Observations on Paludestrina ulvse and venlrosa. 525 



LX. — Observations on the Succession of the Gastropods 

 Paludestrina ulvre and ventrosa in Brackish Water. By 

 G. C. Kobson, B.A. 



(Published by permission of the Trustees of the British Museum.) 



In the marshes immediately west of Leigh-on-Sea, in Essex, 

 there is a system of tidal ditches in which can be seen a 

 gradual transition from an estuarine to a brackish fauna and 

 flora. Two of the most prolific members of this fauna — 

 the Gastropods Paludestrina ulvce and venlrosa — were selected 

 for ecological study in May of the present year, and observa- 

 tions were made upon them p riodicaliy until S sptember. In 

 the hitter month some of the ditches were cleared out for 

 agricultural or sanitary purposes, and, as a consequence, the 

 observations were discontinued. Although a year's observa- 

 tions are desirable in such cases, certain of the distributional 

 phenomena recorded were so constant and weli marked us to 

 justify publication. 



Due west of Leigh the first marsh forms a narrow plain 

 between low cliffs and a lateral channel of the Thames known 

 as ''Leigh Ray," The marsh appears to be rather lower 

 than the high-tide mark of the river, and is protected from 

 the latter by a high sea-wall. 



Inside and parallel to the latter is a ditch about 8-9 feet 

 wide which receives supplies of sea-water at intervals (v. infra) 

 through a drain piercing the wall. From this main ditch are 

 given off at right angles a number of secondary ditches, 

 which traverse the marsh and receive a certain amount of 

 surface-drainage. These ditches are obviously artificial, and 

 give the impression that the marsh was at one time either 

 cultivated or used for sewage-disposal. At either end the 

 main ditch bends at right angles and forms two secondary 

 ditches, of which there are six in all. Four of these are in 

 open communication with the main ditch, though they appear 

 to be drying up. The two others were cut off from the main 

 ditch during the period of observation. It is the relation of 

 their fauna and flora to those of the main ditch that is the 

 matter of special interest. 



Of the two closed ditches, one — the westernmost of all — 

 was separated by a considerable patch of dry land developing 

 (probably through local elevation) in what was once obviously 

 a continuous channel. The other closed ditch — the fourth 



Ann. & Mag. N. Hist. Ser. 9. Vol. vi. 3b' 



