190 
THE ESSEX NATURALIST. 
failing supply of the mollusc. It seems to have supplied Lister 
in 1678 with his type, 0 strewn vulgare. 
The bay of Cromarty and the Moray Firth still produce a 
few of medium size if my Banffshire specimens are representative, 
as they seem to be. The shells from the drift of Burstwick, 
Kelsey Hill, etc., in the Holderness district, have much affinity 
to these, and are, I imagine, of the same group. 
Prof a E. Forbes, in his paper “ On British Marine Zoology/’ 
Rep. Brit. Assoc. Adv. Sci., 1850, p. 268, wrote “ That the 
diffusion of Lusitanian forms along our southern shores and for 
some distance up the St. George’s Channel is due to the action 
of southern currents, and their climatal influences, must be 
evident to any person who will compare the range of those species 
with the course and extension of Rennell’s current.” This 
“ affects an area extending from our S.W. English province round 
the western coast of Ireland, impinging on the western shores 
of N. Scotland,” along which many organisms are found that 
are rare or absent in the central portion of the Irish Sea. This is 
particularly noticeable in the foliated oysters which extend from 
the Mediterranean to Norway. Tenby, Swansea, and the S.W. 
English Channel are particularly favoured in this respect. 
Some other colonists will be referred to in due course. 
From very early days currents seem to have been at work, 
not only in transporting exotics to our shores, but also in dis¬ 
seminating loose floating oyster-spat, and forming fresh scalps 
elsewhere. This will account for the presence of forms other 
than those common to the locality, most places yielding two or 
more varieties or forms. 
Oysters vary much in the colour and composition of their 
shells, ranging from a dead chalky white, to a very delicate pearly 
opalescence, and are often tinged from pink to a dark purple, 
especially in western shells, where the muscle-mark also is 
darker than in those of the eastern coast. This is certainly more 
than an accidental variation. A true Whitstable native has 
no stain inside, or only one of a very faint blue. Foreign spat 
laid down in these beds produces a shell marked by deep stains, 
and of a chalky whiteness. 
The external sculpture has its uses in determining the several 
groups, thus examples I have from the Nar Valley, Brancaster, 
Durham (Roman), Findhorn (Moray), and other places in the 
