116 Tlie Mollusca of Cambridgeshire 



Limnaea stagnalis is more frequently met with in waters 

 below Cambridge, as at Burwell, Wicken and Ely. Limnaea 

 truncatula seems rather local in the Cam, but it is not un- 

 common in ditches at Cherryhinton and in the Ouse at Ely. 

 Vivipara contecta also does not seem common in the river 

 at Cambridge, but it may be found abundantly in weedy 

 backwaters near Clayhithe, and from there down to Ely. 

 Bourne Brook, which runs from Caxton to the Cam at 

 Grantchester, is also a good locality for this species. F. 

 vivipara is apparently not an inhabitant of the Cam or is 

 rare, though it occurs within the county. 



Turning from the river itself we find much of interest in 

 the molluscs to be found in the numerous ponds and ditches 

 close to Cambridge. Just above the town are the pasture 

 lands known as Sheep's Green and Coe Fen, and through 

 these the Cam now flows in a course which is partly artificial. 

 Hence many of the ponds, especially of Sheep's Green, are 

 remains of the old bed of the river. In a paper on local 

 Pleistocene species Mrs McKenny Hughes 1 has pointed out 

 that in these ponds occur several of the less common species 

 and that it is a striking fact, as shown by an investigation by 

 Mr J. K. Le B. Tomlin, that the various patches of water 

 exhibit great differences in their molluscan fauna, while some 

 of them also contain forms which are either rare or else 

 apparently absent from the river in its present condition. 

 Thus in these pools there occur Valvata cristata, Bithynia 

 leachii, Planorbis crista, P. fontanus and Aplecta hypnonini. 

 One specimen of Planorbis glaber has also been found in a 

 Sheep's Green pool. Of the forms which are found in the 

 river, Velletia lacustris, Planorbis albus and Segment i>i. 

 nitida also inhabit Sheep's Green. Carychium minimum 

 is numerous on the grass at the margin of the pools. 

 Mrs Hughes points out that the conditions above described 



1 "On the Mollusca of the Pleistocene Gravels in the neighbourhood 

 of Cambridge," Geological Magazine, Dec. 3, v., 1888, p. 193. 



