PELECYPODA. 



35 



Fig. 22. 



E-F.] 



'[Cases 

 19g-20a.] 



are joined together and fringed at the ends, and the pallial line is 

 more or less sinuated. Mactra solidissima, the largest species 

 found on the coast of the United States, is a common article of 

 diet. 



The genus Chama consists of tropical species, which are found [Case 19, 

 fixed to corals, rocks, &c. Neverthe- 

 less, they have a small bent foot, but 

 what purpose it serves is difficult to 

 conceive. 



Some of the " Cockles " {Car- 

 diidce) from warm latitudes are highly 

 coloured and adorned with most 

 beautiful sculpture. Probably the 

 majority are eatable, as the com- 

 mon cockle (Cardium edule) of the 

 British coast. The foot of these 

 mollusks is very large, bent, and used 

 for leaping. The siphons are short 

 and fringed at the margins. 



The Tridacnidce, or true Clams, 

 differ from other Bivalves with united 



Common British Cockle 

 (Cardium edule). 



foot ; b, exhalant siphon ; 

 c, branchial or inhalant 

 siphon ; d, edge of mantle ; 

 e, ligament ; /, nmbones or 

 beaks of the shell. 



[Case 20, 



A-B.] 



mantle-margins in having but a cen- 



tral adductor muscle. In the typical species the animal is attached 

 to the rocks by a " byssus," a strong fibrous structure which passes 



Fiff. 2:;. 



Left valve of the Giant Clam (Tridacna gigns). 

 Length, 36 inches. Weight, 154 lb. ; weight of the two valves, 810 lb. 



