers, of _Lepas anatifera_ are worthy notice, and in a state of motion must be yet more beautiful. (Plate 2.) The common acorn-shell, _L. balanus_, is to be seen very frequently upon the shells of muscles, oysters, periwincles, whelks, in large groups. I see that you are examining the different appearance of the pedunculated and the sessile _Lepades_. Linnæus made two divisions; later writers have separated them into several distinct genera, which will be noticed when we attend to Lamarck's system. The species are forty-five, of which several are found on the British shores, as _L. tulipa_, _L. diadema_, _L. tintinnabulum_, _L. balanus_, _L. anatifera_, &c. The Indian, American, and Atlantic oceans afford numerous species.

[Sidenote: PHOLAS.]

"Third genus, _Pholas_. Generic character: shell bivalve, gaping or divaricated, with several smaller hinges situated upon the hinge; hinge recurved, with an incurved tooth."

"Father," said Charles, "I must say that the _Pholas_ shell is very unlike those of the multivalve division: I think it should rank with bivalves."

"So many conchologists have judged; nevertheless it possesses more than _two_ valves, and, according to the system, it must be forced into the division of multivalves.

"The _Pholades_ are found in company, but each individual occupies a distinct habitation, which the animal excavates for itself, either in rocks, in wood, coral, or sponge; but the finest specimens are usually to be seen in chalk. In proportion as the animal increases in size, it enlarges the cavity in which it is stationed. The animal is supposed to effect this operation by means of a corroding fluid that is secreted in the body, and which it has the power of ejecting upon the substance into which it has entered.

[Sidenote: PHOLAS.]

"The _Pholas_ has the power of emitting a phosphoric liquor, which shines with brilliancy in the dark.

"I must remind you that the accessory valves are fixed to the margin of the shell by a gelatinous substance; this decays after the death of the animal, and consequently the smaller valves are frequently wanting.

"The number of species is twelve. Several of them are found on our coasts. The _Pholas_ genus is without colour, but the reticulations in some species are exceedingly delicate. _Pholas candida_ (Plate 2) is found on the shores of Kent; you will be pleased with the shells. _Pholas dactylus_ is larger and coarser, and not at all uncommon.

"We have now finished our first division, and must proceed to the bivalves."