Clare Island Survey — Marine Mollusca. 22 7 



western. The eastern or mainland division, as already stated, has 53 littoral 

 species to 51 in the western or Clare Island division. If we exclude from these 

 totals the Nudibranchs and Ascoglossaus, whose general range in Europe is 

 hardly sufficiently well known to permit of their species being assigned to 

 definite distributional groups, we find on analysis that the mainland or 

 eastern littoral fauna thus reduced to 46, includes 6 austral species, or more 

 than one-eighth of the total, while of the western or Clare Island littorals 

 reduced to 38, only 2 species, or one-nineteenth of the total, belong to the 

 austral group. 



It may be well to state here the precise meaning attached to the terms 

 austral and boreal in the comparisons already made, or about to be made. 

 By austral species of mollusca, we mean such species as range from the 

 British Isles south to the Mediterranean; by boreal species, such as range 

 from the English Channel northward to within the Arctic Circle. It must be 

 admitted that there is a considerable element of uncertainty in the constitution 

 of these groups, since they are based on a mass of records often lacking in 

 detail and of unequal value. They may, nevertheless, be accepted as useful 

 guides to the general character of our local molluscan faunas. 



The present contrast in the physical conditions of the eastern and western 

 divisions of the Clew Bay area would appear to be sufficient explanation of 

 the absence from the island of the following group of species, which is 

 characteristic of the molluscan fauna of the opposite or eastern shores of the 

 bay :— 



Gibbula magus Modiolus barbatus Tapes decussatus 



Lunatia catena Axinus flexuosus T. aureus 



Paludestrina stagnalis Tellina balthica Cardium exiguum 



Tornatina obtusa Scrobicularia piperata My a arenaria 



Akera bullata Mactra subtruncata M. truncata 



The presence of these species in the east and their absence from the west 

 merely show that each division exhibits in its marine fauna the defects of its 

 physical qualities, and the whole group, or a large part of it, may at one time 

 have inhabited the Clare Island shores, when the conditions there resembled 

 those which now obtain on the opposite mainland shores. And it is not 

 improbable that such conditions may have obtained on the island shores when 

 the glacial deposits which at one time appear to have choked up the bay were 

 in process of removal by erosion and denudation. 



If these absences from the Clare Island fauna are just such as we might 

 expect, it is quite otherwise iu the case of a widespread west Irish species, 

 Trochoclea lineata (Trochus). No vestige of this could be found on any part 



