236 Scientific Proceedings, Royal Dublin Society. 



found on soil as acid as pli 48 over quartzite rock. Other Hyalinias are found 

 at pH 8. The genus therefore covers a very wide range. Apparently species 

 with hyaline shells can inhabit districts free from calcium salts, or where at least 

 very minute amounts are found. On the other hand those with markedly 

 calcareous shells, such as Hellicella caperata and H. virgata, appear to be 

 strictly confined to regions which are predominantly alkaline. 



Again, there are a number of species which are plentiful in a neutral or 

 slightly acid habitat, though they are found in alkaline regions also; these 

 include Hygromia hispida, Pupa ^lmllilicata, and Helix aspersa. When one 

 recalls the fact that snails possess powers of locomotion, and that within a 

 narrow area considerable differences in reaction may be found, it would be 

 unreasonable to expect very hard and fast rules as to distribution. For 

 example, owing to more thorough leaching out of calcium carbonate, a gully 

 is usually more acid than the land it drains, and, even on alkaline land, plant 

 remains and leaching may together render the surface soil slightly acid. 

 It is precisely on such soil that //. hispida abounds around grass roots. 



The results for various species are shown graphically in fig. 1, as are also 

 those for the numbers of species at different pF values in fig. 2. 



The authors are indebted to Mr. F. W. R. Brambell, of Trinity College, 

 Dublin, for collecting specimens on and near Bray Head, Co. Wicklow ; also to 

 Miss Worsnop for certain Plymouth specimens. Mr. J. W. Taylor, of Leeds, 

 kindly identified the species collected by Mr. Brambell. 



Not included in the list of Table 2 are the following : — Limnaea truncatula 

 at pH 7, L. stagnalis in abundance, in a pond at Kew (it is absent from Devon 

 and Cornwall), at pH 7. It must be added that the pond contained hard water, 

 rich in calcium salts. This could become much more alkaline, about j>H 9, 

 through abstraction of carbonic acid by water plants. Other ponds near were 

 more alkaline, being richer in water plants. 



Bathyomplialus contortus also was abundant in a marshy pool at Plymouth, at 

 pH 8. One specimen of Valvata piscinalis was obtained from the same situation, 

 in which it may be remarlied the large ciliate Spirostommn ambiguum was very 

 plentiful. We are indebted to Mr. W. C. De Morgan for this identification. 



It has not been possible to collect many of the aquatic species as yet, but it 

 may be said that they are either very poorly represented, or altogether absent 

 from the upper reaches of the R. Yealm and the more acid waters listed in 

 Table 1. 



As is well known, snails are more abundant in limestone regions than in 

 most others. The hydrogen ion determinations put this observation on a 

 quantitative basis ; furthermore, many situations upon calcareous sandstones and 

 shales are quite as alkaline as similar situations upon limestone. It may be 

 mentioned that boulder clay, covering limestone rock, is often acid, as it is 

 genetically unrelated to the underlying strata. Through leaching by rain 

 most limestone regions are acid in parts, so a greater variety of soil reaction 

 may be met with in them than in those where the rocks are granitic or quartzite. 

 Thus Roebuck (1921) in his census records 97 species of land and fresh water 

 mollusca from Co. Dublin, where limestone lowland and plutonic uplands are 

 found. But the neighbouring Co. Wicldow, which is also maritime, has only 

 74 recorded species, Co. Wexford, on the other side of Wicklow, having 87. The 

 absentees from Wicklow include Limnaea stagnalis, Valvata piscinalis, and 

 BatJiyomiyhalus contortus, though these are present in Wexford and Dublin. 

 The records for these three species, as shown in this paper, are from alkaline 

 water. Wicklow contains no extensive limestone areas, if indeed any limestone 

 is to be found. The uplands consist of plutonic rocks, and most of the rest is 

 made up of altered and unaltered Silurian strata, with the Bray Head Cambrian; 



