23 8 Proceedings of the Royal Irish Academy. 



than had heen known hitherto. In this we were not disappointed, for 

 Pisidium hibernicum, which up to the present year was unknown from any 

 part of the world but "West Cork and South Kerry, has been recognized by 

 B. B. Woodward amongst specimens of this genus sent to him for identifica- 

 tion. 



These examples were taken in Lough Gowlanagower on Inishbofin. Two 

 other species of Pisidia found in the district were unknown from Treland at the 

 time this survey was commenced. These — P. lilljcborgi and P. steenhichi — 

 appear to be of ancient origin, and in Ireland are probably most- abundant 

 along the west coast, though both are known also from the central plain. 



Next in importance to the above discoveries, I regard the finding of 

 numerous forms of Zimnaea pereger, which show an almost complete gradation 

 between normal Irish specimens and those doubtful species L. involuta 

 Harvey and L. praetenuis Bowell (see Plate II, figs. 1-39). The anatomy 

 of these West Mayo Limnaeae has not yet been examined. When this 

 has been done, I feel sure that the radula or genitalia will show the 

 same liability to vary as the shape, size, and texture of the shell. 

 Before, however, one's opinion upon this delicate subject could carry 

 any weight, it would, in my opinion, be necessary to collect and dissect 

 examples from many different habitats, each season, for a number of years. 



The finding of Limax cinereo-niger upon Clare Island makes an interesting 

 addition to the molluscan fauna of our western Irish islands ; while the 

 presence of Vertigo angustior and Succinea dblonga upon the mainland at 

 Dooaghtry may be included among the best discoveries made during the 

 survey. 



One remarkable feature of the district, worth drawing attention to here, 

 is the luxuriance of the fauna of the sea-cliffs, which on some of the islands 

 proved the chief strongholds of the land mollusca (see Plate I). 



That precipitous cliffs, overlooking the western ocean, can shelter a fauna 

 typical of our native woods upon the mainland, is a fact which must be taken 

 into consideration when we are speculating on the means of dispersal of these 

 '•specialized" animals. 



6. THE QUESTION OF DISPEESAL. 



The lists of species found on the islands off the western coast of Mayo 

 resemble closely those which may be compiled upon the opposite mainland. 

 To some this fact will tend to confirm the idea that shells are carried across 

 barriers such as the sea by chance means, as by winds, currents, birds, or 

 insects To others, such as myself, it points to a former connexion with the 

 mainland by land-bridges. 



