196 THE DISPERSAL OF SHELLS. 



growing, " filled with Physa gyrina Say " of a different 

 form from those found in his neighbourhood ; ^ and some 

 years ago, Dr. Alford Nicholls found fresh-water shells 

 of two species inhabiting tubs in which he was cultivat- 

 ing aquatic plants, gathered some time previously in a 

 pond in the island of Antigua, and carried from thence 

 to Dominica. The digging of canals, of course, as 

 Forbes long ago remarked, has largely facilitated the 

 spread of these animals.- In America and in other 

 large land areas, it will be remembered, " the waters of 

 streams which differ widely in their fauna? " are some- 

 times thus connected.'' 



Land-Shells. 



It seems likely that land kinds are frequently carried 

 in ships' ballast, collected, as it often is, near the shores 

 of one country and discharged on those of another. 

 Dr. Binney in a discussion on the introduction of foreign 

 species into the United States referred to this cause as 

 a " common mode of introduction ; " ^ and at least three 

 species which have occurred alive in England are 

 believed to have been thus imported. Discoveries of 

 living shells in freshly discharged ballast would be 

 of much interest, but I am not aware that any records 

 of the kind have been made. Miss Esmark mentions 



' W. W. Westgate, " Conchologists' Exchange," ii. (1888), p. 98. 

 » Edward Forbes, " Mem. Geol. Survey of Gt. Britain," i. (1846), 

 p. 398. 

 3 W. B. Marshall, " Nautilus," v. (1892), pp. 133-4. 

 '* See "Terrestrial air-breathing Mollusks," i. (1851), p. 151. 



