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their being very flat, of a thin, transparent, and foliace- 

 ous texture, with the edges generally fringed, constitute 

 the remarkable characters of this singular shell. It is 

 sometimes orbicular, and very flat; one of the species 

 has the valves generally of a square form, waved on ei- 

 ther side in large folds, and finely striated, or wrinkled 

 longitudinally: ligament attached to the two ribs at 

 the hinge. Interior with one muscular impression, 

 similar to that of the Ostrea. The very small space 

 between the valves when they are closed, indicates the 

 animal inhabiting them to be of an extremely thin sub- 

 stance. 



This genus immediately precedes that of the G. Ano- 

 mia, with which it was blended by Linnaeus, though 

 it bears but little resemblance to the shells of the lat- 

 ter genus in form; unlike them also, it is never affixed 

 to other bodies, and the organization of the animal is 

 likewise perfectly distinct. 



Placuna sella Placuna papyracea 



Placuna placenta 

 [There is also one fossil species.] 



