PLATE XLV. 



" This Oyster," says Da Costa, ** hitherto only proposed and 

 described by Dr. Lister, is a very different species from the common 

 Oyster, but has been always overlooked as the same kind." Dr. 

 Lister observes that it is found in plenty at the mouth of the 

 river Tees, in Yorkshire, and says he first eat of it at Bourdeaux, 

 in France, where it is greatly esteemed and called Rock Oyster, 

 being found among the rocks. 



The figure of this shell in the plate of Da Costa above quoted, is 

 so very indifferent and devoid of true character, that were we not 

 in possession of the specimen he represents, it would be difficult to 

 ascertain it. In the general description he says the outside is a 

 little uneven, but not rugged nor of a leaved or fla-key structure 

 as the common Oyster : he adds that the ridges are longitudinal, 

 about the thickness of a thread, very numerous, irregular, and run 

 one into another ; but towards the bottom always furcate or divide. 

 This description is accurate but does not accord with the figure, in 

 which the longitudinal ridges appear of a flakey structure or like 

 laminae, and not numerous, irregular threadlike striae as in the shell. 

 We have selected several characteristic specimens of this species in 

 the annexed plate. 



This shell is thick, strong, and nearly opake : it is usually about an 

 inch in diameter ; the valves unequal, the under one being very con- 

 cave, the upper one flatfish. Within, it is of a livid green and 

 rather glossy, the hinge broad, deep, somewhat triangular and stri- 

 ated transversely. In many shells there is a remarkable white mark 

 exactly resembling a thick spot of white oil paint, placed a little 

 below the hinge, this spot always appears in radiated wrinkles from 

 the centre^ and is formed by the muscle of the shell. 



