MEANS OF DISPERSAL. 39 



long time out of water, the author proceeds to remark, 

 by way of answering the latter half of his question, that] 

 " many fresh-water species will live in brackish water, 

 as is well known. Neritina Veclivata, found in Florida, 

 seems indifferent as to whether it lives in fresh water or 

 that which is more or less salty, and A^. lineolafa and 

 viacrostoiua, which I found in Honduras in the mouths 

 of rivers, often extended into the sea. Planorbis 

 tumidus was often found in slightly brackish water in 

 Florida, and the Lhnncsas in the Baltic and some 

 places on the British coasts mingle with the Littorinas. 



''Ampullan'a caliginosa, a Mexican species, closely 

 related to A. depressa^ if not identical with it, Planorbis 

 tumidus and havanensis (identified by Mr. H. A. Pilsbry, 

 who has made a special study of the genus Planorbis), 

 and a few other tropical species found to-day in the 

 Southern States, may have been introduced, I think, 

 in the manner of which I have spoken. Of course such 

 voyages of living snails, with a successful termination, 

 could only happen rarely, but it must be remembered 

 that countless ages have elapsed since the present 

 species have occupied the earth." ' 



That both adult shells and their ova are frequently 

 carried out to sea with drift timber, brushwood, &c., 

 cannot be doubted for a moment, but it is not actually 

 known, I believe, that any mollusc from a truly fresh- 

 water habitat can endure much exposure to the 

 waters of the ocean. More direct experiments on the 



' C. T. Simpson, " Conchologists' Exchange," ii. (1887), 50-1 



