Prof. E. Forbes on the Mollusca of the British Seas. 71 



the full contemplation of such wondrous charms is reserved 

 for immortal and invisible admirers. 



Although, in consequence of the great number of mollusks 

 that are common to all parts of the British seas, provided we 

 compare localities where conditions of sea-bottom and depth 

 are similar, it might seem that there is little evidence of 

 a peculiar distribution within the limits of our area, if we 

 regard its shell-fish either in mass, or analyse the relations 

 of the several species to foreign and surrounding regions, 

 we shall find very distinct manifestations of peculiarities 

 within the boundaries of our own. Were a conchologist de- 

 sirous of accumulating personally and rapidly a complete 

 collection of British shells, he would fail in his object if he 

 confined his researches to any one locality, even though it 

 embraced a considerable reach of coast and variety of sea- 

 bottom. Four districts, at least, would have to be visited. 

 To the Channel Islands he would have to go for several forms 

 that are almost extra British. On the south-west coasts 

 of England he would find a few shells that he would seek 

 for in vain in more northern or eastern seas. Only on the 

 west coasts of Scotland, many species of great interest and 

 peculiarity could be readily obtained. In the extreme pro- 

 vince of the Zetland Isles he would gather some of our most 

 remarkable rarities ; and possibly, after all, he would have 

 to visit as much of the northern half of the German Ocean 

 as may be claimed for our natural history province, and the 

 west coasts of Ireland, before his cabinets could be fairly 

 filled. 



In reality, our molluscan fauna is a composite assemblage, 

 in which immigrants from the north and from the south inter- 

 mingle with the aboriginal inhabitants, and descendants of a 

 pre-adamite fauna survive amongst them. Those forms that 

 have travelled northwards, and those that have journeyed 

 southwards, have not all made their way with equal speed. 

 Consequently, as we proceed either way, we find a number of 

 species gradually disappear, and differences instituted, both 

 positive by the presence of peculiar types, and negative by 

 the absence of others, that serve to mark a sub-division of 



