River Cam 115 



closer search than the Fenland to the north, and for the 

 former our information is perhaps fairly full, though it is 

 possible that one or more forms found in the Pleistocene 

 gravels of the district which still live in other parts of 

 England may prove to have survived in the county. 



Nevertheless there is much to reward the searcher in the 

 neighbourhood of Cambridge. In the Cam as it passes through 

 the Backs and immediately below the town many of the 

 commoner freshwater forms are to be met with, though it 

 appears probable that the increasing pollution of the water 

 has driven some species down stream, in the same way that 

 the lamprey has been ousted from the river at and above 

 Cambridge in recent years. The departure of the Unionidae 

 is noticeable, and for them and also for Dreissensia the 

 neighbourhood of Upware is a more favourable locality 

 than Cambridge. Anodonta cygnea 1 occurs abundantly in 

 the Cam above Grantchester, and Sphaerium rivicola may 

 be found in many places. Pisidium fontinale lives in the 

 backwaters of the Cam in the Backs and in the river 

 itself lower down, while P. amnicum occurs more frequently 

 near Upware. Among weeds in the backwaters of the river 

 as it passes behind the Colleges there will be found in fair 

 abundance Bithynia tentaculata, Valvata piscinalis, Limnaea 

 2)ereger, Planorbis corneus, P. albus, P. carinatus, P. umbili- 

 catus, which is very common, and P. wrtex. Limnaea auricu- 

 lar -ia, L. palustris and Segment ina nitida occur more sparingly. 

 Ancylus fluviatilis and Acroloxus lacustris are also to be met 

 with without much search. Neritina Jtuviatilis is fairly 

 numerous on the bases of the College bridges. Physa 

 fontinalis is not uncommon, and Succinea elegans may 

 be found anywhere among grass and wood on the banks. 



1 The nomenclature throughout this article is that employed by 

 Mr B. B. Woodward in his "List of British Non-Marine Mollusca" 

 (Journal of Conchology, x. p. 353, October, 1903), and adopted by the 

 Conchological Society in their revised list. 



82 



