PELECYPODA. 



41 



The TridacnidcB, or true Clauis, differ from other Bivalves with Cases 195- 



196 

 united mantle-margins in having but a single 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. eJongata, is eaten by the natives, and the shell employed 

 for the manufacture of lime. Tridacna i/igas, the largest known 

 bivalved mollusc, sometimes weighs over 500 lb., that exhibited in 



Fie 



Left valve of the Giant Clam (Tridacna gigas). 

 Length, 36 Inches. Weight, 1.54 lb. ; weight of the two valves, 310 lb. 



the upright cases at the entrance of the G-allery being 310 lb, in 

 weight. A large pair bordered with gilt copper are used as henifiers 

 or holy-water vessels in the church of St. Sulpice in Paris. Tridacna, 

 are found associated in large numbers in lagoons, among coral-reefs 

 in the Eastern and Pacific Seas. The animals are described as pre- 

 senting a beautiful iridescent glare of blue, violet, and yellow 

 variegated with fantastic markings. 



The genus Chama consists of tropical species, which are found Case 196. 

 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 Hippuritidm and 

 Radiolitidce, 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 C'hamid(B, 

 but their true position as regards living Mollusca is very pro- 



