F. S. Wright . 137 



is that, as one generation dies and becomes buried (or, perhaps, becomes 

 buried and dies), a new generation grows up, which anchors itself to the 

 shells of its predecessors. So the bank grows, each successive generation 

 accumulating more silt, and, in this manner, covering those beneath^. 

 It is not improbable, however, that in natural conditions, it may survive 

 for a somewhat longer period than that mentioned above. Whatever 

 future research may prove to be the true facts, I am convinced that the 

 above factors account, in a large degree, for the poor flesh content of 

 those shellfish living on certain beds which are liable to silting. Covering 

 by sand causes feeding to be interrupted, often for fairly extended periods, 

 whereas, in better situations, it proceeds ceaselessly. There must be 

 a considerable amount of sand held in suspension in the water near the 

 bed of the river, and it is suggested that, often for some hours during 

 each tide, this factor is serious enough to prevent the mussels feeding. 

 They are compelled to remain closed in order to prevent the entrance 

 of sand into the branchial cavity. The views expressed above receive 

 a certain degree of confirmation (although perhaps this is not very 

 definite) from the evidence afforded by the mussel's orientation with 

 regard to the current, when this is a swift one. My curiosity and 

 scepticism were aroused by the following paragraph, as my own experience 

 had led to results opposed to the views stated by its author. In an 

 jnaugural address to the Malacological Society^, B. B. Smith Woodward 

 says : " Mytilus, which also comes of a family having a long pedigree, 

 has not a particularly stout test capable of resisting heavy blows, but 

 it meets the waves with its outwardly directed, sharp, wedge-shaped 

 shell and cleaves them instead ;...." On the Ro-ddu be(i, however, 

 where the mussels are buried head foremost in the sand in order to 

 secure a holdfast, I found that, in the majority of cases, the valves, or 

 sides, opposed the currents. Thus, one valve faces upstream, and the 

 other downstream. In this manner, they are probably able to feed for 

 rather longer periods than would be the case if they were placed as stated 

 in the quotation, because of the sand danger. Mussels may, and do, 

 -meet the current with their sharp siphonal extremity, but this is probably 

 because they have little choice in the matter, or because the velocity of 

 the current is not great. The quotation is, without doubt, very mis- 

 leading. 



As Mytilus does not appear to thrive in purely marine conditions the 



' See also article by R. L. Ascroft, op. cit. 



^ "What Evolutionary Processes do the Mollui?ca Show?" Proceedings of the 3Iola- 

 colo-jical Societ'h Vol. vn, Part .5, June 1907. 



