12 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 ob- 

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

 ped 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 with a 

 thick brown or green epidermis; which, when removed, 

 seldom exhibits any other appearance than that of a livid 

 or wan-colored surface ; except in those cases where the 

 substance coniposing the shell is of a pearly nature, then 

 the removal of the epidermis exposes the hidden beau- 

 ties of the shell, which, when polished, aflfords.thc 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. 



There are some of the species of Mya which are desti- 

 tute of the property of generating pearls. 



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

 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 afford 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, 



