Geographical Distribution of Mollusca. 379 



All these species are usually somewhat dwarfed in size, and 

 are stained and corroded by the peat. The dominant species 

 is P. spirorbis, which is usually very abundant. This species 

 would seem to be restricted to the marshy ends of non-calcareous 

 ponds, which are a dense mass of grass and small water plants, 

 and to peat-drains and marshes. It ranges throughout the 

 country in habitats of this kind. The form prevailing through- 

 out this district is the var. leucostoma of Millet. 



§ II. — Upland. 



We have as yet no records of these. 



WOODLANDS. 



The woodlands of the district fall into two distinct types — 

 the Ash type, characteristic of the Mountain Limestone, and 

 the Oak-birch type, dominated by Quercus sessiliflora, charac- 

 teristic of the Silurians and Millstone Grit Series. 



The former type prevails between the valley bottoms and 

 the pavements on the plateaux of the scar limestone. Below, 

 the soil is damp and deep, and the shade noticeable, above, the 

 soil is shallow and liable to drought, and the shade little better 

 than that of scrub. The lower woods shew an admixture of 

 Oak, the upper scrubs have little high growth, and are con- 

 stituted of the shrubby associates of the Ash wood of the lower 

 mid-slopes. Woods of this type are the most luxuriant of 

 our British associations, not only in individuals, but also in 

 species. The physical conditions are wholly favourable to an 

 abundant life. 



Beech woods are not native to the district, but where sub- 

 stituted in the Ash woods of the deeper and damper soils, 

 show life conditions of a fairly distinct character, which justify 

 a separate consideration. 



The Oak-birch type of woodlands is that which prevails 

 over most of the Pennine areas off the Limestone. It includes 

 many wood associations though of a lower grade than is the 

 case with the Ash wood type. In the higher associations there 

 is lacking the variety of the scar woods, w^hile in the dry heathy 

 woods which stand at the bottom of a series of wood associa- 

 tions, the life conditions of soil and exposure are distinctly 

 unfavourable to plant life, except of very specialised character. 

 This poverty is reflected in the molluscan association. 



1909 Nov. I. 



