238 



Scientific Proceedings, Royal Dublin Society. 



species of snails, they are fully aware that other factors may also be important 

 or dominant. The amount of salt in the soil is tentatively put forward as one 

 of these for land species near salt marshes, as the concentration of salt obviously 

 is for water mollusca. Furthermore, the dry or wet nature of a habitat is also of 

 much importance, though hard to express quantitatively. This factor and the 

 relation between various plant associations and snails have been studied recently 

 by Kendall (1921, 1922). The plant associations are themselves regulated by 

 the soil reaction. Salisbury and Tansley also (1921) have pointed out that the 

 moUuscan fauna of certain woods on the Wenlock limestone are intermediate 

 in character between the jjlentiful fauna of a calcareous beech wood, and the 

 restricted list of species obtainable from an acid oak wood (Salisbury, 1918). 



In order to examine possible factors limiting the distribution of snails, 

 other than the hj'-drogen ion concentration, a detailed study was made of three 

 localities, similar in soil reaction and general situation. From these large 

 numbers of snails Avere collected, and since the sites were calcareous it was 

 possible to get the specimens in a small area. For each site the pll value was 

 determined, also the salt content as shown by the electrical conductivity of a 

 mixture of one part of air-dry soil with five of water. The soil was sieved, 

 and the portion passing the hundred mesh to the inch was used in the determin- 

 ations. The mixtures were shaken, at intervals for three hours on a rotating 

 wheel, by which time it was judged that all the readily soluble salts had gone 

 into solution. On standing, however, the conductivity increased and attained 

 an equilibrium value, probably due to the solution of calcium carbonate as 

 bicarbonate, occasioned by bacterial production of cai-bonic acid. The conduc- 

 tivity was measured at 0°C., and the apparatus was standardized against one 

 hundi-edth normal potassium chloride. The snail population is given 

 in percentages, the total number collected being also shown. 



The sites were as follows : — 



A. Plymouth Hoe, grassy bank, near laboratory, gentle slope, south aspect. 



B. Plymouth Hoe, grassy bank, near sea, steep slope, south aspect. 



C. Fort Stamford, near Plymouth Sound, east side, grassy and stony bank, 

 with moss and red valerian {C entrantlius ruber), at east end, south aspect." 



D. Oreston Quarry, near Plymouth, on estuary of R. Plym, limestone rubble 

 and grassy banks, aspects south and east. 



A. 

 8 per cent. 



Helicella caperata . 

 H. virgata . . . 

 Pupa uvibilicata 

 Helix aspersa. . . 

 Helix nemoralis 

 H. pulchella . . . 

 Hygromia hispida . 

 H. striolata . . . 

 Pyraniidula rotundato 

 Hyalinia nitdula . 

 H. crystallina . . 

 Coclilicopa lubrica . 

 Vitrina pellucida . 

 CocMicella barbara 



Total count . . 



Species . . . 



pH 



C X 10', 3 hours 



C X 10=, 11 days 



79 

 6 

 2 

 

 

 

 



1 



4 

















 117 



6 



7-8 

 12 

 37 



