88 THE DISPERSAL OF SHELLS. 



of May 8th, 1890, at Birch, near Manchester, about fifty 

 yards from the nearest water. 



Ancylus fluviatilis and allied molluscs frequently 

 adhere to the shells of the large fresh-water mussels, 

 Vnio and Anodonta ; for instance, Mr. Baker Hudson, 

 who has collected large numbers of pearl mussels {Unio 

 margaritifer) from the Yorkshire Esk, has found 

 A. fluviatilis to occur on at least sixty per cent, of 

 them,' and this suggests another possible mode of dis- 

 persal which may have come into operation once every 

 now and then in the course of ages, for a bird carrying 

 a large bivalve clinging by closure to its toe might 

 obviously transport these limpet-like shells also, for 

 short distances at any rate, so that it is just within the 

 range of possibility that a newly formed pool might be 

 simultaneously stocked by a bird with a univalve and a 

 bivalve, molluscs as widely dissimilar as are Ancylus 

 and Unio or Anodonta. 



Fresh-water limpets and inoperculate pond-snails of 

 various kinds are known to crawl occasionally upon the 

 backs or limbs of frogs and other aquatic or amphibious 

 creatures. Mr. W. Thompson, in 1841, mentioned 

 that he once saw numbers of Limncea peregra (a 

 common pond-snail) attached to the backs of some 

 turtles kept in a pond at Fort William, near Belfast, 

 and they appear to have held on with some firm- 



^ And see A. M. Norman, "Zoologist," xi. (1853), 3763; Tate, 

 "Land and Fresh-water Mollusks," (1866), p. 207; H. Pollard, 

 "Naturalist," 1887, p. 138; H. A. Pilsbry, " Nautilus," iv. (1890), 

 48. 



