36 BIVALVES MACTRA. 



direction; they being ridged or plaited longitudinally, 

 whereas in the Mactra, the elevations and their adjacent 

 grooves are placed transversely. 



The generality of Mactrae are of a delicate construe^ 

 tion, and afford a semipellucid appearance. They are, 

 for the most part, thin, brittle, and remarkably light; 

 ind in some species the valves are open or gaping. 



The most prevailing color is blueish or yellowish-white, 

 but some have, upon a brown ground, delicate rays of 

 purple, heightened with rich tints of the same color : 

 others, again, are of a brilliant lilac, passing into a deli- 

 cate blue. 



The hinge of the Mactra is its best distinction from all 

 the other genera of Bivalves, for the middle tooth is al- 

 most invariably complicated, and of a triangular form, 

 liaving a small hollow on each side of it ; the lateral teeth 

 are remote from the beaks and inserted into each other. 

 The hinge of the Mactra, though very articulate, is re- 

 markably thin and delicate; in some cases, the teeth 

 which compose it are much thinner than paper. The 

 MactrsB are mostly equivalves; the interior margin is 

 rarely crenate or toothed. 



The Northern and European seas supply many of the 

 species of Mactra. 



The Indian and American oceans also produce them ; 

 some are found in the Mediterranean, and others are in- 

 debted to the Cape of Good Hojie and African shores for 

 their subsistence. They are also frequently found at the 

 moutjis of rivers. 



