330 Scientific Proceedings, Royal Dublin Society. 



ranging from 59-294 X 10-^ and L. pereger from 72-227 X lO"". When 

 taken in conjunction with the failure to find the snails in the R. Yealm, where 

 the water was at C = 32 X 10"% it seems justifiable to conclude that the 

 lower limits recorded have an approximate value as setting a genuine bound 

 to the habitats of these species. The greatest abundance of L. truncatula was, 

 however, in a highly calcareous water near Malldraeth Marsh. 



On considering the hydrogen ion concentration it is seen that L. truncatula 

 has been found in water between pH 6-4 and 78, and L. pereger from pH 6-6- 

 7-7. No differential significance can be attached to these figures, but it seems 

 that L. pereger can endure deeper and less well-aerated water than can 

 L. truncatula, which is amphibious. The records show that snails may be 

 infected even in the most acid of the waters studied. 



It is, however, noticeable that none of the records of soil are more acid 

 than pH 58. Streams draining Dartmoor may be even more acid than 

 the R. Yealm, pH <oA:, which receives tributaries that lessen its acidity from 

 the Culm Measures. Values such as pH ^A above the Culm, on granite, and 

 pH 5-0, for a bog pool, have been observed ; and among the China-clay pits 

 pH 6-2 to 6-4, with the low value 31 X lO"" for conductivity. It appears, 

 therefore, that there must be great stretches of upland pastures in which 

 the water is too acid and too poor in salts for either of these species to 

 exist. The acidity of the soil in these districts is even greater than is that 

 of the water. Wet peat bogs, where cotton grass abounds, may be as acid 

 as pH 4-1 ; heather bogs, pH 46 ; and the siliceous soil of uplands, good 

 permanent pastures with wild white clover, cocksfoot, sheep's fescue, crested 

 dogstail, and bent, may be as acid as pH 5-4. It is, therefore, not unpleasing 

 to find that such habitats appear to be too acid for L. truncatula to live. It 

 is evident, therefore, that the habitats given in Roebuck's census (1921) 

 "L. pereger: fresh and stagnant water generally" and "L. truncatula: ponds, 

 ditches, and wet places generally," require modification in the direction of 

 the exclusion of fresh water of very low conductivity, and also of high acidity 

 in water or soil. It may also be pointed out that L. truncatula, has been 

 recorded (1900) by one of us as occurring in quantity on the mud between 

 the tidal limits of the upper part of the R. Tweed, where the water was 

 slightly brackish. The nature of the habitats makes it clear that though both 

 species can harbour F. hepatica, yet L. truncatula is the moi'e likely to cause 

 infection. 



Summary. 



1. The habitats of L. pereger and L. truncatula appear to differ in the fact 

 that whereas the former is truly a water-snail, and can endure even somewhat 

 stagnant water; the latter is amphibious, and can live either in shallow, well- 

 aerated water or on moist land, or even on cliffs in a region of high humidity. 



2. The observed ranges for the two species are almost identical as regards 

 acidity and salt content of the water, those for L. pereger being pH 6-6 to 77 

 and C = 72-227 X IQ-'^ at 0° C, and for L. truncatula, pH 6-4-7-8 and 

 C — 59-294 X 10"^. It is noticeable that the records do not include upland 

 waters of very low salt content, with conductivity 20-30 X 10"*', nor regions of 

 high acidity, more acid than pH 64 for water or pH 58 for land records. 



