CH. Ill] MOLLUSCA OF PONDS. 141 



of fresh-water Mollusca can be studied in its most simple 

 aspect by a consideration of the faunas of artificial ponds 

 and of " dew ponds." The latter are, it should be explained, 

 artificial ponds dug and lined with concrete or chalk mud 

 for the convenience of cattle ; they are left to be filled by 

 rain, dew, and the condensation of mist. At first devoid 

 of inhabitants such ponds gradually become stocked, thus 

 proving a capacity for active or passive migration on the 

 part of the Mollusca. Careful and successive observations 

 have proved in a few cases the actual time in which a 

 given pond may become populous. An isolated pond 

 near Leeds investigated by Mr Nelson during the years 

 1860-63 yielded two bivalves, Sphcerium lacustre and 

 Pisidium pusillum, and two Gastropods, Planorbis nautilus 

 and Lymncea peregra. About 1873 an additional species, 

 Planorbis corneus, was discovered ; and finally ten years 

 later the Molluscan fauna was increased by six other 

 species. Now this particular pond was in no communi- 

 cation even in seasons of flood with any other pond or 

 stream ; hence its inhabitants must have arrived by some 

 other conveyance than running water. To be able to 

 travel at all, and we shall enquire immediately into the 

 means of dispersal, implies considerable tenacity of life, 

 principally in withstanding the fatal effect of a too thorough 

 desiccation. That this tenacity of life need not by . any 

 manner of means be underestimated in dealing with the 

 problem under review, is shown by the fact that an 

 Australian Unio " having already survived in a dry drawer 

 for 231 days, packed up (after being tested in water) and 

 forwarded to England, reached Southampton in a living 



