Praegeii — On the Raised Beaches of N.E. Ireland. 47 



I believe that if to the above Table were added those species 

 whose distribution is mainly northern or southern (instead of entirely, 

 as in the Table) the changes in the character of the fauna would be 

 rendered still more conspicuous ; but the groups of shells used above 

 will sufficiently serve the purpose. 



From an inspection of the above Table the Arctic character of the 

 Ballyrudder fauna, and the northern character of the fauna of the 

 boulder-clays, is at once apparent. Not less striking is the distinctly 

 southern character of the estuarine clay fauna, and of the raised 

 beaches, when contrasted with the columns showing the facies of the 

 existing fauna. If we add up each column, and reduce the results to 

 percentages of the total fauna of each deposit, this result is still 

 more striking — 





o 

 o 



< 



o 

 S?5 



s 



"3 



o 



Ballyrudder, 



25 



15 



3 



Glacial Clays, 



13 



5 



3 



Estuarine Clays, 







2 



6 



Eaised Beaches, 







3 



9 



Present Seas, 







4 



3 



This result may be expressed graphically, as shown below. In 

 fig. 1 (p. 48) horizontal distance represents time. We have no data for 

 arriving at even a rough comparison of the relative intervals between 

 the periods under consideration, so they are assumed to be equal. On 

 one side of a base-line the percentage of northern or southern species 

 in each fauna is marked off. "We thus get three curves, representing 

 the increase or decrease in the northern or southern character of the 

 fauna of the north-east of Ireland, from glacial times to the present 

 day. 



And furthermore, if, as in fig. 2 (p. 48), we let vertical distance on 

 one side of the base-line represent percentage of northern forms, and on 

 the other, percentage of southern forms (the one-being, so to speak, of 

 opposite sign to the other), and draw a curve, which is the mean of the 



