60 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



distribution of the various organisms responsible for the 

 discolouration. The patches were examined daily from 

 July 28th to August 17th, and from September 6th to 

 September 28th, and were found to vary greatly in extent. 

 This variation showed a periodicity corresponding with the 

 tides, but in the opposite direction from that recorded in 

 1911 and 1912. The various organisms were present in 

 enormous numbers at spring tides, so that the greater part of 

 the south end of the beach was discoloured, while, at neap 

 tides, the patches were small, isolated, and much less clearly 

 marked. Another point on which this year's observations do 

 not agree with those made in 1911 and 1912 is the alternation 

 of different organisms on the same patch of sand. Throughout 

 the time when the beach was examined this summer, although 

 neighbouring patches were produced by different organisms, 

 each patch was always characterised by one predominant 

 organism, so that it was possible to predict the cause of 

 discolouration in any sample of sand, provided its position 

 on the beach were known. Again, while most of the patches 

 disappeared about half an hour before that area of sand was 

 reached by the rising tide, a few of them were always visible 

 on the surface, even when covered by water. According to 

 the previous reports, the Amphidinia showed positive helio- 

 tropism. Experiments made this year all seemed to show that 

 the movements were negatively heliotropic, but it was evident 

 that they were affected more by variations in the amount of 

 water present than by different intensities of light. 



Several species of Diatoms were found forming patches 

 which could generally be distinguished by their rather bright 

 brown colour and by the fact that, for the most part, they 

 remained on the surface at high tide. 



The Dinonagellata were represented by at least three 

 species of Amphidinium, one or more species of Gymnodmium, 

 Polykrikos, and a few unidentified, colourless, naked Dino- 



