CXIV INTRODrCTION. 



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 are common to the 

 ISIediterrauean and the Arctic seas : the greater portion 

 of them are probably northern forms which have migrated 

 southwards ; and more than half arc 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 tj 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, 



* Sucli as Cdbdaria Peachii, Menipea fernata, and Scrupocellaria scabra. 



t Such as Lepralia folita, Schizoporella sinvosa, Mucronella laqucata, 

 Escharoides romcea, Stomatopora compacta, 8. diastoporides (also at Wick), 

 Hornera lichenoides. 



\ Crihrilina annulata has migrated from tbe Arctic seas to our south- 

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

 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 communica- 

 tion between the Atlantic and the Mediterranean, by which the fauna 

 became diffused." — Shetland Dredging Bepori, 1807. 



