324 



PALUDESTRINA CONFUSA (FRAUENFELD) IN THE 

 WAVENEY VALLEY. 



By C. OLDHAM. 



(Read before the Society. September 7th, 1921), 



It is some i8 years since Mr. Tomlin announced the discovery in the 

 flood-debris at Oulton Broad, near Lowestoft, of dead shells of a 

 small pectinibranch mollusc which the late Dr. Boettger referred 

 with some hesitation to Pseudanmicola atiatina (Drap.), a species 

 doubtfully separable in his opinion from P. similis (Drap.). Mr. 

 Tomlin tells me that although at the time he thought the Oulton shell 

 differed from the one that used to occur in the marshes of the Thames 

 between Woolwich and Greenwich, and which Jeffreys identified with 

 Draparnaud's Cyclostoma simile, he now regards the two as local 

 varieties, i.e. of Amnicola confusa of Frauenfeld. 



So far as I know Oulton Broad is still the only recorded East 

 Anglian station for P. confiisa, but my experience of it in the Waveney 

 Valley, last May, suggests that it will be found in the brackish tidal 

 waters of other sluggish rivers between the Wash and the Thames. 

 The reed-beds at Oulton are sometimes flooded by brackish water, 

 but at the time of my visit they were dry except for a little water in 

 holes and shallow trenches, and there I found P. confusa living with 

 Limncea pereger, L. trtincatiila, Platiorbis marginatus, P. leucostoma, 

 Bithynia tentaculata and Pahidestrina jenkinsi, and — on the reed- 

 stems just above water — Hygromia hispida and Succinea putris. 

 P. confusa occurs not only in these reed-beds but in the Waveney, 

 which is connected with the broad by a wide gut ; and although I 

 failed to find it in the marsh drains it was abundant at the muddy 

 margin of the stream, in wet places between the retaining banks and 

 the actual channel, and as dead shells, in the flood-debris. The land 

 within the retaining banks is under water when the river is in flood or 

 there is an unusually high tide, and as the distance from the sea 

 increases the diminishing salinity of the water is apparent in the 

 changes of the vegetation and in the associated mollusca. At a spot 

 just below the swing-bridge which carries the Great Eastern Railway 

 across the river near Beccles, the flood water, although brackish, is 

 less so than at Oulton Broad ; the plant-association indeed suggested 

 a fresh-water marsh, its chief constituents being Arinido phragmites, 

 Iris psetidacorus, Thalictruin flavuiu, Spircea ulmaria, Caltha palustris, 

 Menyanthes trifoliata, Cardamitie prafensis, C. amara, and Valeriana 

 officinalis. Here P. confusa was living with Limncea pereger, L. 



