ROBSON : MOLLUSCA ON THE DOGGER BANK. 



l77 



factor in this case. On the other hand, none of the Lamellibranchia, 

 except the Spisula, were plentiful. Four other forms referred to 

 (p. 176) were frequently met, but normally only four or five living 

 examples in a haul,^ and never in the plenty that one finds elsewhere. 

 It is on just such a bottom that one would expect to find quantities 

 of Ensis, Cardivm, Modiolaria, etc. 



In default of objective evidence to account for this poverty of 

 molluscan life, I am tempted to stress the absence of any organic 

 debris that might afford food for an average molluscan population. 

 Every haul I inspected contained only the clear fine sand and shell 

 fragments, with the occasional particles of mud or gravel. Whether 

 no debris settles there at all, or, if it does, whether it is swept away 

 by a strong scour and deposited elsewhere, it is impossible to say. 

 In any case, the mollusca that do live there must be, like the Naticas, 

 carnivorous, or they must subsist entirely upon planktonic organisms 

 or bottom protozoa and diatoms. 



As previously stated, the bottom in this area is covered with 

 a layer of sheir fragments. Since there is so little hard bottom, it 

 is most improbable that these are remains of molluscs, that have 

 died in situ, abraded by tidal action. In several examples of dab 

 and haddock trawled on the spot the stomach was found full of 

 similar fragments, and Captain Davis and I are of the opinion that 

 this layer is derived from molluscs swallowed whole by these fish 

 and voided as comminuted remains in the faeces. With the exception 

 of the patches of Spisula, these fragments are always vastly in excess 

 of the living animals taken on the spot, and it remains to account for 

 these. 



The conclusion one would come to is that these fragments represent 

 the bulk of the indigenous molluscan fauna that has been demolished 

 by the vast shoals of dab, plaice, and haddock that are found on this 

 ground in certain seasons of the year. If this is true, then, with the 

 exception of the Spisula, it is plain that periodically the molluscan 

 fauna is enormously impoverished by the attacks of fish. Whether 

 this destruction is counterbalanced by abundance of molluscan 

 life, so that after the periodic raids it is replenished by a single 

 breeding season, or whether the molluscan fauna is in danger of 

 partial extinction in this area, it is impossible to say. 



The enormous quantities of Spisula subtruncata are very hard to 

 explain. It is evidently palatable at least to the dabs, as examples 

 were found in the stomachs of the latter. For the time being, one 

 can only fall back on the suggestion that it is a newcomer that has 

 found the Bank very suitable as a breeding-ground, a suggestion 

 supported by the fact that it does not figure in Hargreaves' list as 

 coming from the Dogger up to 1910. 



^ The maxima obtained were : Tellina fabula, 8 per haul ; Mactra stuUorum, 

 11 per haul. [F. M. D.] 



