INTRODUCTION. 



CXVll 



these rather less than half are common to the two 

 countries. 



About 100 British species occur on the more south- 

 erly coasts of Norway^ and about 70 within the Arctic 

 region ; no distinctively southern forms^ I believe, are 

 met with amongst them. About 50 of our species range 

 to the North-American coast, almost all of which have 

 been recorded from the Arctic seas *. 



It is interesting to compare the number of species 

 found in three well-marked districts (each of which has 

 been thoroughly investigated) — the south-west coast of 

 England, Shetland, and portions of the north-east coast 

 of England f. 



137 



139 



89 



Turning to the distribution of our British species in 

 time, we find that a considerable number of them existed 

 in the Tertiary period. Many are recorded by Busk ; 

 and about 40 are included in the list of the Upper Tertiary 



* We may expect a flood of light to be thrown on the subject of distribu- 

 tion by the results of the ' Challenger ' expedition, when published. In 

 their absence it would hardly be safe to indulge in generalization. 



t Alder, ' Catalogue of Zoophytes of Northumberland and Durham ' and 

 Supplement. Norman, " Shetland Polyzoa," Eep. Brit. Assoc. 1867. Hincks, 

 " Catalogue of Zoophytes of South Devon and Cornwall." The Shetland 

 and South-west lists have been revised; the North-east is as given bv Alder 



