CHAPTER IX. 



ON THE FRESH-WATER AND LAND MOLLUSCA IN- 

 TRODUCED INTO THE BRITISH ISLES BY HUMAN 

 AGENCY. 



Man's agency, as Forbes long ago remarked, "may 

 materially affect a fauna, and has affected that of 

 Britai n.*^ ^ Of the forty-six fresh-water species included 

 in the Conchologlcal Society's list of 1883, however, 

 only two, the zebra mussel {Dreissena polymorpha) and 

 an American coil-shell [Planorbis dilatatus), can be 

 reasonably regarded as human importations ; and, as 

 far as I know, only one other, SphcErium ovale, has ever 

 been looked upon as even doubtfully indigenous. 



Sphceyiiun ovale, which is local in this country, happens 

 to occur in company with Planorbis dilatatus in canals 

 at Gorton, Pendleton, etc., so that Mr. Jeffreys was led 

 to suggest, in 1869, that it might possibly be the 5. 

 iransversuni of Say introduced, like P. dilatatus, from 

 America ; but he observed that it had long been known 

 in this country, and that he possessed a specimen which 

 was in Dr. Turton's collection of British shells more 



' Edward Forbes, " Report, 9th meeting, British Association, 

 1839," (1840). P- ijo- 



P 



