The _Tridacnidæ_, or true Clams, differ from other Bivalves with united mantle-margins in having but a central adductor muscle. In the typical species the animal is attached to the rocks by a “byssus,” a strong fibrous structure which passes through an aperture at the upper part of the shell. A species found in the Red Sea, _T. elongata_, is eaten by the natives, and the shell employed for the manufacture of lime. _Tridacna gigas_, the largest known bivalved mollusc, sometimes weighs over 500 lb., that exhibited in the upright cases at the entrance of the Gallery being 310 lb. in weight. A large pair bordered with gilt copper are used as _bénitiers_ or holy-water vessels in the church of St. Sulpice in Paris. _Tridacnæ_ are found associated in large numbers in lagoons, among coral-reefs in the Eastern and Pacific Seas. The animals are described as presenting a beautiful iridescent glare of blue, violet, and yellow variegated with fantastic markings. [Illustration: Fig. 33. Left valve of the Giant Clam (_Tridacna gigas_). Length, 36 inches. Weight, 154 lb.; weight of the two valves, 310 lb. ] [Sidenote: Case 196.] The genus _Chama_ consists of tropical species, which are found fixed to corals, rocks, etc. Nevertheless, they have a small bent foot, but what purpose it serves is difficult to conceive. In this place attention should be called to the _Hippuritidæ_ and _Radiolitidæ_, very remarkable extinct families of bivalved molluscs which occur abundantly in the cretaceous strata of southern and eastern Europe, Egypt, etc. They are remarkable for the solidity of the shells, the relatively small space occupied by the animal, and the complicated character of the hinge and processes bearing the adductor muscles. They are usually classed near to the _Chamidæ_, but their true position as regards living Mollusca is very problematical. A fine series of these shells is exhibited in Gallery VIII., wall-case 5, in the Geological Department. [Sidenote: Cases 198–199.] The _Myidæ_, popularly known as “_Gapers_,” on account of their valves being open at one or both ends, have the mantle united all round, except where the small foot is protruded. The siphons are very long, united almost to the ends, and covered with a coarse wrinkled outer skin. They bury themselves in mud and sand at low-water mark or in shallow water. The species are few in number, and chiefly from the shores of northern countries. _Mya arenaria_ of our own coasts is largely eaten in some parts of Europe and North America. [Illustration: Fig. 34. British Gaper (_Mya truncata_). _a_, foot; _b_, siphon-sheath; _c_, exhalant siphon; _d_, inhalant siphon; _e_, umbones or beaks; _f_, anterior, _g_, posterior end of shell. ] [Sidenote: Case 198.] The _Corbulæ_ (Case 198) have one valve larger than the other and are like little _Myæ_, but the valves _are almost closed_ and their siphons are very short. [Illustration: Fig. 35.