28 BIVALVES.. MYA. 



it; others, again, have two or three teeth, and, in some 

 instances, small crenulations supply the place of a regu- 

 lar hinge. 



The form of the Mya varies exceedingly, some are obr 

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

 off; others, again, are more orbicular or round; and ma- 

 ny 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 epidennis; and when this is re- 

 moved, no other appearance than that of a livid or wan- 

 colored 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 quanti- 

 ties 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. glycimeris, for instance, is often found from ten to 

 twelve inches broad. Others, again, as the M. crassa, 

 &c. are remarkable for their excessive weight and thick- 

 ness; and their weight often appears out of proportion to 

 their dimensions. 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. They al- 

 so frequent algae, &c., adhering to them by a byssus or 

 beard. 



