138 TEANS ACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



to two (or more) feet below the level of the surface ; their 

 steep eroded clay banks are occupied by colonies of Scrobi- 

 cularea piper ata. 



A factor that probably plays a part in causing unequal 

 distribution of fauna is the set of currents and eddies affecting 

 the floating larval stages (spat). Kellogg* gives good examples 

 of this. He states that sometimes the set (of Lamellibranch 

 spat) is quite evenly distributed, mostly it is irregular and thin 

 but very dense in a few spots. These variations are due to 

 sharply defined currents, or an eddy (or its margin) may sweep 

 together astonishingly great accumulations. Two patches 

 were studied, situated in a current and divided by a mat of 

 eel grass, they were two or three yards wide and 200 yards long. 

 In the middle of the spawning season they were as densely packed 

 as it was possible for them to be, but before the end of the 

 summer all died through overcrowding. In another spot, 

 young clams settled 1,000 to 1,500 to the square foot, and 

 from a similar area 1937 individuals about J inch in length 

 were taken ; while from another, where they had not com- 

 pletely buried 2,486 were sifted. In some areas various ages 

 occur, which although crowded were not densely packed. 

 There, therefore, appears to be a balance — the greatest possible 

 number existing and attaining but slight growth. 



In many cases the numbers of cockles present may have 

 been affected by commercial gathering, but on the Dyfi beds 

 this is of little account at present and, therefore, overcrowding 

 is more likely to occur. 



Arenicola must be of great ecological importance on the 

 cockle beds, owing to its abundance and the work it performs. 

 Ashworthf states that they are present in abundance where 

 a rich diet of decomposing matter is available, and their 

 absence or relative scarcity depends on two factors : (1) Purity 



* Op. cit., pp. 307-8. 



fAshworth, J. H., "Catalogue of Chaetopoda of British Museum— 

 Arenicolidae, 1912." 



