126 
The Irish Naturalist. 
June- 
but on the sandy area between Downhill and Magilligan 
Point, the xerophile species are well represented. Several 
species which occur in the Holocene deposits in these. sand- 
hills have not, up to the present time, been taken in the 
county in the living state. 
In those districts in the north, south or west of Ireland, 
between which and the great central plain occur barriers — 
such as mountains, where fresh-water species do not find 
congenial dwelling-places — one always finds a scarcity of 
fresh-water shells. This cannot be explained by the absence 
of suitable habitats and one is forced to the conclusion that 
(i) after the Glacial Period had passed away, the fresh-water 
mollusca in Ireland took up their headquarters in the great 
waterways and marshes of the central plain, and (2) from 
this centre they are still spreading or attempting to spread 
north, south, and west. Where they dwelt during the Ice 
Age, and by what route they subsequently reached the 
central plain, are matters which cannot even be commented 
upon in such a small paper as this. The Roe Valley and 
the marshes about its estuary furnish similar evidence in 
favour of the above conclusions to those districts of Kerry, 
Galway, Mayo^ and Donegal, which are in a hke manner 
isolated from the central plain. Ubiquitous species, such as 
Limnaea pereger, L. palustris, L. truncatula, Ancylus fluviatilis, 
Planorbis leuco stoma, P. crista, Valvata piscinalis, Aplecta 
hypnorum and various species belonging to the genus 
Pisidium, are of course to be met with here ; but of 15 species, 
Acroloxus lacustris, Limnaea stagnalis, Amphipeplea glutinosa, 
Planorbis albus, P. carinatus, P. umbilicatus, P. contortus, 
P. fontanus, Physa fontinalis, Bithynia tentaculata, Valvata 
cristata, Anodonta cygnea, Sphaerium corneum, S. lacustre 
and Pisidium anmicum, which occur in the basin of the Bann 
to the east and whose distribution in Ireland is mainly 
central or their occurrence more common in the central 
counties, only five — Planorbis contortus, Physa fontinalis, 
Valvata cristata, Bithynia tentaculata and Sphaerium corneum 
. — have succeeded in estabhshing thernselves. I would explain 
■•■See Report on L. and F.W. Mollusca, Qare Island Survey. Pro- 
ceedings R. I. Acad., vol. xxi., part 23, pp. 44, 43. 
