158 MARINE PROVINCES. 



Arctic mollusca are or have been residents, whilst many of the 

 species originating within its bonndaries extend fnrther south- 

 wards. 



III. Celtic Pi'ovince. 



Tlie Celtic Province, as described by Prof. E. Forbes, includes 

 the British island coasts, Denmark, Sweden, and the coasts of 

 the North Sea and Baltic. The fauna of this region (which 

 includes the principal herring fisheries) is essentially Atlantic ; 

 many of the species are of ancient origin and occur fossil in the 

 Pliocene. 



Fischer admits that this province does not fulfill the zoological 

 conditions, as it contains too large a proportion of boreal species, 

 and very few peculiar to its region. 



The British marine mollusca described by Forbes and Hanley 

 amount to nearly 500 species. 



Of this number two-thirds of the Nudibranchiata, and a few 

 marine univalves, and bivalve shell-fish, are, at present, only 

 known in British seas ; but as most of these are minute or 

 " critical " species, it is considered they will yet be met with 

 elsewhere. 



A few of the species belong to the Lusitanian province, whose 

 northern limits include the Channel Islands, and just impinge 

 upon the English coast. They appear mostly to have emigrated 

 northwards after the glacial epoch, as they are not included in 

 the Crag mollusca. 



Of the Gastropoda 54 are common to the seas both north and 

 south of Britain ; 52 range farther south, but are not found 

 northward of those islands ; and 34 which find here their southern 

 limit occur not only in Northern Europe, but most of them in 

 Boreal America. Nearly half of the bivalves range both north 

 and south of Britain ; 40 extend southward only, and about as 

 many more are found in Scandinavia, 27 of them being common 

 to North America. (Forbes.) 



According to Mr. M'Andrew's estimate in 1850, 406 British 

 shell-bearing mollusca were then known, of which 



211 or 53 per cent, were common to Scandinavia. 



246 or 61 " " ' North of Spain. 



227 or 56 " " S. Spain and Medit. 



97 or 24 " " Canary Islands. 



The wide expanse of the Baltic afl'ords no shell-fish unknown 

 to the coasts of Britain and Sweden. The water is brackish, 

 becoming less salt northward, till only estuary shells are met 

 with, and the Littorinas and Limnseans are found living together, 

 as in many of the British marshes. 



