MEANS OF DISPERSAL. 3 1 



diffusion of fresh-water shells had been chiefly effected 

 by " streams and land-floods ; " ^ but the influence of 

 these agencies, it should be remembered, must be con- 

 fined, to a great extent, within the limits of river-basins ; 

 in some parts of the world, however, floods are known 

 to connect low watersheds. Unusual rushes of water 

 must certainly wash away large numbers of shells and 

 carry them down stream, possibly to great distances. 

 Every shell collector knows that ditches and pools 

 lying near to rivers, and liable to be affected by over- 

 flowSj are generally stocked with more species than 

 those on high ground, far from the larger water-courses. 

 In Nottinghamshire, for instance, Mr. Musson has 

 remarked, the greatest number of species can be 

 obtained from those parts of the Trent Valley which 

 are subject to flooding, and, in fact, nearly every species 

 recorded for the county can be collected in the meadows 

 between Colwick and Beeston, while on the other hand, 

 the species found in ponds unconnected with the 

 general drainage system are comparatively few. Many 

 facts illustrating transportal by floods might have 

 been got together. A variety oi Ancylus fluviatilis was 

 noted at one time as very abundant on stones in shallow 

 places in the River Went, but a strong flood, it is said, 

 " swept nearly all the stones and shells away." " Mr. 

 L. E. Adams has described a flood in the Thames which 

 brought down large quantities of shells, comprising 



1 Tate's " Land and Fresh-water MoUusks," 1866, p. 188. 



2 J. Wilcock, as quoted by Mr. George Roberts, "Zoologist," (3), 

 ix. (1885), 475. 



