Page:Natural History, Mollusca.djvu/289

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277
VENUS SHELLS.

hinge are approximated beneath the beak in a single group. In general they are more flattened and wider in proportion to their length than the Cockles, from which they differ conspicuously also in having the ribs never radiating towards the margin from the beaks, but always, where present at all, in concentric lines parallel to the margin. The ligament often leaves behind the beaks an oval depression, commonly called the corslet. The muscular impressions are strongly marked, and the pallial, or that which indicates the adhesion of the mantle, is much sinuated in its course.

The animal has two siphonal tubes, capable of being protruded in a greater or less degree, sometimes united to each other; their extremities are fringed, though very slightly in some species. The mantle is widely open in front, for the projection of a large compressed foot, which serves for creeping.

This family is peculiarly rich in the number of its species; the genera Venus and Cytherea alone contain about a hundred each still existing, besides half as many more which are fossil. They are found in almost all seas, generally at a moderate distance from the shore, but extending from tidemarks to great depths. Our native species, Venus striatula and ovata, for example, live indifferently at low-water mark, and at the depth of a hundred fathoms. The tropical regions afford, as usual, the greatest number of species, and those most remarkable for beauty of form, sculpture, and colour, for all which, however, the majority of the family may be considered as preeminent among bivalves; whence the various appellations of the goddess of beauty, and similar names have been selected for the genera.