158 



fixed. It is Ibmetimes found on the fouth coad of Devon; 

 the finefl fpecimens we have taken by dredging in Sal- 

 comb bay, adhering to the infide of old bivalve fhells, 

 efpecially the Cardium Itzvigatum. 



Thefe have the under valve convex, and the upper 

 valve flat, or fomewhat concave; but when the furface is 

 flat to which they adhere, the lower one conforms to it, 

 and the upper one becomes a little convex. 



A variety is marked with rufous-brown between the 

 ribs, towards the margin. 



This fliell differs from the aculeata in being larger, more 

 pellucid, and in the ribs or ftriee being more diltant, un- 

 dulated, and never furniflied with fpines. 



GEN. 



