TRIDACNA, a genus of molluscs of the family Tridacnidre, comprising the genera Tridacna and Hippopus, the shells of which are the largest known amongst acalephous molluscs. T. gips is sometimes 4f feet long, weighing 500 lbs. T. safrana is of a beautiful blue round the edges ; third species is 'I'. squamosa. There are known six recent and one fossil species. Tridacna shells are used in some of the islands of the Pacific Ocean, at Lesson Island, at Ualan, at some of the Live Islands, and in New Guinea, to form adzes. —Woodward ; Figuier.
Tridacna gips, the gigantic clam shell, was formerly of such value, that the Republic of Venice presented one to Francis I., who gave it to the church of St. Sulpice in Paris, and it is still used there as a. basin for holy water. It is the Benitieeof the French. Its shell is transversely oval, with great imbricato-squainous ribs, the scales short, arched, and lying near together; the interstices of the ribs are not striated. The size and weight of this immense bivalve, the largest and heaviest known, combined with the beautiful marble-like appearance and whiteness of the inside of the valves, have always caused it to be sought for as an ornament for grotto-work or for garden fountains; and, indeed, the valve of a large individual forms a very picturesque basin for catching the clear falling water, and transmitting it through the deep interstices of its indented edge to the reservoir below. This species can
hardly have been the Tiiilacna of Pliny (Nat. Hist. xxxii. 6), but his Pedalia or oysters, a foot long, from the Indian Sea, may have been one of the Tridacnm of modern authors. At Carteret Harbour, New Ireland, the natives obtain many very large individuals, whose flesh they eat raw.
This species occur at Tongataboo, at the Moluccam, at Tinior, and at IN'aygiou, and appear to inhabit nither shallow water.
Ilippopus maculatus, Lam.
Mama hippopus, Linn. I Trldacna maculate, Quoy. Ilippopus maculatus, Lam. I Ilear's paw clam.
This well-known but beautiful species, now much used in the ornamental arts for inkstands, etc., has a traverscly ovate shell, which is of moderate size, ventricose, ribbed, subsquamous, and white spotted with red or purple; the lunule is heart-shaped and oblique. The naturalists of the Voyage of the Astrolabe found this species at Carteret lIarbour, New Ireland, and also at Vani kero, where they collected specimens left dry on the reefs. It spins a byssus. —Eng. Cyc.; ltrood ward, p. 453.