FRESH-WATER SHELLS. 9 



or dew-ponds of the Lincolnshire Wolds, dug out of 

 the chalk and puddled with clay, though often or usually 

 dry in summer, do not essentially differ, I imagine, from 

 those of the Downs. Recently my brother showed me 

 a pond of this kind, the most isolated he could find 

 about Louth, situate on one of the highest of the rolling 

 chalk hills in the neighbourhood, at a great distance 

 from any other water, and in this were bivalves of two 

 kinds, SphcBriiun lacustre and Pisidiiun pusilluin, the 

 former in good numbers. In ponds on lower ground 

 and less remote from other water (but perfectly isolated 

 and out of the reach of floods), where the conditions of 

 existence are probably more congenial, small bivalves 

 certainly seem to occur with some frequency. Uncon- 

 genial conditions, Mr. Reid remarks, have probably 

 much to do with the poverty of the fauna and flora in 

 dew-ponds. Mr. Musson, who, also, has paid some 

 attention to the subject, speaks of Sphcerium lacustre as 

 a species commonly found in "upland ponds" [ponds 

 without inlet or outlet, dependent directly on rain-water 

 for their supply, as distinguished from such as are 

 affected by floods and rivers], in which also, as he adds, 

 Sphcerium corneum, more generally found in rivers, 

 canals, &c., occurs occasionally. A small "upland" 

 horse-pond, near the Hemlock Stone, near Nottingham, 

 examined by him during 1883 and 1884, was inhabited 

 by six kinds of molluscs, of which three were bivalves : 

 Sphcermm lacustre and two species of Pisidium ; and 

 three univalves : Planorbis nitidus, PI. naictileiis^ and 

 Ancylus lacustris — a strange set of species, as he remarks. 



