26 The hish Naturalist. February, 



the lower estimate, and set down the depth of water during 

 the Thracia cojtvexa period as some forty feet or thereabouts 

 at the site of the present floating dock. There are two 

 reasons for this preference ; one reason that influences me 

 is that in my own experience I have met many of these 

 species living in water one or two fathoms shallower than 

 has been recorded, and secondly I have found that these 

 shells occur more abundantly about the upper limit assigned 

 by the Dredging Committee, than they do near the lower 

 limit. 



(4). When subsidence had gone on to this extent, another 

 change occurred — a movement of elevation which finally 

 brought up to the surface a sea-bottom which had once 

 been covered with at least six to seven fathoms of water. 

 The shells of the deeper water become more and more 

 scarce, Thracia convexa disappears, and at the top of the 

 bed we meet only littoral shells such as Mya arenaria, 

 Cardium cdule, Cerithium reticnlaUim, Littorinae and mussels. 



If I have been able to prove that the previous depression 

 occurred as stated, and that this clay was deposited as 

 mud at a considerable depth, it will not be necessary to 

 dwell on the proofs of elevation, as it follows from the 

 present position of the bed. The upward movement was 

 slow and long-continued, and possibly interrupted by breaks 

 or pauses of longer or shorter duration. A remarkable bed 

 of sea-urchins appears to indicate such a pause. On this 

 bed, which is situate some six or seven feet below the 

 surface, there is spread out like a sheet a continuous covering 

 of the tests of Echinus miliaris, a small echinoderm that is 

 common in our bay. The shells are placed side by side, 

 and in most cases so close as to be either touching, or 

 nearly so. I have not dredged over any ground on which 

 any sea-urchin was so plentiful as this must have been on 

 the estuarine clay. Nor do I know of the species living 

 here in such great profusion at present, and I consider that 

 a pause is indicated, giving time for the aggregation of so 

 many on one surface. The shells were not piled up there 

 by currents, but appear to have lived and died as found, 

 side by side. 



I have now to connect or correlate these phenomena 



