LAMELLIBRANCHIA. 
41 
The Tridacnidce, or true Clams, differ from other Bivalves with Cases 195- 
196 
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 gi'gas, the largest known 
bivalved mollusc, sometimes weighs over 500 lb., that exhibited in 
Fig. 33. 
Left valve of the Giant Clam {Tridacna gigas). 
Length, 36 inches. Weight, 154 lb. ; weight of the two valves, 310 lb. 
the upright cases at the entrance of the Gallery being 310 lb. in 
weight. A large pair bordered with gilt copper are used as benitiers 
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 Hippuritida 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 Chamidcc, 
but their true position as regards living Mollusca is very pro- 
