XII BOREAL AND CELTIC SUB-REGIONS 365 



European, the reason being that on the American coasts the cold 

 Labrador current, which hugs the land, bars back the advance of 

 southern genera, but allows Boreal genera to spread southwards, 

 while on the European side the warmer conditions produced by 

 the Gulf Stream keep the Boreal species back, and allow more 

 southern forms to spread northwards. 



Many of the Boreal species occur on both sides of the Atlantic, 

 and thus support the theory of a more continuous fringe of con- 

 tinental land once existing along the north of the Atlantic. 

 Among the prominent genera, besides several of those mentioned 

 under the Arctic Sub-region, are Purpura^ Ohenopus^ Littorina^ 

 G-ibbula, JVatica, Patella.^ Tectura^ Chiton, Doris, Aeolis, Tellina, 

 Thracia. 



(3) The Celtic Sub-region includes the British Islands (except- 

 ing perhaps the Shetland Islands), the coasts of the North Sea 

 and the Baltic, with N. France to Cape Ushant. The absence 

 of any cold or warm current exerting direct influence upon the 

 coast-line of this sub-region causes a very gt-adual change in the 

 conditions of life as we move either southward or northward. 

 The fauna of the British seas contains a decided mixture of 

 northern and southern forms. The following are among the 

 common Boreal species which attain their southward range on 

 our coasts : Tectura testudinalis Miill. (to Dublin Bay and 

 Scarborough), Trichotropis borealis Brod. (to the Dogger Bank), 

 Margarita helicina Fabr. (to Yorkshire and Dublin Bay), M. 

 groenlandica Chem. (western Scotland), Natica montacuti Forb. 

 (to Cornwall), Trophon truncatus Str. (to Tenby), Chiton mar- 

 moreus Fabr. (to Dublin Bay and Scarborough). Buecinum 

 undatum and Littorina littorea become very scarce on our extreme 

 south-western coasts. Among Lusitanian species which reach our 

 coasts are G-ibbida magus L. (to Orkney and Shetland Islands), 

 Phasianella pullus L. (to Caithness), Gralerus chinensis L. (to 

 Milford Haven), Galeomma Turtoni Turt. (to Weymouth), Car- 

 dium aculeatum L. (to Isle of Man), Solen vagina L. (to north 

 Ireland). 



It appears from the Mollusca of our Crag formations that at 

 the time of their deposition the temperature of our seas must 

 have been considerably warmer than it is now. Thus in the 

 Crag we find many species and even genera (e.g. Mitra, Fossarus, 

 Triton, Vermetus, Ringicula, Chama') which now occur no farther 



