36 BIVALVES. MYA. 



long and truncate, as if part of the shell had been chopped 

 off; others, again, are orbicular or round; and many are 

 angular and eared. 



With regard to their general colouring, little can be said, 

 for the greatest part of them are covered with a thick 

 brown or green epidermis ; and when this is removed, no 

 other appearance than that of a livid or wan-coloured surface 

 is exhibited; except in those cases where the substance 

 composing the shell is of a pearly nature, then the removal 

 of the epidermis exposes the hidden beauties of the shell, 

 which, when polished, affords the most brilliant mother-of- 

 pearl imaginable. This pearly appearance is confined to 

 the fresh water species. 



The M. margaritifera is famous for the production of the 

 finest pearls, and was formerly found in great quantities in 

 the river Conway, in Wales. The creation of pearls is 

 said to originate in a disease of the animal. 



Some species of this genus grow to a large size, the M. 

 glycymeris, for instance, is often found from ten to twelve 

 inches broad. Others, again, as the M. crassa, &c. are re- 

 markable for their excessive weight and thickness; and 

 their weight often appears out of proportion to their di- 

 mensions. Rivers and cataracts afford heavy and thick 

 specimens. 



In some places the Mya constitutes a considerable por- 

 tion of food, not only for man, but also for aquatic birds. 

 They have a propensity to burrow in the sand and mud, 

 and are wholly or partially concealed therein. 



Some species of the Mya inhabit the East Indian seas ; 

 others are found on the coast of Africa; several are com- 

 mon in the European and Northern seas ; and some species 

 are to be found in the South seas ; they also occur in con- 

 siderable abundance in the neighbourhood of Sheerness and 

 Sandgate Creek. 



