3i8 Geographical Distribution of Mollusca. 



Planorbis umbilicatus (Miiller). Associated with : — 



Limnsa pereger Miill. I Aplecta hypnorum L. 



Planorbis spirorbis L. | Pisidium piisilhim Gmelin. 



It is the var. rhombea Turton of this species which occurs 

 near the Lune Estuary in the drains and ditches which are 

 in all probability subject to an inrush of salt water in flood 

 seasons. Var. rhombea also occurs in the salt-marshes of the 

 Thames Estuary, in salt-marshes at Blytheburgh (Suffolk), 

 and in the marshes between Lewes and Newhaven (Sussex). 



§ 2. Sand Dunes — Xerophile. None in the district. 



DRY GRASSLANDS. 



Calcareous Pastures. Sub-Xerophile and Calco-phile. 



I. Carboniferous Limestone. (Examples at Silverdale, 



Grange, etc.). 



Pyramidula rupestris Drap. Associated species : — 



Agriolimax agrestis L. I And locally 



Pupa cylindracea da Costa. | Pupa muscorum L. 



This is one of the most characteristic species of the Lime- 

 stone region, and has an uninterrupted range. It is essentially 

 a Helix of the rock surfaces which it much resembles. Even 

 during the heat of the day it will remain exposed to the sun's 

 rays while the Pupfe retreat into the crevices of the rock or the 

 roots of the grasses. Ptipa muscorum is more abundant on the 

 lower sea-ward pastures than on the higher ground. Pyramidula 

 rupestris with Pupa cylindracea is a distinctive feature of 

 Limestone pasture at high altitudes, though P. rupestris alone 

 even extends to the summit of Skiddaw (3054 feet) on Ordovi- 

 cian Grits. 



Helicella caperata Mont 



(a) In conjunction with Thymus serpyllum is associated 



with :— 



Agviolimax agrestis L. I Helicella. itala L. 



Avion intevmedius Norm&nd (rare), j Hygromia hispida L. 



In conjunction with Tanacetum vulgare is associated 

 with : — • 



Agriolimax agrestis L. 

 Avion ater L. 

 Viivea cellavia Miill. 

 ,, nitidula Drap. 



Hygvomia vttfescens Penn. 



,, hispida L. 



Helix aspevsa jMiill. 



Naturalist, 



