THE EUROPEAN SEAS. 



223 



Trichotropis borealis, Natica Grcenlandica, Astarte 

 elliptica, Nucula pygmcea, Terebratula caput ser- 

 pentis, Crania Norvegica, Emarginula crassa, Lottia 

 fulva, Pecten danicus, Nemra cuspidata, N. costata, 

 and JV. abbreviata, being such as are met with to- 

 gether in the far north (pp. 49-58). 



The explanation which Ed. Forbes gives of these 

 " outliers " is as follows : — When the bed of the 

 sea of that period when in our latitudes the fauna 

 was more northern than it is now was upheaved, 

 the whole was not raised into dry land, but tracts 

 of greater depth, and which consequently were te- 

 nanted by peculiar forms, still remained under 

 water, though under different depths. In these 

 changes a portion of a fauna wo aid be destroyed, 

 but such species as could endure alterations in ver- 

 tical range would live on. 



If the following diagram, A, represents the relation 

 of sea to land for the period of the northern fauna, 



A 



the next, B, may represent it after the partial up- 

 heaval of the sea-bed. In this last, the unshaded 

 interval below the water-line will be that in which 

 the newer fauna has established itself in shallower 

 waters, and the shaded part that in which the 



