58 PECTINIM. 



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 

 semifossil state, but occasionally retaining their beau- 

 tiful pink colour, are not unfrequently dredged upon 

 both sides of Scotland and off the coasts of Shetland, 

 close to land and also at various distances from it, at 

 depths of from 30 to 80 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 in- 

 habited 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 eleva- 

 tion 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 abun- 

 dant in every part of the Arctic Ocean at depths varying 

 from 15 to 150 fathoms. It has not been recorded as 

 living south of Drontheim ; and Malm says that it does 

 not now exist anywhere on the Swedish coast, although 

 it is common there in a fossil state. This species is not 

 unlike the variety nivea of P. varius in shape and the 



