INTRODUCTION. CXlll 



Distribution in Space and Time. 



The number of species described in the present work 

 as occurring on the British coasts is 235. Of these, 69 

 have not so far been found elsewhere ; but of course no 

 inference as to their range can be drawn from this nega- 

 tive fact. For 28 species Shetland is the only British 

 locality : of these, 8 have not been found (recent) beyond 

 it ; and the remaining species are almost all Arctic forms, 

 which appear to have here their southern limit. Two of 

 the number, however, are exceptions, and range much fur- 

 ther south — one {Cellaria Johnsoni) being a rather abun- 

 dant IVfediterraneaii form, and extending to Madeira. 

 In this case Shetland appears to be the northernmost 

 limit in the range of an essentially southern form, the 

 curious point being that so far it has not occurred at 

 any intermediate station between south and north". 

 Another of them [Porina borealis) is common to the 

 deep water of the ^' Outer Haaf" and the Arctic seas, 

 and to great depths off the coasts of Florida, Portugal, 

 and the Azores. In this case we have to do Avith one 

 of the "abyssal^' forms, wliich are remarkable for their 

 extended distribution. 



The^'Shetland sea, as Mr. Norman has remarked, seems 

 to be " in an especial manner the meeting-place of northern 

 and southern types. ^^ Arctic forms, as we have seen, 

 'descend to it, and in many cases go no further south ; 

 southern forms have made their way up to it, and gene- 

 rally have not become naturalized in the colder waters 

 beyond it. Out of 68 species recorded in the present 



* 



Norinan mentions other cases of a .similar kimi (■• Shetland Dredo-ino- 

 Eeport," Rep. Brit. Assoc, for 18G7, p. 248). 



I 



