BIVALVES. MYA. 25 



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

 instances, small erenulations 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 

 many are angular and eared. 



With regard to their general coloring, little can be 

 said, for the greatest part of them are covered w ith a 

 thick brown or green epidermis; which, when removed, 

 seldom exhibitsany other appearance than that of a livid 

 or wan-colored surface ; except in those cases whe e the 

 substance composing the shell is of a pearly nature, tlien 

 the removal of the epidermis exposes the hidden beau- 

 ties of the shell, w hich, when polished, affords the most 

 brilliant mother-of-pearl imaginable. 



The Mya margaritifera is famous for the production of 

 the finest pearls, and used formerly to be 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, tlie 

 Mya glycemeris, for instance, is often found from ten to 

 twelve inches broad. Others, again, as the Mya 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 afibrd heavy and 

 thick specimens ; but they are, for the most part, more 

 fragile, and less weighty than those of the sea. 



In some places the Mya constitutes a considerable por- 

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

 &c. &c. They have a propensity to burrow in the sand 

 and mud, and are wholly or parti;dly concealed therein. 



