Native oysters--those that are reared on artificial beds--are of course removed as soon as they are ready for the market, but those that live on natural banks are often left undisturbed till their shells are thick with age. The latter, too, are often destroyed in large numbers by the boring sponge (p. 124), which so completely undermines the substance of the shell that it finally breaks to pieces. In the genus _Anomia_ the lower valve is concave, and perforated with a large oval hole very near the hinge, while the upper one is very convex, but the shell is very variable in shape, since the animal sometimes clings permanently to an object, and the shell, during its growth, accommodates itself to the surface of that object. The use of the hole is to allow of the protrusion of a set of muscles which proceed from the upper valve, and give attachment to a plug or button, more or less calcified, by which the animal clings. [Illustration: FIG. 153.--1. _Anomia ephippium._ 2. _Pecten tigris._ 3. _Pecten_, ANIMAL IN SHELL] One species (_A. ephippium_), known as the Saddle Oyster, is common on some parts of our coast. It is seldom found on the beach at low water, but the empty shells are often washed up by the waves. The same family includes the Scallops, which constitute the genus _Pecten_. In these the shell is nearly round, with ears on each side of the umbones, those on the anterior side being generally much more prominent than the others, and both valves are ornamented by prominent radiating ribs. The shell is often very prettily coloured, and the animal rests on the right valve, which may be distinguished from the left by its greater convexity, and by the presence of a notch under the anterior ear. The hinge is straight, with a very narrow ligament, and the internal cartilage is situated in a central pit. [Illustration: PLATE V. MOLLUSCS 1. Solen ensis 2. Trivia Europæa 3. Trochus umbilicatus 4. Trochus magnus 5. Littorina littorea 6. Littorina rudis 7. Haminea (Bulla) hydatis 8. Tellina 9. Capulus hungaricus 10. Chrysodomus antiquus 11. Buccinum undatum 12 & 13 Scalaria communis 14. Pecten opercularis 15. Pecten varius 16. Pecten maximus] The mantle of the animal is free, with double margins, the inner of which forms a finely fringed curtain all round, and on this curtain are a number of black eyes surrounded by very fine tentacles. The gills are in the form of very thin crescents, and the foot is shaped like a finger. Although the majority of scallops are inhabitants of tropical seas, several species are to be found off our coasts, where they range from depths of about four to forty fathoms, and the empty shells, often in the most perfect condition, are frequently found on the beach.