THE EUBOPEAN SEAS. 



263 



changes of definite date, the best proofs he can 

 have of the extent of these old seas, and of the 

 modifications they have undergone. 



Mitra Groenlandica is an eastern, or Old- World 

 mollusk, in its generic relations, as are some others 

 of the Greenland fauna ; or as, in the case of the 

 flora of the connecting land (p. 255), the long line 

 of coast which once stretched from Greenland to 

 Scandinavia, presented a fauna in which the assem- 

 blages of the opposite sides of the Atlantic were 

 represented and blended. 



Many considerations, however, make it probable 

 that the species of our Boreal and Celtic provinces, 

 which have western relations, exceed numerically 

 those of the North- American Boreal region, which 

 have had an eastern origin. This question will 

 have to be treated somewhat in detail, when we 

 shall describe those past conditions of the Atlantic, 

 as well as those stages in its fauna which are indi- 

 cated by the crag and other tertiary sea-beds. It 

 may suffice, in the present place, to state that the 

 somewhat indistinct features of the Arctic and 

 Boreal faunas, if mere lists of species are taken, is 

 the result of the migration outwards of the Arctic 

 species. The Boreal fauna can be cleared of its 

 mixed character, by separating all those species 

 which at present have a wide range within the 

 Arctic basin; and then the residual Boreal forms, 

 which are not Arctic, and of which so many are 



