Ill K NAL TILLS. 



A MOLLUSCAN STAMPEDE. 



31 



BY T. VAN HYNlNci. 



Iii the summer of 1899, while collecting shells in the vicinity of 

 Des Moines, Iowa, I made ;in observation which has seemed to me 

 to be of more than ordinary interest. Along the margin of a hayon, 

 adjacent to the Des Moines river, 1 discovered what was quite ap- 

 parently an old, abandoned cray-fish hole beside an old stump. Clear, 



cold spring water was bubbling up from within the hole, overflowing 

 and running down the slope about ten feet to the bayou. All the 

 way up the little stream, from the bayou to the hole, Physa gyrina 

 wa.s very numerous and in all stages of development, from very 

 minute young ones up to well-developed adults. All were headed up- 

 , stream and moving slowly towards the hole. Around th j margin of 

 the hole, and down in it as far as I investigated, about fourteen 

 inches, the surface or wall of the hole was covered with them one 



