16 MOLLUSCA FROM THE CRAG. 
described by Deshayes as a new species, under the name of O. Bodlayer, 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‘Andrew, very near to Gibraltar. The common Oyster of North America, 
called O. 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 dorealis, 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. 
