KENPAII. : TirF. Mni.IJTSfA OF nUNDIK. 243 



nuiiieious and nearly all very small and shallow, and so subjected 

 to great variation of temperature. Possibly this has something to do 

 with the paucity of molluscan life in many of them. As a rule the 

 water is clear and there is little vegetation except at the edges of the 

 pond, and when masses of floating Water ('rowfoot {Ramiticulus 

 aijnaticd) occur. The ponds on the Oxford Clay show much more 

 vegetation than those on the Oolitic Limestone and Upper Estua- 

 rine Clays, and are proportionately more prolific of mollusca. 



A typical association of the Hard-Water Pond is: — Linmcea pereger, 

 Plano7-bis crista, Sphceriitm conieinn, S. lacustre^ and Pisidium obhisale, 



L. peregcr is sometimes replaced by Z. stagnalis or (in very shallow 

 ponds) by L. truiuatula ; but where it does occur it is the dominant 

 species. The most constant members of the association are P. crista 

 and P. obtusah^ and when occurring they are extremely abundant. 

 In one shallow pond is found in large numbers a form of P. obtiisale, 

 which Mr. Stelfox informs me approximates very closely to Pisidium 

 scholizii. I may here say that the great bulk of my collections of the 

 Pisidia has been examined and identified by Messrs. A. W. Stelfox, 

 R. A. Phillips, and C. Oldham, to whom my sincere thanks are due, 

 as without their invaluable assistance the lists of aquatic species 

 would have been very incomplete. 



In these ponds occur other species more or less frequently, viz., 

 Planorbis fontanus^ Velletia lacustris, Pisidium subtruncatuni, P. 

 7nilium, and P. Jienslowanuvi. And, when the pond or one part of it 

 is shallow and much overgrown with rushes and grasses, appear 

 Planorbis spirorbis, Aplexa hypnoruin, and P/iysa fofitinalis. 



In the damp margin of the pond occur some or all of the following: 

 Agriolimax agrestis, A. Icevis, Hyalinia radiatula, Punctum pyginceum, 

 Hygromia Iiispida, Vallofiia costata, V. excaitrica, Succinea putris, and 

 6". elcgons. 



§ 3. Soft- Water Ponds. 



These ponds are few in number, occurring here and there in the 

 River Alluvium. They present quite a different facies of molluscan 

 life, far richer both in number and variety than the hard-water ponds 

 of similar size and depth. The striking feature of the Soft-Water 

 Pond is the predominance of PIa?iorbis, especially of the larger 

 members of the genus, while BitJiynia tentaculata appears as a strong 

 competitor in the associated life. 



Dominant : — Planorbis corneus and Bithynia tentaculata. 



Associated with : — LimncBa pereger, L. palustris, Planorbis nmbili- 

 caius, P. spirorbis, Physa fontinalis, Sphcerinin corneuni, and Pisidium 

 milium. 



