c\iv INTRODUCTION. 



work as common to Britain and the Mediterranean, about 

 40 have occurred in Shetland. Of these, some are species 

 enjoying a very wide range, and a large proportion are 

 generally distributed on the British coasts. 



About 24 of the British species arc common to the 

 Mediterranean and the Arctic seas : the greater portion 

 of them are probably northern forms which have migrated 

 southwards ; and more than half are also inhabitants of 

 the North- American coast. 



It is interesting to trace the course of the northern 

 forms in their migrations. Some have moved southward 

 and found their terminus in the Shetland sea ; others 

 have extended their range down the north-eastern coasts 

 of Scotland and England*; others, again, have spread 

 themselves to the Hebrides and west coast of Scotland, 

 and to the opposite coast of Antrim, but apparently not 

 further f; some are widely distributed on the English 

 coasts ; and a certain number have found their way far 

 south J. 



If we turn now to the southern forms, we find a nearly 

 parallel series of facts. A number of Mediterranean 

 species reach our seas, which do not pass much beyond 

 the south-western district, a few of them ranging for a 

 greater or less distance up the western side of England, 



* Such as Cellularia Peachii, Menipea ternata, and Scrupocellaria scabra. 



t Such as Lepralia polita, Schvsoporella sinuosa, Mucronclla laqueata, 

 Escharoides rosacea, Stomatopora compact a, S. diastoporides (also at Wi.-k ). 

 Hornera lichcnoidea. 



\ Cribrttina annulata has migrated from the Arctic seas to our south- 

 western shores (Cornwall), and has also made its way down the north- 

 eastern side of Scotland (St. Andrews). 



Jeffreys gives a list of 76 species of Mollusca, usually considered northern. 

 which are common to the North Sea and the Mediterranean. He adopts 

 Forbes's view, that at some former period " there was an open coinimmic;i- 

 tion between the Atlantic and the Mediterranean, by which I lie famm 

 became diffused." Shetland Dredging Report, 



