MYA. 107 



those of the preceding species, are rough and clumsy. 

 Fig. 3, and 4, are a variety of the shell, somewhat differ- 

 ing in shape, but the same in every other respect as the 

 larger one ; perhaps the difference is owing merely to 

 age. This species is remarkably thick. Its country 

 is unknown. Cu 



PEARL GAPER. 

 PL 23./. 1, 2, 3. 



30. Mya margaritifera. M. testa ovata, anterius coarctata ; cardinis 



dente primario conico, umbonibus decorticatis. Linn. Syst. Nat. ed* 



12. p. 1112. Linn. Gmel. p. 3219. 

 Shell oval, compressed on the fore part ; primary tooth of the hinge 



conic, umbones decorticated. 

 Mya testa ovali-oblonga, apertura sinuata, natibus decorticatis. Mull. 



Zool. Dan. Prodr. 2961. 

 List. Anim. Angl. App. t. 1. f. 1. Conch, t. 149. f. 4. Knorr. 



Vergn. 4. pi. 25. f. 2. Schr'dt. Flussconch. t. 4. f. 1. Born. Test. 



p. 21. Penn. Br. Zool. 1812. pi. 46. f. 2. Da Cost. Brit. Conch. 



pi. 15. f. 3. Chemn. Conch. 6. t. 1. f. 5. Encyclop. Method, pi. 249. 



f. 5. Donov. Brit. Shells, t. 73. Mont. Test. Brit. p. 33. Linn. 



Trans. 8. p. 40. 



The M. margaritifera is a strong oblong-oval shell, 

 covered with a thick, black epidermis. It is always 

 more or less decorticated about the hinge, and fre- 

 quently worm-eaten; the epidermis is concentrically 

 striated, and rather overlaps the margin. The inside 

 is of a bluish green colour, and pearly lustre; the 

 hinge has a prominent tooth in one valve, which locks 

 into a corresponding sulcus in the opposite valve; the 

 muscular depressions are very strong and rugged, par- 

 ticularly the one next the hinge. 



This shell is confined to the rapid rivers of northern 

 climates. It is found within the Arctic circle, in Lap- 



