MEANS OF DISPERSAL. I4I 



animals were within the shells^ four floated. An 

 individual of H. aspersa in the same condition, and 

 having the mouth closed with an epiphragm, also 

 floated ; and it is worth noting, perhaps, that when left 

 in water for four hours, though it slightly forced the 

 protecting epiphragm, it did not extend its body. The 

 reader will remember, of course, that individuals which 

 from their weight could not be floated by slowly-flowing 

 rivers in their ordinary action, might be carried to con- 

 siderable distances by streams with powerful currents, 

 and by the rushes of water which occasionally occur 

 in many slower streams in times of flood. Prodigious 

 numbers of shells are known to be carried down by 

 rivers, and they are frequently collected from alluvium 

 (as dead-shells) in great quantity. Very large numbers, 

 for instance, some evidently from a great distance, 

 were collected by Dr. Scharff in March and April, 1879, 

 after the subsidence of an over-flow of the river 

 Garonne, comprising no less than 148 specimens of 

 Pupa miiscoriim, and 352 of Vertigo pygmceal^ I 

 hoped to have found records of the discovery of living 

 shells in such situations, but have not done so, 

 and it is perhaps hardly likely that evidently river- 

 carried individuals should often be noticed, for unless 

 they happened to be in a state of hibernation they would 

 be sure to crawl away almost immediately from the 

 debris with which they were stranded. None of the 

 specimens from the Garonne were seen to be alive, but 



■ R. Scharff, " Journ. of Conch.," ii. (1879), PP- 3 15- 16. 



