VARIETIES OF OYSTERS. 169 



it was a valve of P. opercularis, in the same condition, and 

 as large as northern specimens. E. Finmark to Bergen, 

 where it becomes dwindled, in 5-50 f. (Sars). 



P. Islandicus, Miiller, once lived within the area which 

 now constitutes the more northern part of the British seas, 

 and nearly the whole of Scotland. It is, however, no 

 longer an inhabitant of our coasts. Dead shells in a semi- 

 fossil state, but occasionally retaining their beautiful pink 

 colour, are not infrequently dredged up on both sides of 

 Scotland and off the coasts of Shetland, close to land, and 

 also at various distances from it, at depths of from 30780 

 fathoms. It is not uncommon in pleistocene beds, on the . 

 west of Scotland and in the Moray Firth. 



The best explanation I can offer for its never having 

 been found alive in any part of our seas is by suggesting 

 that the ancient sea-bed which it inhabited during some 

 part, if not the whole, of the glacial epoch, was afterwards 

 upheaved above the level of the sea, so as to cause the 

 extinction of this and other arctic species, and that at a 

 subsequent period a great part of this district was slowly 

 submerged, and is now again covered by the sea. 



We know that this process of elevation in some and 

 depression in other parts of the Atlantic sea-bed is still 

 going on. Sweden and Greenland are instances of the 

 former phenomenon ; and to the latter may be referred the 

 discovery by Dr. Wallich of star-fishes belonging to a 

 species which usually inhabits shallow water, living at a 

 depth of 1260 fathoms, as well as the occurrence of Nassa 

 incrassata and other littoral kinds of mollusca, in nearly 80 

 fathoms, off the coast of Shetland. P. Islandicus survives 

 and is abundant in every part of the Arctic Ocean, at 

 depths varying from 15-150 fathoms. It has not been 

 recorded as living south of Drontheim, and Malm says 



