64 NORMAN: ON THE MOLLUSCA OF BERGEN FIORDS. 



the Northern Arctic and temperate seas, originating and 

 luxuriating under glacial conditions, but they nevertheless 

 extend far down into the temperate regions of both Atlantic 

 and Pacific Oceans. Some forms seem to have migrated 

 southwards {e.g., Tectum tesiiidi7ialis), but the larger number 

 are, I believe, retiring northwards under the altered and less 

 Arctic climate of the North Atlantic. As a rule in their 

 more southern habitats, they Avill be found in a dwarfed form, 

 resultant from the unfavorable conditions in which they now 

 find themselves, and are gradually becoming extinct. As 

 species which have apparently very recently (geologically speak- 

 ing) become extinct in the British Seas, may be mentioned 

 Hypothyris psittacea, Columbella rosacea,, Astarte crenata, Leda 

 pernula, Molleria cost ii lata. 

 In order to complete my views as to the gi'eat groups into 



which the Fauna of the North Atlantic ought to be divided, I add 



the remaining groups though they have no immediate bearing on 



the present paper. 



M . — Mediterranean. 



Cn. — Canarian. Species which have originated on the Western 

 coast of Africa or the contiguous cluster of Islands as far 

 south as St. Domingo, lat. lo. N. 



E. A. — Eastern American. Littoral forms which have origin- 

 ated on any part of the N.E. American coast as far south as 

 (say) Cape Hatteras, lat. 35 N., migrating thence N. and S. 

 As far as our present knowledge goes the migration of such 

 littoral forms as are found both in Europe or America, but 

 have not a common Polar origin, seems to have been from 

 east to west. This may probably be accounted for by the course 

 of the main ocean currents, which partially assist the migration. 



Q)q_ — Caribbean. Species which have their home in the Gulf 

 of Mexico and among the West Indian Islands, from Cape 

 Hatteras, lat 35. N. to Trinidad lat. 10. N. 



J.C, iL, Feb., 1S79 



