410 



CHAMACE^E; CHAMA, TRIDACNE. 



each other, that they cannot be detached without breaking the 

 shells. These are subject to changes not only of shape but of colour, 

 in accordance with the accidents of their position. The attached 

 valve is very irregular, and takes the form of the surface to which 

 it is applied ; and it is usually much less coloured than the other. 

 The valves have a series of foliations^ or leaf-like projections, on 

 their surface ; and the luxuriancy of these depends upon the 

 stillness of the medium in which the animal exists. If it inhabit 

 deep and placid water, the expansions will generally be of con- 

 siderable size ; whilst those of the individual that has borne the 

 buffetting of a comparatively shallow and turbulent sea, will be 

 poor and stunted. This genus is confined to the warmer seas ; 

 the Mediterranean being the locality of the lowest temperature 

 where any species have been hitherto found ; the shells have 

 been observed at various depths, ranging from points near the 

 surface to seventeen fathoms. 



952. The Tridacne, of which one species is the largest known 

 Conchiferous Mollusk, is still more restricted to w r arm localities ; 



the East Indian and 

 Australian seas alone 

 supplying specimens 

 of it. This is readily 

 distinguished from 

 the Chama by the 

 equality of the valves; 

 since, instead of being 

 fixed by the adhe- 

 sion of one of these, 

 it is attached, during 



part of its life at least, by a tendinous byssus that passes out 

 through a channel in the anterior part of the mantle, which forms 

 a well-marked groove in the shell. The Tridacne, or Giant 

 Clamp-shell, sometimes attains an enormous weight as well as 

 dimension. There is a pair in the Church of St. Sulpice at 

 Paris, which are used as " Benitiers" (receptacles for holy- 

 water), and weigh more than 500 pounds ; Lamarck mentions 

 a specimen in which each valve measured three feet by two ; 

 so that the story of an oyster which furnished a dinner to a 



FIG, 596. TRIDACNE. 



