528 Observations on Paladestrina ulvce and ventrosa. 



out of the vessel coutaiuiug it, even if the salinity of the 

 water was noimal. 



It is clear from this account that there must be some factor 

 limiting the distribution of P. iilvce as compared to that of 

 P. ventrosa. It is plain that it has the opportunity of gettinj^- 

 into the last closed ditch and the upper waters of the fourth 

 ditch, but is never found in the latter and only very rarely in 

 the former. Salinity cannot be the limiting factor, as we 

 have setn that it tolerates low salinities and it also occurs in 

 other places — e.g., the Exe estuary (i) — where it must expe- 

 rience considerable daily alteration in salinity. Temperature 

 need scarcely be considered, as all the ditches are adjacent 

 and very shallow. The main ditch is usually rather deeper 

 than the others; but P. ulvce has been found elsewhere 

 swarming- in water as shallow as that of the closed ditches. 



On the whole, it seems more likely that the presence or 

 absence of a food-plant is the limiting factor. P. iilvce is not 

 limited to Ulva lactuca, as it occurs at Leigh on Schlerocki'oa 

 mariliina, on Ulva, Edteromorphoj and Zostera plentifully in 

 the Exe estuary (i), and on Ulva and Zostera in the Paghara 

 lagoon (Robson MS.). But it is plain that in this area 

 nothing liad tempted it out of the Ulva-wsiiev to colonize in 

 water from which Ulva was absent even in August when 

 Enteromorpha had replaced liuppia. 



One must therefore conclude that Ulva is the limiting factor 

 in the present area and that the mollusc has not yet adapted 

 itself to the other available plants, though elsewhere it is not 

 limited to Ulva lactuca. 



It would be interesting to know if the chief plants of this 

 area show the succession usually found, and so to show that 

 the succession of the two molluscs depended ultimately upon 

 the physical factors determining the distribution of the plants. 

 Untortunately the minimum salinity which Ulva lactuca 

 tolerates is apparently unknown, nor are there apparently any 

 British records tor the exact distribution of Pw/^/'/a maritima. 

 It has been shown that the latter occurs with Zoslera and 

 apparently Ulva lactuca in tidal waters in America (3). 



It should also be pointed out that in the main ditch there 

 were signs of pollution (either natu;al or from sewage), which 

 might easily disturb the ordinary plant succession and account 

 for the fact that in two ditches only differing in a slight 

 degree of salinity there is such a marked difference in the 

 flora. It is well known that Ulca lactuca tends to thrive in 

 polluted water (2). 



The author is indebted (0 Mr. F. J. Lambert of Leigh for 



