MY A. 99 



rugged appearance, and more or less indented at the 

 margin. Inside white, with a broad somewhat trian- 

 gular tooth in each valve. The shell in some degree 

 resembles the 31. prcetemtis, but the margin is waved, 

 and the end is not truncated as in that species. It is 

 about three quarters of an inch long, and an inch broad. 

 Mr. Montagu found it lodged in hard limestone at 

 Plymouth, into which it had bored like a Pholas ; and 

 which he thinks was probably the occasion of its dis- 

 torted growth. 



DOUBLE-TOOTHED GAPER. 



16. Mya bidentata. M. testa subovali, compressa, cardinis dentibus 

 antrorsum porrectis, obliquis. Linn. Trans. 8. p. 41. 



Shell somewhat oval, teeth at the hinge projecting forwards, oblique. 

 Mont. Test. Brit. p. 44. Penn. Brit. Zool. 1812.— 4. p. 166. 



A very small species, an eighth of an inch long, and 

 somewhat more in breadth. Of a dirty white colour, 

 compressed, and smooth ; two broad diverging teeth in 

 one valve, but none in the other. 



This was found by Mr. Montagu, burrowed in old 

 thick oyster shells from Salcomb Bay, Devonshire. 



DECUSSATED GAPER. 



17. Mya decussata. M. testa ovata, decussata; margine undata. 

 Shell oval, decussated; margin waved. 



Mont. Test. Brit. Suppl. p. 20. pi. 28. f. 1. Penn. Brit. Zool. 1812. 

 4. p. 167. 



A white oval shell, with irregular concentric ridges 

 decussated by regular longitudinal striae, which form 

 tubercles at the anterior end; the margin is waved. 

 Inside white, with a broad erect tooth in one valve, and 

 a projecting indented plate in the other. 



