44 Proceedings of the Royal Irish Academy. 



loTver or Scrobicularia zone of the estuarine clay, a littoral deposit 

 which, imderlies a deposit of deeper water, the upper estuarine clay, 

 and, in other places, the raised beach or sea-bed. 



Ve are now in a position to compare the faunas of the successive 

 deposits of the series. This will yield the best and most instructive 

 results if. eliminating all those species which range both north and 

 south of the British area, we select the species which are of distinctly 

 northern or southern type — those which either have now their habitat 

 altogether outside British waters, or have the boundary (northern or 

 southern) of their area of distribution within this region. This should, 

 if the material at our disposal be sufficient for such an analysis, give 

 us a key to the northward or southward fluctuations of the fauna 

 during the periods of deposition of the beds under consideration, I 

 may add that nowhere else in Ireland could such a comparison be 

 instituted, nor do I kaow of any area of the same size in England or 

 Scotland where the glacial, post-glacial, and recent molluscan faunas 

 are all so completely represented and so available for comparison. 

 The Ballyrudder gravels and Belfast "Waterworks boulder- clay are the 

 most fossiliferous glacial deposits in Ireland. The estuarine clays, 

 with a total fauna of 340 species, present, so far as I am aware, the 

 richest post-glacial fauna in the British Isles ; and the raised beaches, 

 with a fauna of about 130 species, also abundantly represent the life 

 of the period. Lastly, the extensive researches of Thompson,^ Hynd- 

 man,- and Dickie^ furnish us with full information regarding the 

 existing molluscan fauna of the north-east. 



For the purposes of this comparison it is necessary to assume that 

 the shells found in the various beds lived in the neighbouring seas at 

 the time of the deposition of the beds. There is, of course always the 

 chance of derived fossils, but this chance is small, to judge from the 

 very small percentage of derived forms in recent dredgings or on our 

 existing beaches. As regards the present fauna, however, this risk 

 has been obviated by admitting only such species as have been taken 

 alive in the district. 



The Ballyrudder gravels yield a percentage of exotic forms so 

 much higher than that of the local glacial clays, that it has been 

 thought advisable to give this deposit a separate column. The term 



1 " Xatural History of Ireland," iv., 1856. 



* "Eeports of tlie Belfast Dredging Committee," Brit. Assoc. Reports, 1857, 

 1858, 1859. 



' " Eeport on the Marine Zoology of Strangford Lough," Brit. Assoc. Eeport^ 

 1857. 



