526 Mr. G. C. Robson on the Succession 



from the west — was separated only by a few feet of dry land 

 rising an inch or so above the water-level at the point of 

 junction. 



There can be very little doubt that both these ditches 

 receive water from the main ditch, but only at intervals when 

 the latter has received an exceptionally large supply from the 

 river. It is impossible to be certain whether the main ditch 

 gets filled at every high tide and the secondary ditches only 

 at every spring tide, or whether the main ditch is untouched 

 by the neaps and filled by the springs, and the secondary 

 ditches only added to by exceptionally high springs. It is 

 certain, however, that the closed secondary ditches do not 

 receive as much sea-water as the main ditch. Variation in 

 the amount of water in the former, and, as a consequence, in 

 its temperature and salinity, must therefore be more marked 

 in the closed than in the main ditches, at least in the summer 

 and early autumn months. 



Tiie fauna and flora of the three ditches is indicated in the 

 following lists. They are by no means exhaustive, but 

 Indicate the chief forms or associations observed : — 



A. Main ditch (eight stations : May-September) : 



Nereis diversicolor. 

 Paludestrina idvce (swarming). 



vetitrosa (rare). 



Alderia modesta. 



Limopontia (sc.) nigra. 



Cordium edule. 



Carcinus mcenas, 



Gobiics sp. 



Ulva lactuca. 



Vaucheria sp. 



Salinity : 2-76 7o-2-95 % NaCl. 



B. Westernmost secondary ditch (closed) (four stations : 



May-September) : 



Paludestrina nlva (very rare indeed, only in August). 



-> ventrosa (swarming). 



Palcemo?ietes varians. 



Carcinus mcenas. 



Ruppia niariiima (May-July). 



Enteromorpha intestinalis (August). 



Vaucheria sp. (August). 



Salinity : 2-47 °U-2-lb % NaCI. 



