24 THE DISPERSAL OF SHELLS. 



occurrence in waters derived exclusively from the rain- 

 fall of the immediate neighbourhood, however, has often 

 been regarded as surprising and mysterious. But we 

 must not fall into the error of concluding that the 

 creatures are necessarily introduced, after the appear- 

 ance of the water, by accidental means of dispersal, for 

 it is stated that they are able to remain alive in a torpid 

 state for long periods buried in the mud, and that on 

 the return of water to the surface they are ready to 

 burst into activity with remarkable rapidity. Their 

 almost immediate appearance after the occurrence of 

 rain, in tropical countries, in swamps and hollows which 

 have long remained dry is, I understand, quite familiar 

 to the naturalists of those regions. Adanson states that 

 a minute fresh-water shell (which he calls Btilzvms) is 

 to be seen only from September to January in the 

 marshes of Senegal which are formed by the rains of 

 June, July, August, and September ; when the marshes 

 dry up the animals disappear, a few empty shells alone 

 remaining upon the surface^ but they never fail to return 

 with the rainy season.^ Major Becher, in a paper on 

 the mollusca of the Maltese Islands, mentions the find- 

 ing oi Lhnncea trinicattila in small hollows, the mud of 

 which, in summer, " must be baked till it becomes 

 almost brick." ' Mr. W. A. Marsh records the finding 

 of Liinncea humilis (said by some to be the same as 

 L, truncatuld) in ponds on his land, in Illinois, which 



" Hist. Nat. du Senegal," p. 7, as quoted by Dr. Johnston in 

 "Loudon's Mag. Nat. Hist.," vii. (1834), 115. 

 - E. F. Becher, " Journ. of Conch.", iv. (1884), 231. 



