Cxii INTRODUCTION. [CH. 



sixty miles from the Bill of Portland^ and about thirty- 

 five miles from Cape Carteret on the coast of Brittany. 

 All the six species which I have above mentioned are 

 found on our side of the Guernsey coast. 



Exotic and spurious species. — The fauna of any par- 

 ticular country (although isolated^ like Great Britain) 

 cannot be satisfactorily studied by itself and without 

 reference to the fauna of other parts of the same district. 

 The habit of observing and comparing the Mollusca of 

 different countries is of undeniable advantage; and it 

 may be favourably contrasted with the tendency of local 

 naturalists and collectors to exaggerate trifling differ- 

 ences^ which would have disappeared on a more extended 

 survey. The enlargement and increase of such expe- 

 rience have the same beneficial effect on a mind inclined 

 to the cultivation of science, as travelling in a foreign 

 land, with one's eyes open, has in expanding the intellect 

 and impro\4ng our social nature. By such means our 

 notions become in each case less contracted ; and (which 

 is perhaps of more importance) our ideas with regard to 

 the labours of other naturalists are imbued with a spirit 

 of greater liberality and charity than if we had pursued 

 the selfish course of working in our own sphere without 

 any intercourse or sympathy with them. 



The " index expurgatorius," containing the species of 

 Mollusca which are termed "spurious" (being those 

 which have been admitted into catalogues of British 

 shells, but have not been proved to be indigenous to 

 this country), is now very small, owing to the labours of 

 Dr. Gray in revising the list of our land and freshwater 

 shells, and of the authors of the ^ British Mollusca ' in a 

 similar revision of our marine shells. The casual intro- 

 duction of tropical or foreign shells by means of ship- 

 wrecks or ballast is not so frequent as has been supposed. 



