32 THE DISPERSAL OF SHELLS. 



twenty-three fresh-water and twenty-four land species ; 

 the greater part were " dead and worthless," but some 

 of the aquatic kinds, it appears, were still alive.^ LimncBa 

 trimcatida, Mr. Jeffery has remarked, " has a habit of 

 following the flow of water during floods," and often 

 gets left high and dry." During the summer of 1881, 

 Professor Thomas, studying the life history of the liver- 

 fluke, and anxious to try infective experiments with 

 this snail, searched repeatedly for it in the neighbour- 

 hood of Oxford, trying the localities in which he had 

 formerly found it, and those given in Whiteaves' list, 

 but only empty shells could be discovered. In 1882, 

 however, there were floods in July, and the creature 

 came down in vast multitudes with the waters of the 

 Isis ; so numerous was it that, several times, a single 

 sweep of a small hand-net gave more than 500 

 examples, and this in a ditch where the year before not 

 a single specimen could be found ; all along the margins 

 of the ditches it occurred in the greatest profusion, and 

 it was found in numbers on the land after the flood- 

 waters had retired.^ Mr. W. A. Marsh, in his valuable 

 notes on the land and fresh-water shells of Mercer Co., 

 Illinois, mentions that Liinncea reflexa (common in the 

 small lakes of the county and in some of the larger 

 ponds in the Mississippi River bottom) is carried out 

 of the lakes in times of very high water, and may then 



^ L. E. Adams, "Science Gossip," xvii. (1881), 118. 

 2 W. Jeffery, "Zoologist," (3), ii. (1878), 181. 

 ^ A. P. Thomas, " Quart. Journ. Micro. Sci.," (n.s.), xxiii. (1883), 

 IC5-6, 130-1. 



