F. S. Wright 131 



the individuals in such a unit will not be of the same dimensions, but, in 

 practice (and when checked by other methods) the results have proved 

 fairly accurate. 



I found that the only place which fulfilled most of the requirements 

 stated above, as well as certain others to be detailed presently, was the 

 Ro-ddu seed area, at Aberdovey. At low water of spring tides, certainly, 

 many areas bearing mussels are readily accessible, but in the majority 

 of them the individuals are very unevenly distributed. It has been said 

 before that the mussels with which we are chiefly concerned here are 

 those from 2" to 2-|" in length, as, at Aberdovey, the average growth made 

 by a two-inch mussel during the close season is about half an inch 

 (actually f|v"). 



Therefore, the conditions required in order to obtain this " population 

 factor" were, not only the greatest number of individuals in, say, one 

 square foot, but that all within such unit of measurement should be 

 between 2" and 2|" in length. 



The sea mussel is fixed in a variety of ways (Fig. 3) ; if in clusters, the 

 long (antero-posterior) axis may lie in any plane without inconveniencing 

 the shellfish, and the same may be said of it when attached to stones 

 and other solid objects. When, however, it is found living on sandy or 

 muddy bottoms, and more especially when crowded together, or if any 

 force of current is experienced, it generally lives with its anterior or 

 "head" end buried in the substratum, the posterior, or siphonal end, 

 projecting slightly above the surface. The mussel apparently favours 

 these conditions, and I have noticed that newly transplanted mussels 

 so.on gather mud and sand around themselves, as described above. In 

 sluggish streams especially, such a posture confers certain advantages 

 with regard to feeding. The siphons will thus be slightly raised above 

 the level of the floor, and, therefore, in a better position to receive the 

 detritus and other food material which washes about there, than if they 

 were more elevated. In this way, a bed of mussels will retard food 

 particles passing through it. 



This condition prevails on the Ro-ddu Scar, which is occasionally 

 accessible at very low spring tides. By actual count of the individuals 

 (all from 2" to 2|" in length) falling within one square foot, I found that 

 the population factor in this place lay somewhere between 121 (11 x 11) 

 and 144 (12 x 12). These figures represent crowded, though not ex- 

 cessively crowded, conditions^. 



^ For details of overcrowded conditions on mussel beds, etc. see article by Andrew 

 Scott and Thomas Baxter, "Mussel Transplantation at Morcambe," in the Report for 1905 

 of the Lancashire and Western Sea Fisheries Laboratory. 



