DREDGING NOTES. 13 



snails with the apex up and the opening facing 

 you, that opening may seem to be either on the 

 left or on the right ; that is, it may be either a 

 sinistral aperture or a dextral one. If the former, 

 the snail belongs to the PJiysa division. You 

 can see such snails floating wrong side up in these 

 pools, taking a swim after their own peculiar 

 fashion. I think that while on these swimming 

 excursions the snails are sometimes brought into 

 contact with the polliwogs. I saw a pond-snail 

 swimming once when a polliwog was very persis- 

 tent in his attentions to it as it floated on top of 

 the water. A boy who was with me was much 

 interested in the performance, thinking, I sup- 

 pose, that the snail was about to be eaten. 



The snails themselves (I am now speaking of 

 PJiysa) are black, or very nearly that color. One 

 can observe the flesh when the snail is swimming 

 or walking. When it is dead its shell is of a 

 lighter shade, a grayish or yellowish color. 



The method of swimming has been variously 

 described as " clinging with their foot to the sur- 

 face of the water," and as a " creeping on the 

 under surface of the air," that is, on the layer of 

 air next to the pond. It is a queer performance 

 to watch, for the shell and the creature in it look 

 weighty enough to sink to the bottom of the pool, 

 yet the swimming or floating goes on. 



There are much smaller snails in this brook. 

 One finds them only occasionally, at least at cer- 



