The genera Arca, Pectunculus, and Nucula, form a family formerly included in the genus Arca of Linnaeus. The animals of the genus Area have a pedal ligament, with which they adhere to different bodies. The animal of the Pectunculus, on the other hand, is furnished with a large compressed foot, with which it is able to crawl. The inhabitant of Nucula is unknown.
The animals of this section have two adductor muscles. The cloak is open in front, with a separate aperture be hind, for the passage of the excrements. The shells are equivalve. The species were formerly included in the genus Mytilus.
In the restricted genus Mytilus, represented by the common mussel, the beak is terminal. The margin of the cloak, at the rounded angle of the front of the shell, is fringed with branched tentacula. In the genus Mo diolus, represented by the 'Myelitis modiolus of Linnaeus, the beak is removed a little way from the anterior ex tremity. In the Lithodomu8 of Cuvier, which includes %taus lithoMagas, both extremities of the shell are equally rounded, whereas in the Mytilus and Nlodiolus. the anterior is the most pointed. In these three genera there is a foot and byssus.
The genera Anodonta and Unto are inhabitants of fresh water, resembling each other in the form of the animal, but differing in the structure of the hinge of the shell, the former being destitute of the teeth possessed by the, latter. The foot is large and compressed. The posterior end of the cloak is fringed with small tentacula. The Anodonta is represented by Illytilua anatinus, and Unio by 'Ilya margaritifera of Linnmus. M. Cu vier considers the genera Cardita, Venericardia, and Crassatella, as nearly allied to the genera of this section.
This section includes at present only two genera, Tridactina and Hippopus. The cloak is furnished with three openings, all of them near the anterior extremity. The first of these is the largest, and serves for the pas sage of the byssus. The second admits the water to the branehix, and the third is opposite the anus. The valves are closed by one adductor muscle.
The animals of this section resemble those of the last in the three apertures to the cloak. The first is large, and the two last are sometimes produced into separate or united tribes. The foot issues from the anterior open
ing, while the two posterior apertures serve for respira tion and the passage of the excrement. There are two adductor muscles for closing the shell, one near the mouth, and the other at the anus.
In the genus Cliama, the two posterior apertures are in the form of short tubes, the anterior one is small, and indicates the corresponding size of the foot. The foot of the animals of the Isocardia is much larger, and the anterior aperture is large in proportion.
In the common cockle (Cardium,) prized by many as an agreeable article of food, the foot occupies a large share of the cavity of the shell. It is bent in the middle, with the point directed forwards. Cockles are considered in season during March, April, and May. They are sold in this country by measure, and eaten either raw, or boiled and pickled.
In the genera Cyclas, Tellena, Donax, and Venus, the foot is long and tongue-shaped, and the posterior tubes, in general, considerably produced, and more or less united at the base. In the genus Loripes, the foot is small and cylindrical, and the tubes short and united. In the Mactra the tubes are likewise short, but the foot is compressed.
In the animals of this section the cloak is united in front, with an opening at the anterior extremity for the passage of the foot, and a production at the other ex tremity in the form of two united tubes. The cuticle of the cloak is continuous with that which invests the shell. Hence, when the animal is removed, the cuticle fre quently remains on the margin of the valves in loose membranes.
The animals of the different genera are constructed nearly on the same plan, so that it is impossible, without the aid of the shell, to give them any definite arrange.
mcnt. The following genera are considered as belong to this section : Mya, Lutraria Anatina, Glycymeris, Panopea, Pandora, Gastrochxna, Byssomia, lliatella, Solen, Sanguinolaria Pholas, Teredo and Fistularia. They all appear to prefer concealment, lodging in the sand, in stones or wood. Many of them arc used as food, as the Razor-fish (Solen,) and Gapers (Mya.)