CO^GBYLIA—DITHYRA. 50. -291 



Montagu, Test. p. 153, and Suppt. p. 580. 

 Donovan, British Shells, ii. tab. 45. 

 Ostrea subviridis- Lister. Conch, tab. 202, fig. 36. 

 Mus. nost. On old shells, and the crevices of rocks. 



Shell an inch or more in diameter, very variable in figure and 

 consistence, sometimes very convex and opake, often thin trans- 

 parent and flat ; sometimes the under valve is more convex than the 

 upper, which is rarely concave ; but always furnished with numerous 

 radiating strice which cause the margin to be crenate : inside of a 

 rich and splendid green^ with an iridescent blue margin : the beak 

 is generally terminal, but in the flatter varieties not ([uite so : the 

 perforation is lai-ge, and sometimes quite entire. 



On the under valve, on one side of the perforation, is a triangular 

 striated cavity, resembling that which is seen on the valves of the 

 Pecten, as if it had contained an additional ligament for the more 

 safe attachment. 



The plug terminates in a thin oval layer, strongly striate trans- 

 versely, and crossed with fine longitudinal lines, freij[uently found on 

 the valves of the Pinna ingens. 



The varieties figured at 12 and 13, we have taken from the cavi- 

 ties of rocks in Torbay. They are extremely thin and brittle, re- 

 sembling brown paper ; the upper valve a little concave, and clothed 

 with thin greenish-white scales ; the under valve very convex and 

 fragile : the longitudinal strise very obscure. 



Anomia testa orbiculari, valvd superiori punctis elevatis, inferiori P""';*'''''- 



punctis concavis. 

 Shell orbicular, with raised dots on the upper valve and concave 



ones on the under valve. 



