22 8 Proceedings of the Royal Irish Academy. 



of the carefully explored eastern or southern shores of the island, though it is 

 frequent and well developed on the outer islands of the eastern coast of the 

 bay some 8 miles distant. 



Having touched on these negative characteristics of the Clare Island 

 marine mollusca, attention may be drawn to what appears to be a positive 

 distinctive feature in the fauna of its shores as compared with those of the 

 mainland — the presence of two interesting Ascoglossans or close allies of the 

 Nudibranchs, Hermaea dendritica and Elysia viridis. Both of these occur in 

 profusion between tide-marks on the island ; and their prominence in its marine 

 fauna, if one may be permitted to associate the idea of prominence with such 

 elusive, protectively coloured species, is perhaps conditioned by the abundance 

 on the island shores of their food-plant, the green seaweed, Codiwin tomentosum. 

 The absence of this weed from the greater part of the uncongenial eastern 

 shores of the bay explains the absence therefrom of the two Ascoglossans. 

 Yet it is not improbable that further research may prove that these species 

 occur on the imperfectly explored rocky coast from Koonah to Old Head, 

 where Codium is abundant. 



Amongst the species found in the middle or deeper water division of the 

 area, but not in either of the other divisions, the following are probably excluded 

 by insufficient depth of water : — 



Calliostoma Montagui Cyliehnina nitidula Astarte sulcata 



Scalaria Trevelyana Volvulella acuminata Cyprina islandica 



Rissoa cancellata Dentalium entalis Diplodonta rotundafca 



Odostomia conoidea Lima Loscombi Saxicavella plicata 



In this small group, though we find that mingling of northern and 

 southern forms which occurs all round the Irish shores, the southern element 

 in it is still perceptibly superior, as it is throughout the Irish shallow-water 

 areas in general. 



(b) Comparison with the East Coast Mollusca. 



"When the molluscan fauna of Clew Bay, as a whole, and so far as we know 

 it at present, is compared with that of the shores and shallow waters of 

 Co. Dublin on the east coast, we do not find the austral element directly pre- 

 ponderant in the western area, as we have found it in comparing one part of 

 that area with another, and as we should find it in comparing the Irish west 

 coast as a whole with the east coast. But we find the boreal element much 

 stronger in the east than in the west ; and it is in the relation of the boreal to 

 the austral element in each of the faunas that we shall find the best standard 

 of comparison. The western or Clew Bay area has 5 boreal species to 31 

 austral; the eastern or Dublin area has 12 boreals to 37 australs; so that 



