Clare Island Survey — Land and Fresh-water Mollmea. 23 9 



Thus, in the case of Clare Island, the molluscan fauna of parts of the 

 mainland such as Old Head, Dooaghtry and Curraun, is so similar to that of 

 the island, that I believe a land-connexion must have existed between these 

 localities during the period when this part of the fauna was dispersed. The 

 geologists engaged upon the Survey are willing to allow that a post- Glacial 

 Boulder-clay land-bridge existed between the island and the mainland. 

 While this probably accounts for the presence on Clare Island of Hycdinia 

 cellaria, Zonitoides nitidus, Hclicella intersecta, H. itala, Hygromia hispida, and 

 Helix aspersa, as well as most of the fresh-water species, the arrival of the 

 remainder seems more than likely to have been pre-G-lacial. If there 

 existed any place of retreat during the Ice Age on the west coast of Ireland, 1 

 the great sea-cliff of Croaghmore would strike one as being a most probable 

 refuge ; facing, as it does, away from the mainland and towards the temperate 

 waters of the Atlantic, the ice descending from the mainland could not surely 

 have affected the conditions prevailing upon the cliff-face to any great 

 extent. 2 



The hypothesis that shells are carried by birds or insects (see " The 

 Dispersal of Shells," by H. Wallis Kew) is founded mainly upon evidence 

 afforded by the fact that water-beetles, birds, &c, have been found to 

 transport fresh-water snails. There has been little evidence to support such 

 a theory of dispersal in the case of the terrestrial species; yet it is a fact that 

 while Clare Island and the other islands included in this survey contain 

 almost all the land-species which, from a study of habitat, would be expected 

 to occur on them, they are singularly deficient in species of the fresh-water 

 group. This does not appear to me to be altogether due to the want of suitable 

 habitats, for the lakes on some of the islands, such as those in Achill, are very 

 similar to those of the Louisburgh district. The latter, however, contain 

 twenty-four fresh-water species, while in the former only fourteen have been 

 found, eight of those from Achill, and six from Louisburgh, being of the genus 

 Pisidium, some at least of the members of which are capable of spreading 

 across almost waterless tracts of land (p. 10). Sphaerium cormum, which 

 must be considered one of the most likely species to be carried b}' animals 

 (see H. Wallis Kew, op. cit.), is unknown on any of the western islands. 



1 It is often urged by those who do not believe in the pre-Glacial origin of our fauna and flora 

 that the proximity of glaciers or snow-fields signifies an "Arctic climate," in which only species now- 

 regarded as of Arctic origin could live. Many mollusks, however, which are abundant in the west 

 of Ireland, are also common in close proximity to glaciers in western Norway. These are protected 

 during the long, cold winters in the latter country by a thick mantle of snow, a covering which 

 nature seldom provides for our western Irish animals. Thus our winter is perhaps more likely to be 

 injurious to some species than that of semi-glaciated districts such as parts of Norway. 



2 These remarks cannot apply to some of the smaller islands, where no places of retreat are visible. 

 Thus it appears that their molluscan faunas must be of post-Glacial origin. 



E.I. A. PROC. VOL. XXXI. B 23 



