SEA-FISHERIES LABORATORY. 131 



flourishing (Pwllcrochan, Milford Haven), but in the latter 

 (Langum, Milford Haven) the shells were not numerous 

 and were massive and coarse. As regards physical medium 

 they occur most abundantly in estuarine sands in which a dark 

 underlying layer of decaying organic matter occurs, though the 

 largest shells have come from the purer sands of more marine 

 conditions. 



2. Current. 



A moderate and constant flow seems essential, as upon 

 this food supply, aeration and cleansing must to a great extent 

 depend. Lack of flow must lead to stagnation and adverse 

 conditions, while on the other hand too strong a current will 

 lead to destruction through removal of the substratum or 

 smothering of the cockle, etc., by deposition. 



3. Stability. 



A relative stability is therefore essential, and excessive 

 current or wave action will be fatal. A study of maps or 

 charts on which cockle beds are indicated will show that such 

 shelter is supplied either by estuarine conditions or by pro- 

 tecting sand banks. 



The conditions governing the Dyn Estuary have been 

 studied by several scientists, but the data given seldom relate 

 to the actual a^ea of the cockle beds. Geological and botanical 

 information of great interest is given by R. H. Yapp, I). Johns 

 and 0. T. Jones, in " The Salt Marshes of the Dovey Estuary," 

 Journal of Ecology, V, 1917. Vaughan Cornish, in " Waves of 

 Sand and Snow," 1914, gives the results of a series of observa- 

 tions on the shifting sand areas of the Traeth Maelgwyn during 

 May-June, 1900; while some winter salinities are given i«>r 

 the Dovey and Mawddach Estuaries by F. W. Durlacher, in 

 connection with " Sewage Drift in Relation to Mussel Beds " 

 (Trans. Biolog. Soc., Liverpool, XXVI11, 1911). It is 



