I92I. HiNCH — Post-Glacial Climatic Optimum in Ireland. 95 



In certain localities the most recent deposit is a blackish 

 clay, with sandy layers characterized by the presence of 

 My a arenaria in beds of thousands. 



As this paper is only a tentative effort to elucidate the 

 Climatic Optimum in Ireland, it is not proposed to deal with 

 every available instance in which the present distribution 

 of the mollusca of the Estuarine Clays does not coincide 

 with that of late post-Glacial time. Much still remains 

 to be done in the standardisation of present-day distribution, 

 and it is felt that a general statement is all that is desirable 

 now. The method which has been adopted was to exclude 

 all species which have been classed as rare or rather rare in 

 either the Scrobicularia or Thracia Clays, and only to deal 

 with those which can safely be classed as frequent, abundant 

 or in profusion in these late post-Glacial deposits. In this 

 way we get rid of species based on the occurrence of one or 

 a few specimens, and the mention of which might give an 

 erroneous impression regarding the broad general movement 

 of animal life since that time. Where there is any doubt 

 as to the deposit from which a species has come that par- 

 ticular species has also been excluded. As a result we find 

 that in the Scrobicularia Clay there were present in abund- 

 ance or in profusion the following five species, Tapes decus- 

 satus, Scrobicularia piperata, Solen vagina, Rissoa alhella, 

 Jeffreysia opalina, and in the Thracia Cla}' the following 

 eight species, Lima hians, Montacuta hidentata, Axinus 

 flexuosus, Lucinopsis undata, Solen vagina, Thracia convexa, 

 Odostomia minuta, Nassa pygmcea. 



Now a comparison of the present-day habitat of these 

 twelve species, which occur in the Estuarine Clays in abund- 

 ance or in profusion, shows that a great general change of 

 distribution has taken place since late post-Glacial time. A 

 number of these species are now practically extinct in the 

 older headquarters in the north-east of Ireland, being 

 represented there at present-day only by occasional living 

 specimens, by single valves, or as the result of dredging in 

 considerable depths. The inclusion in present-day distri- 

 bution of specimens obtained from the Turbot Bank, which 

 is probably a fossil shell deposit, has tended to obscure 



