BITS BY POND, RIVER, AND SEASHORE. 179 



they appear to be as stationary as the crags upon 

 which they live, it is surprising what an amount of 

 wandering about some of them really do. I once 

 had some rocks under observation for a week, and 

 was astonished at the way in which a number of 

 purpura shifted from place to place. One day a 



SHELLS ON SEASHOKK. 



rock would be covered with them, as seen in our 

 illustration on page LSI, and the next hardly a single 

 specimen could be seen upon it. 



The large shells in that illustration are popularly 

 called dog periwinkles, or purple shells ; the latter 

 name refers to a purple dye that may be obtained 

 from them. This was formerly used in manufac- 

 tures, but the difficulty of collecting it in sufficient 

 quantities has caused its use to be abandoned. 



