VI.] INTRODUCTION. CXiii 



— although it sometimes occurs^, and I have several times 

 picked up on the sea-strand, near a port resorted to by 

 foreign vessels, shells which had evidently come from 

 ballast. Strangers of this kind may, however, be de- 

 tected without much difficulty by the application of in- 

 trinsic evidence. A much more fertile and perplexing 

 source of error, as regards the introduction of spurious 

 species, consists in collectors of Mediterranean, as well 

 as British, shells not taking sufficient care to keep these 

 collections separate; and too much praise cannot be 

 given to Mr. M ^Andrew, whose labours and experience 

 in the investigation of the European MoUusca are so 

 well known, for his extreme accuracy in the above re- 

 spect. 



' 'Sea-side sketch, — Having offered this imperfect view of 

 the British Mollusca, with regard to their structure and 

 habits, and their relation to other animals and ourselves, 

 as well as to their distribution, I cannnot refrain from add- 

 ing another page to this unusually long introduction, to 

 exhibit a charming and truthfal picture by my lamented 

 and highly gifted friend. Professor Edward Forbes : — 

 " To sit down by the sea-side at the commencement of 

 ebb, and watch the shore gradually uncovered by the 

 retiring water, is as if a great sheet of hieroglyphics — 

 strange picture-writing — were being unfolded before us. 

 Each line of the rock and strand has its peculiar cha- 

 racters inscribed upon it in living figures, and each figure 

 is a mystery, which, though we may describe the appear- 

 ance in precise and formal terms, has a meaning in its 

 life and being beyond the wisdom of man to unravel. 

 How many and how curious problems concern the com- 

 monest of the sea-snails creeping over the wet sea- weed ! 

 In how many points of view may its history be considered ! 

 There are its origin and development — the mvsterv of 



