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states in his British Conchology, his opinion, "that minera- 

 logical conditions have very little to do with the habitat of any 

 of the mollusca, nor with their comparative abundance or 

 scarcity in any locality." It seems reasonable, however, to 

 suppose that the connection of certain species with certain 

 formations, even though they may not be restricted to them, 

 is not entirely attributable to accident. As an example of 

 this, we are apt to associate Cyclostoma elegans, Miill., with 

 limestone and chalky districts ; but it occurs fairly commonly 

 in Jersey, where there are no calcareous strata at all. It is 

 generally the case that when species commonly found on 

 chalk occur in other districts the texture of the shells is 

 thinner ; and we assume, from the form found on the chalk 

 being far more abundant, that it is the typical one, and 

 that the thin shell is adapting itself perforce to unnatural 

 conditions. Snails can exist on any soil that contains some 

 amount of calcareous matter, and I think very few soils are 

 entirely devoid of this. 



Beginning in the North West : — The first district is what we 

 will call the Upper Thames division, which may be taken 

 roughly as including the whole of Berkshire. The greater 

 portion of this district is on the chalk formation. It is a very 

 little worked district, and I have but few records. Amongst 

 these, however, are Paludina cojitecta, Millet., which occurs in 

 a few places; and Linincea glutinosa, Miill., one of the rarest 

 of the Limncsas, is found at Reading, and nowhere else in the 

 district till we come to East Kent. When living, the shell is 

 completely covered by the mantle, which renders it readily 

 distinguishable from all other Limncsas, where the mantle only 

 reaches to or slightly beyond the outer lip. 



The Lower Thames Region. — This district includes the 

 south side of the river from Windsor to the borders of Kent, 

 and such part of Surrey as lies north of the chalk downs. 

 This part of the river is essentially the home of those species 

 which like mud and a sluggish water, as Paludina vivipara, L., 

 Liimicea auricidaria, L., Unios, and Anodontas, though these 

 never attain any size like they do in quiet pools. D. poly- 

 morpha, Pall., is common in the upper parts of the river, 

 where the water is clearer ; also in adjacent streams and 

 ponds. Neritina Jiuviatilis, L., adheres to stones through- 



