23 2 Proceedings of the Royal Irish Academy. 



allies in North America or Siberia. These two groups together comprise what 

 I shall refer to afterwards as the " western " fauna. 



Some of the western species appear to have formerly occupied a much 

 greater area than they now inhabit, and they have probably been replaced 

 . by a newer fauna from the east in all but the most isolated districts of 

 western Europe. The migration of others would appear to have been 

 aided by a continuous coast-line from the Iberian peninsula northwards 

 during the period of their dispersal. Some of the former have been preserved 

 by the mountains and other places of retreat on the mainland of the Con- 

 tinent, but the British archipelago has served as a retreat for several species 

 which otherwise would have been exterminated ere long. 



The land and fresh-water Mollusca of Ireland may be roughly divided 

 into two groups, namely, " western " and " eastern." These terms are used 

 since they briefly express the distribution of the species in western Europe, 

 although in Ireland the eastern group is rather " central," while the western 

 element is frequently dominant in parts other than in the west. Thus the 

 greater part of the fauna of Wicklow is distinctly " western." 



In parts of Ireland separated by natural barriers from the central lime- 

 stone plain it is usual to find almost identical molluscan faunas. Thus, that 

 of north-west Donegal does not differ essentially from that of south-west 

 Kerry, the west division of Cork, or parts of Wicklow. The greater part of 

 the ground included in the Clare Island Survey is isolated from the central 

 plain, but it embraces in the Castlebar and Clew Bay districts western exten- 

 sions of the limestone area. Hence the list of the land and fresh-water 

 mollusks of the district is considerably greater than otherwise would have 

 been the case. That some of the species included in this list have inhabited 

 this country since pre-Glacial times, there is, I think, sufficient evidence. The 

 apparent lack of this evidence in the case of others belonging to the eastern 

 group tends, in my opinion, to throw considerable doubt upon the supposed 

 pre-Glacial arrival of this part of the fauna. If it be true that we have species 

 of pre-Glacial and others of post-Glacial arrival, it would be expected that 

 some of the more widely distributed species would have reached us during 

 both periods. It is therefore interesting to find that some of these present 

 two forms or races with different geographical ranges in Ireland, the one 

 corresponding with that of species of the western group, while the other 

 resembles the distribution of the eastern fauna. 



When the country has been closely re-surveyed, and we have expunged 

 from our lists all records founded upon specimens collected in gardens, towns, 

 or other similar situations, where there is the slightest possibility of their 

 having been introduced by man ; and when the marls containing fresh-water 



