Page:Natural History, Mollusca.djvu/290



This extensive genus, including some of the most highly prized ornaments of cabinets, is distinguished by a shell usually somewhat solid, rounded in outline, or broader than long, equal-valved, but somewhat unequal-sided. The surface is generally adorned with many concentric ridges, often rising into strong ribs, and sometimes divided by radiating furrows. The margins are crenate, or scalloped; the beaks prominent. The hinge is formed of three diverging cardinal teeth in each valve. The muscular impressions are round, and well marked; the pallial sinus wide and pointed.

The animal is thick and globose, or oval; the edges of the mantle, which is open, are fringed. The siphons have fringed orifices, generally separate. The foot is not grooved.

About half-a-dozen members of this beautiful genus inhabit the British seas, which are considered as nearly the northern limit of its geographical range. Of these the finest is Venus casina, of which the valves are about an inch and a half in diameter. They are marked with strong concentric ribs, white, or occasionally painted with two or three crimson radiating bands.

Though this is a rare British shell, it has been taken at various localities all round the coast. I have obtained specimens from deep water off the Isle of Portland.

In some respects these shells present close resemblances to those of the last family; but their