Observations on PahKlestvina ulvre and veiiiro3;i. 525 



LX. — Observations on the Succession of the Gastropods 

 Paliulestrina ulvoe and ventrosa in Brackish Water. By 

 G. C. KoBSON, B.A. 



(Published b}' permission of the Trustees of the ]>ritish Museum.) 



In the marslies immediately west of L^iiili-oii-Sea, in Essex, 

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

 gradual transition from an estnarine to a braeki.sli fauna and 

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

 the Gastropods PaluJestv'ma ulvce and ventrosa — were selected 

 for ecological study in May of the pres(;nt year, and observa- 

 tions were made upon them pi-riodicaliy until September. In 

 the latter 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 distiibutional 

 phenomena recorded were so constant and well marked as to 

 justify publication. 



Due west of Leigli the first marsh forms a narrow j)lain 

 between 1 iw cliffs and a lateral channel of th-j Thames known 

 as ''Leigh E,ay." 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 tiiis 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 artiticial, and 

 give the ini|)ression that the niarsh was at one tune 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 ciiainiel. The other closed ditch — the fourtli 



Ann. d: M<iy. X. Hist. Srr. 9. Vol. vi. 3(j 



