FRESH-WATER SHELLS. 7 



mud, or sometimes by a lining of concrete, such " dew- 

 ponds " retain a store of water, except during droughts 

 of exceptional duration, derived from "the rain, dew, 

 and condensation of the mists which often hang on the 

 tops of the hills." Many occur at the distance of a mile 

 or even two or three miles from the nearest stream or 

 marsh, and, as the Downs rise to 800 feet, their average 

 height is fully 200 feet above other water, so that the 

 aquatic animals and plants found therein must certainly 

 have been " transported uphill as well as across un- 

 congenial tracts of dry grass,^"* or be descended from 

 ancestors thus transported. Notwithstanding this ex- 

 treme isolation, however, the ponds are often found to 

 be inhabited by fresh-water shells. Two, at least, of the 

 three examples given by Mr. Reid were thus inhabited : 



(I.) " Large dew-pond on the open Down nearly a 

 mile east-south-east of Amberley Station ; height, 310 

 feet above the sea, and 300 feet above the marshes of 

 the Arun ; nearest water, the marshes of the Arun, 

 distant half a mile. Noted 15th November, 1884. 

 This pond has evidently been made several years, and 

 is now full of water-plants. The species observed, were 

 Junciis (not in flower or fruit, though very abundant), 

 Potanwgeton densiis (very common) , Ra7tu7tai/us aqua- 

 tilisy Cha7'a, and a single small, but vigorous tuft of 

 Elodea canadensis, ... (I have apparently omitted to 

 note the mollusca from this pond, if there were any). 



(2). " Redlion Pond, on the open Down, three miles 

 south-east of Lewes ; height 540 feet above the sea, and 

 530 feet above the marshes of the Ouse ; nearest water, 



