S3 10 Proceedings of the Royal Irish Academy. 



While many naturalists have sought for means by which shells could be 

 dispersed by chance, few appear to have studied the power of fresh-water 

 species to travel over " dry land," such as from one river-basin to another. 

 It seems probable that during wet weather fresh-water snails may travel 

 considerable distances, aestivate during intervening spells of drought, and 

 proceed once more when rain again falls. 1 



The power of some of these mollusks to live out of water for considerable 

 periods, especially when young, is considerable. 2 From personal observa- 

 tion it seems to me almost certain that such species as some of the more 

 widely distributed Pisidia, Limnaea pereger, L. trimcatula, L. palustris, Ancylus 

 fluviatilis, and Planorbis spirorhis would have little difficulty in overcoming 

 most barriers, excepting sea or waterless tracts of land. It is therefore 

 not surprising to find that these species are generally to be met with in the 

 most isolated peninsulas and on the western islands of Ireland. These habitats 

 appear to differ only by the fact that the land-bridges connecting the 

 peninsulas to the mainland are still in existence, while in the case of the islands 

 the connecting links have disappeared. 



7. INFLUENCE OF MAN UPON THE FAUNA. 



In cultivated areas, and more particularly near seaports and towns, man's 

 influence upon the flora of a country is well known. Couchologists, however, 

 have, up to the present time, scarcely studied the effect of man's presence 

 upon the distribution of the mollusca. Thus a shell gathered in a garden, in 

 many cases, has been considered sufficient proof that the species to which it 

 belongs is a native of the area in which the garden is situated. 3 



1 Since writing the above, I have received the following notes from Mr. H. Lamont Oir, of 

 Belfast, which confirm the suggestion that some species of fresh-water shells may cross damp areas 

 of " dryland." "When collecting on Black Mountain [Co. Antrim], 600 feet above sea-level, I turned 

 over a stone lying among the grass and rushes, and I was surprised to see the bleached shells of 

 Pisidia. Upon further examination I found a considerable number of live Pisidia. The situation 

 was a grassy slope, with a gradient of about one in three or one in four, and at no time could have 

 held standing water. I found the living shells at the roots of grass, where the ground was damp, 

 but not splashy. They were full-grown specimens, and must have lived for more than one season." 

 Had a pond been dug in the neighbourhood, and some of these Pisidia found by Mr. Orr at the roots 

 ■ >f the grass made their way into it, and been subsequently discovered by a conchologist, we should 

 have been asked to believe that here indeed was absolute proof of the carriage of shells by birds, 

 insects, or wind. I myself have taken Pisidia in places somewhat similar to Mr. Orr's habitat. 

 Among places where these have occurred to me, I may mention the north-eastern cliffs of Clare 

 Island, the great talus beneath Fair Head in Antrim, and at an altitude of 1,200 feet on the sea-cliffs 

 of Brandon in Kerry. 



2 Young L. stagnalis have remained out of water, sealed against the side of a bell-jar, in my 

 house, from June, 1910, till about January, 1911, except upon the occasions when I pushed them 

 back into the water. 



3 See L. E. Adams, Journal of Conch., xiii, 211-214. 1911, 



