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Report on the distribution of Pidmoniferous Mollnsca in the 

 British Isles. By Edward Forbes, M.W.S., For. Sec. B.S. 



The object of this Report is to ascertain the geographical and 

 geological distribution of Pulmoniferoiis Mollusca in the British 

 Islands. I shall consider the subject under three heads : 



1st. A view of the various influences which affect their distri- 

 bution. 



2nd. A detailed view of the distribution of the indigenous 

 species in the various districts of these Isles. 



8rd. The relations of this division of our native Fauna to the 

 Fauna of Europe, and the distribution generally of the more re- 

 markable species. 



One hundred and one indigenous undoubted species of Pul- 

 moniferous Mollusca inhabit the British Islands. Of these two 

 belong to the genus Arion, five to Limax, one to Testacellus, 

 one to Vitrina, two to Succinea, thirty-seven to Helix, three to 

 Bulimus, two to Achatina, one to Azeca, five to Clausilia, one 

 to Balea, twelve to Pupa, eleven to Planorbis, two to Physa, 

 eight to Lymneus, two to Ancylus, one to Carychium, one to 

 Acme, and five to Auricula. 



These one hundred and two species are not equally distri- 

 buted throughout the country, neither are they most numerous 

 in the south, and decreasing gradually towards the north, or 

 vice versa. Their numbers vary in various places, and on in- 

 quiry we shall find this variation to depend on certain influ- 

 encing causes, the two great primary influences being climate 

 and soil. The influence of climate in Britain is indicated by 

 the reduced number of species found in the more northern 

 and colder districts, as compared with the number inhabiting 

 the provinces of the south and centre. It is also indicated 

 by the disappearance of species which inhabit all soils indiffer- 

 ently as we advance northwards, and by the presence of spe- 

 cies in certain situations in southern and warm districts which 

 usually avoid, or are sparingly found in such localities elsewhere. 

 It is further shown by the tendency of individuals to multiply in 

 temperate situations, and by the superior beauty of colouring 

 displayed by species inhabiting warm districts. 



Were climate the sole influence, the number of species would 

 diminish as we advance northwards ; but, though we find such 

 a diminution very evident on comparing the extremities, the 



