348 ME. r. W. HAEMER ON THE LENHAM BEDS [Aug. 1 898, 



Some years ago dredging was carried on rather extensively 

 under the direction of Committees of the British Association for 

 the Advancement of Science. In every case but one recorded by 

 them, a much larger proportion of living than of dead specimens 

 of mollusca were found. On the Turbot Bank before-mentioned, 

 however, 9 species only of the former were found, as compared with 

 175 of the latter, many of the specimens of the dead lamelli- 

 branchs being double.^ The Turbot Bank stretches from the 

 entrance of Belfast Lough towards the Copeland Islands, and lies at 

 a depth of about 25 to 30 fathoms. It rests against, and gradually 

 shallows towards the shore, extending seaward for a short distance 

 only, and shelving rapidly into deeper water. 



The coast of Antrim is separated from the Mull of Cantire by 

 a narrow channel through which the tidal currents run with great 

 velocity. Consequently no deposition takes place there, glacial 

 strata being still exposed at the bottom of the sea, uncovered by 

 more recent beds. The influence of these currents is felt to a 

 considerable depth, so that the dredging operations were sometimes 

 seriously hampered by them. It is to these currents that the 

 accumulation of dead shells farther south, on the Turbot Bank and 

 elsewhere, is due. The shells are swept up by them from the sea- 

 bottom on which the molluscs live, and redeposited, not where the 

 currents are running strongly, but in comparatively sheltered 

 places where their influence is less felt.^ 



The molluscan fauna of the Turbot Bank is of a character zoologi- 

 cally similar to that of the Coralline Crag, nearly all the existing 

 British species known from the latter being common to the two 

 deposits. The percentage of single to double and of dead to living 

 shells is, however, much larger in the Crag. 



Some beds, composed almost entirely of organic material, princi- 

 pally the shells, often fragmentary, of dead mollusca, extending 

 over a limited area only, have more recently been discovered near 

 the soathern end of the Isle of Man by the Liverpool Biology 

 Committee, and are described in their annual Eeports. To these 

 deposits, which are similar in character to the shelly sands of the 

 Coralline Crag, the term ' neritic ' has been applied by Prof. Herd- 

 man.^ From him, and from Mr. J. Lomas, E.G.S., of University 

 College, Liverpool, I learn that they are due to the strong currents 

 which sweep through the Calf Sound ; that they are not spread 

 evenly over the sea-floor, but occur in the form of banks ; and 

 that beds of large shells are often found in one place, and smaller 

 shells in another. Moreover, dead and living shells seldom occur 

 together, and it happens frequently that in spots where molluscan 

 life is most abundant no fossil record of it is accumulating. Most 

 of the mollusca found in the Irish Sea live at a depth of less 



1 Eeport Brit. Assoc. (Dublin) 1857, p. 230. 



2 Other banks, containing manj^ dead shells and due to the same cause, occur 

 in the immediate neighbourhood, as, for example, ' the Eiggs,' situated a mile 

 south of Donaghadee and a mile from land, in about 20 fathoms. 



^ In a letter to me Prof. Herdman speaks of this material as ' recent crag.' 



