294 Proceedings of Royal Society of Edinburgh. [sess. 
weed until Blackrock and Kingstown are reached, where it is 
found in fair quantity. 
On Plates 2 and 3 we give charts of these two localities (Belfast 
Lough and Dublin Bay), on which we have marked in black those 
areas over which the ulva is chiefly distributed. It must he 
recollected that much of this seaweed is unattached by any stalk, 
and drifts about from place to place. Hence no chart can he 
drawn to represent where it will be found on all occasions, and the 
Plates must therefore, in respect of the occurrence of the weed, be 
regarded merely as diagrams. 
Strangford Lough, which is quite close to Belfast Lough, re- 
sembles the latter in extent of area, and also in the large surfaces 
uncovered in its upper reaches at low water. It differs from it, 
however, in that no large town is situated on its banks. In this 
Lough Ulva latissima is practically absent. 
The above-mentioned facts seem to offer strong prima facie 
evidence that the growth of Ulva latissima is associated with 
sewage pollution of sea. water, and as a consequence that its 
occurrence in quantity in a particular locality may be regarded as 
an indication of sewage pollution. There can, at all events, be no 
doubt as to the nuisance which this seaweed can at times give 
rise to, which closely resembles that proceeding from very foul 
sewage. And there can also be no doubt as to the extraordinary 
powers of nitrogen assimilation which it possesses. 
