Xciv INTRODUCTION. [CH. 



shell [Physa acuta) inhabits the West Indies, and that 

 such Mollusca seem to have a greater aptitude for dif- 

 fusion, or a greater capability of enduring different 

 climates, than land Mollusca, being (as some naturalists 

 would call it) more ^' mundane/^ 



Fossils. — It is sometimes very difficult, if not impos- 

 sible, to distinguish what are called " fossil " from " re- 

 cent ^' shells of marine species, if they are " dead " or 

 found in an empty state. When the shells in question 

 belong to species which are not known to inhabit the 

 locality where they occur, this difficulty may give rise to 

 some interesting questions. In most cases, the nature 

 of these shells is manifest from their dull appearance 

 and greater opacity, contrasted with fresh shells of the 

 same species; and it does not require much experience 

 to determine whether single valves of Pecten Islandicus, 

 which are not unfrequently taken at comparatively great 

 depths in our northern seas, are fossil or recent, although 

 they occasionally retain their coloured markings. This 

 species is abundant in the Arctic regions, and during 

 the Glacial epoch appears to have been diffused over a 

 large tract of the European sea-bed; but I am not 

 aware that it is now found in a li\dng state south of the 

 Bohuslan district of the Swedish coast. But a perplex- 

 ing case has occurred with respect to some shells which 

 were taken by the dredge in the Irish Sea off the coast 

 of Antrim. The locality is a submarine deposit called 

 the "Turbot bank," lying about five miles south of 

 Larne and having a depth of about 20 fathoms at low 

 water. This bank was repeatedly and diligently explored 

 during several successive years by Mr. Waller, Mr. 

 Hyndman, and other naturalists ; and I had the advan- 

 tage of not only examining the produce of their labours, 

 but of taking part in an expedition which was made in the 



