28 BIVALVES. MY A. 



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

 instances, small creuulations supply the place of a regu- 

 lar hinge. 



The form of the Mya varies exceedingly, some are ob- 

 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, uo 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, atfords the 

 most brilliant mother-of-pearl imaginable. This pearly 

 appearance is confined to the fresh water species. 



The ]\I. 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 hi the sand and mud, 

 and are wholly or partially concealed therein. They al- 

 so frequent alga^, tkv., adhering to them by a byssus or 

 beard. 



