DISPERSAL BY MAN. 2ig 



which have been published would be a tedious under- 

 taking, and might occupy the greater part of a lifetime ! 

 According to a " census of the authenticated distribu- 

 tion of British land and fresh-water mollusca," ^ pub- 

 lished in 1889, it appears that specimens collected in 

 the following English and Scottish counties, or vice- 

 counties, have passed under the examination of the 

 Conchological Society^s referees : 



Somerset, North. Staffordshire. 



Surrey. Shropshire. 



Middlesex. Leicestershire. 



Berkshire. Nottinghamshire. 



Oxfordshire. Derbyshire. 



Cambridgeshire. Cheshire. 



Huntingdonshire. Lancashire, South. 



Northamptonshire. Yorkshire, South-West. 



Gloucestershire, East. Yorkshire, Mid- West. 



Gloucestershire, West. Renfrewshire. 



Worcestershire. Lanarkshire. 

 Warwickshire. 



No Welsh or Irish habitats are given. 



The Dreissena is perhaps better fitted for dissemina- 

 tion by man and subsequent establishment than any 

 other fresh-water shell ; tenacity of life, unusually rapid 

 propagation, the faculty of becoming attached by a 

 strong byssus to extraneous substances, and the power 

 of adapting itself to strange and altogether artificial sur- 

 roundings have combined to make it one of the most 



^ By J. W. Taylor and W. D. Roebuck, forming chapter v. of 

 the volume on " Land and Freshwater Shells," in Swan Sonnen- 

 schein's " Young Collector " series, 1889. 



