191 1. Stewart. — Fhicluatioiis of Sea-level. 25 



there has been httle or no change. Ah the abundant shehs 

 are species that are found in both northern and southern 

 areas, and consequently notliing can be inferred from their 

 occurrence. Of the less common forms one has been 

 considered South European, and three or four are of northern 

 type. The indications are therefore to that small extent 

 northern. However, there is very little in the point, as the 

 recent British Mollusca have on the aggregate a northern 

 aspect.^ 



As to the conditions of depth we have positive evidence, 

 that evidence proving a level different to what is now 

 established, though only negative as to the amount of 

 change. We know the limits between which certain shells 

 live, and knowing these limits, we may, when we find them 

 fossil, form an estimate to guide us to the depth of water 

 which existed at the time they lived. Thus by considering 

 the shells, and their known habits, we may come approxi- 

 mately to the result. 



I give you the names of twenty of the most abundant 

 shells of this zone of the clays — I mean the Thracia convexa 

 bed. Examine the dredging reports before-mentioned, take 

 and put down the lowest number of fathoms recorded for 

 each of these twenty species, tot up the whole and divide 

 by twenty. The result by this system will be a depth of 

 about seven fathoms as the depth of the water at the time 

 when the animals enclosed in these shells were living at 

 the bottom of our bay in thousands and tens of thousands. 

 I have selected ten univalve and ten bivalve shells, the ten 

 which I found most abundant of feach group ; by the method 

 I have pointed out you get as the minimum depth of water 

 seven fathoms, and it is remarkable that if you reduce the 

 number, and take the five commonest shells as guiding 

 species, you get by this process the same result, a minimum 

 of seven fathoms. It is possible, however, that a greater 

 depth of water may have existed at that time. It is open 

 to us also to take the maxima of recorded depths, and that 

 would give a depth of about double what we have arrived 

 at by taking the minima. I am, however, inclined to take 



1 Further investigations have revealed a distinctly southern aspect in 

 the fauna of the Thracia convexa bed. — Eds. 



A3 



