Xvi INTRODUCTION. 



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 not a few shells that he would seek for in vain in more 

 northern or eastern seas. Only on the west coasts of Scot- 

 land, many species of great interest and peculiarity could 

 be readily obtained. In the extreme jH-ovince of the Zet- 

 land 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 assem- 

 blage, in which immigrants from the north and from the 

 south intermingle with the aboriginal inhabitants, and de- 

 scendants 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 provinces within our 

 area. Even among many of the species that are widely 

 and almost universally spread throughout our seas, we find 

 the frequency of their occurrence diminishing one way or 

 other according to their origin. As a general rule the 

 northern influence prevails over the southern in the British 

 fauna, and gives greater peculiarities to the zoology of the 

 Scottish than to that of the English seas. The central 

 portion of our area — the Irish sea — is a sort of neutral 

 ground, from which several forms are absent that are to be 



