240 Scientific Proceedings, Royal Dublin Society. 



5. There remain over a number of puzzling cases in which, within an area of 

 two square miles, certain species are altogether absent from one locality, though 

 abundant in others, in spite of similarity in pH value, salt content (as shown 

 Ijy electrical conductivity measurements of soil extracts), and aspect. A 

 difference only in exposure to wind could be noted. 



6. The distribution of some species within the British Isles is probably 

 explained by the ' ' Age and Area ' ' theory of Willis rather than by a limitation 

 through unfavourable ecological factors. Cochlicella harhura appeal's to have 

 a western, and Tlieba caniiana an eastern, origin. 



REFERENCES. 



Atkins, W. R. G. — Some factors affecting the hydrogen ion concentration of 

 the soil, and its relation to plant distribution. Sci. Proc. R. Dublin 

 Soc, 1922, 16 (N.S.), No. 30, and Notes from Bot. School, T.C.D., 1922, 

 3, No. 133-177. 



Kendall, C. E. Y.— The mollusca of Oundle. J. of Conchology, 1921, 16, 

 240-244 and 1922, 16, 248-2.50. 



Roebuck, W. D. — Census of the distribution of British land and fresh water 

 Mollusca. J. of Conchology, 1921, 16, 165-212. 



Salisbury, E. J. — The oak-hornbeam woods of Hertfordshire, Pts. 3 and 4. 

 J. of Ecology, 1918, 6, 14-52. 



Salisbury, E. J. and Tansley, A. G. — The durmast oak-woods {Querceta 

 sessiliflorae) of the Silurian and Malvernian strata near Malvern. 

 J. of Ecology, 1921, 9, 19-38. 



Stelfox, a. W. — A list of the land and fresh-water moUusks of Ireland. 

 Proc. Royal Irish Acad., 1912, 29b, 65-164. 



