CONCHYLIA— DirHYR^. 45. 205 



Shell very irregular, sometimes growing to a large size, when the 

 beak of the under valve becomes much elongated, and transversely 

 striate in the ligamentous cavity. 



OsTREA testa subrotundd vel oblongd leviusculd parasitica, valvd parasitica. 



superiori convexd. 

 Shell roundish or oblong nearly smooth and fixed to other substances, 

 with the upper valve convex. 



Tab. nost. 17, fig. 6 and 7. 

 Ostrea parasitica. Chemnitz, viii. p. 19, tab. 71, fig. 660. 



Turton, Conch. Diet. p. 134, fig. 8. 

 Ostrea orientalis. Dillwyn, Descript. Catal. p 274. 

 Mus. nost. On bivalves, crabs, and submarine substances. 



Shell from half an inch to two inches in diameter, very variable 

 in figure and substance, sometimes very thin and radiate with black 

 or brown, when attached to thePecten opercularis: sometimes thick, 

 with the upper valve semicylindrically tumid, and the under valve 

 concave underneath, as if it had accommodated its shape to a piece 

 of stick of the size of the fore-finger, with the beaks much incurv- 

 ed on one side and not extending beyond the margin : the surface is 

 nearly smooth, of a greenish color with occasionally some dark 

 longitudinal marks ; inside perlaceous, with fine crenulations on 

 each side the hinge. 



A small variety is found fixed to Serpulse, the inside of old Oys- 

 ter-shells, and sometimes in the cavities of rocks, with the upper 

 valve flat and a little scaly ; the under valve very concave and hol- 

 low, especially under the hinge ; the beaks of the concave or under 

 valve often much lengthened, an evidence of age; and it is fre- 



