226 JORDAN: ON THE GENUS FUSUS. 



Some naturalists have suggested a bathymetrical limit, the 

 loo fathoms line, but why loo fathoms more than 50 or 150 

 fathoms ? 



Inthe Mull of Galloway there is a depth of nearly 150 fathoms, 

 and at several localities off the west coast of Scotland there are 

 depths exceeding 100 fathoms, and at one of these, inside the 

 island of Mull, Etnarginula crassa and other species have been 

 dredged by Messrs. Frank Coulson and Alfred Brown, of Glas- 

 gow. To decide that these shells shall not be regarded as 

 British would be most illogical and unsatisfactory. 



Again, in the fiords of Norway, many miles away from the 

 open sea, there are depths greatly in excess of 100 fathoms, in 

 which live a rich and varied fauna. The suggested limit is 

 therefore unsuited to other countries as well as to our own. 

 Moreover, a naturalist could dredge across the English Channel, 

 along the coast opposite northwards to Norway, and in the con- 

 trary direction along the French coast, and probably around the 

 Bay of Biscay to the Iberian peninsula, and yet not go outside 

 the 100 fathoms line. 



If a bathymetrical limit must be adopted the 150 fathoms 

 line would be more satisfactory than 100 fathoms, because it 

 would include nearly all the inter-insular depths ; but in my 

 judgment no limit of depth would be satisfactory, for the reasons 

 stated and because mere depth has but little, if any, influence 

 upon the distribution of moUuscan life, excepting the littoral 

 and sub-littoral species, and some of these even live at consider- 

 able depths. 



It has also been suggested that species shall be considered 

 as British only when obtained within sight of the British Isles, 

 but this would exclude all the rare Dogger Bank and Shetland 

 mollusca, and very greatly reduce the number of species now, 

 by general consent, regarded as British. Moreover, the range 

 of visibility is an ever varying quantity depending upon atmo- 

 spheric conditions, the altitude of the coast, &c. 



Dr. John Murray suggests " that further investigations may 



J.C., vi., July 1890 



