16 MOLLUSCA FROM THE CRAG. 



described by Deshayes as a new species, under the name of 0. Boblayei, does not 

 appear from the representation to be more than a modified form of our very variable 

 shell ; and for my own part, I am much inclined to believe, that Poli was correct, and 

 that it is still an inhabitant of the Mediterranean, as a specimen evidently of this 

 species was lately shown to me by Professor E. Forbes, said to have been obtained 

 by Mr. M' An drew, very near to Gibraltar. The common Oyster of North America, 

 called 0. borealis, by Lamarck, which differs very materially in its varieties, is still con- 

 sidered by some Conchologists as doubtfully distinct. Dr. Gould says, 'Invert. 

 Massach./ p. 138, "The Oystermen maintain that our shell is identical with the English 

 Ost. edulis, and there are certainly forms in which the American and European speci- 

 mens could not be distinguished ;" and although this is described by that gentleman 

 under the name of borealis, it was evidently his impression also, that it was not 

 specifically distinct, as Ost. edulis, Linn, is enumerated in his synonyma. A fossil 

 species, also, from the upper Tertiaries of America, figured and described by Conrad 

 under another name, so strongly resembles our species, as to excite suspicion that it is 

 not really different. It is, however, exceedingly difficult in this, perhaps, more so 

 than in most others, to determine its specific limitation, and every species in this 

 genus seems to possess the character of deviating in a great degree from what 

 might be called its typical form. Sir Charles Lyell, in his ' Second Tour to the 

 United States,' vol. i, p. 312, speaks of the Virginian oyster (Ost. Virginica), as 

 resembling the British shell, when it lives isolated and grows freely under water, but 

 that it loses this more rounded form, and becomes greatly lengthened, when living 

 gregariously on banks between high and low water-mark. Our own oyster will 

 assume a variety of forms, dependent principally upon its peculiar position, but no 

 amount of confinement or lateral pressure will train it into the elongated shape of the 

 Virginian shell. 



I believe, however, the range of this species in the living state may be said to 

 extend from the Mediterranean to the North-Eastern Coast of the United States, 

 although it appears to have selected, for its more favoured abode at the present day, 

 the seas of our own Island. 



The portions of the formation belonging to the Mammaliferous Crag Period have 

 not, to my researches, yielded this species, nor is it enumerated as amongst the Estnary 

 shells of the Norfolk Beds, by Woodward. 



