All the animals of this class inhabit the waters. A few species live in pools, lakes, and rivers, preferring fresh water ; but the sea contains the greatest numbers.
The following arrangement of the genera is the one adopted by M. Cuvier in his Regne Animal." As it stands at present, it brings together into groups nearly allied genera, without, however, distributing them, with much accuracy, into natural families.
In the Conchifera which are brought together in this section, the cloak is open, and the water comes directly in contact with the gills and mouth. The foot, when it exists, is very small ; and, instead of aiding locomotion, appears to be principally employed in forming and fixing the threads of the byssus. Many of the species are ce mented to the rocks from infancy age, others are capable only of shifting to a short distance, by violent ex ertions, consisting in suddenly opening and shutting their valves.
I. TRIBE.—The animals of this tribe is distinguished by possessing only one adductor muscle for closing the valves of the shells, which are inequivalve. The distri bution of the genera into natural families, is at present impracticable, as the animals of few of the species have been investigated with any degree of care.
In the genus OSTREA, represented by the common oyster, the abdomen is prominent, the adductor muscle central, and the branchix united at their summits with the edge of the cloak. The animals of the genera Acardo and Gryphcea, are unknown.
In the animals of the genus Pecten, represented by the common scallop, there is a small foot, supported on a short stalk arising from the abdomen. The margin of the cloak is surrounded with two rows of tentacula, some of which, in the external row, have greenish tuber culated summits. The mouth is surrounded with nu merous branched tentacula, in place of the four ordinary labial appendages. The genera Lima and Pcdum arc nearly related to the genus Pecten, with which they con stitute a particular family.
The genus ANOMIA, as now restricted, is distinguished by the singular character of the adductor muscle, a por tion of which is attached to the corneous or testaceous plate, which passes through the cardinal perforation, and adheres to rocks. There is a small foot, which is capa ble of being likewise protruded through the cardinal per foration.
In the genus SPONDYLVS, the margin of the cloak is fringed with a double row of tentacula, with tuberculated subunits, and the foot is seated on a short stalk, with a large radiated disk.
Besides the genera already noticed, the Placuna, Plicatula, Malleus, Vulsella, and Perna, likewise belong to this tribe. Many of the species are eagerly sought after as food, as the oyster and scallop, and give em ployment to several thousands of the inhabitants of this country, in fishing, and conveying them to the market.
In the revolutions which 'nauseous animals have undergone, the genera of this tribe appear to have suf fered greatly. Multitudes of extinct species, and even genera, are to be found in the solid strata, differing in shape and size from those which are now found in our seas. We are disposed to refer to this extinction of genera, as accounting for the circumstance, that in this tribe almost every genus stands apart, and appears to be but remotely connected with those with which it is now associated, the connecting links having been de stroyed.
II. TRIBE.—Thc valves in this tribe are moved by means of two adductor muscles, one of which is placed near the head, and the other at the anus of the animal.
The pearls produced by the animals of the genus AvicuLA have been long known, and highly prized ; but their anatomy and physiology are still involved in obscurity. The hirundo has been determined by Mr. Sowerby to be a native of the British seas, he having ob tained specimens both from Nlarazion and Bantry Bay. In the genus Crenatula of Lamarck, which is intimately connected with the preceding, there is no mark indicating the animal to have a byssus, which the Avicula is known to possess.
The genus PINNA has been long known and highly prized by collectors. The mouth of the animal is situ ated near the narrow end of the shell. Near the anus there is a conical appendage, susceptible of enlargement and contraction, whose use is unknown. The thread or byssus with which the animal adheres to the sand in which it resides, is collected on the Neapolitan coast ; and when mixed with silk, is woven into various articles of dress, as gloves and stockings.