INTRODUCTION. XXVil 
Helicella barbara is partial to the roots of the Marram 
Grass, coming forth in surprising numbers in rainy weather. 
Jaminia muscorum frequents the roots of the grasses Festuca 
rubra, L., var. arenaria, Fries; Festuca membranacea, Druce ; 
Agropyron junceum, Beauv.; and H. aspersa shews a partiality 
for Euphorbia paralias, Linné, and the less widely distributed 
Iris fetidissima, Linné. 
At the base of the dune slopes the following association 
occurs : 
Jaminia muscorum, Linné (dominant). 
Agriolimax agrestis, Linné. 
Vitrina pellucida, Miller (amongst moss). 
Helicella caperata, Montagu. 
Hygromia rufescens, Pennant. 
Helix aspersa, Miller. 
The chief plants with which these are associated are Curcr 
arenaria, Linné; Festuca rubra, var. arenaria, Fries; Ononis 
repens, Linné, var. horrida, Lange; Lotus corniculatus, Linné ; 
and Thymus serphyllum, Linné; with the last named Helix 
caperata is chiefly associated. 
B. THe LEVELS. 
The flat plain of the Levels consists of tidal, lacustrine, and 
river deposits, protected from inundation at abnormally high 
tides by the dunes, the sea-walls, and the sluices at the mouths 
of the tidal rivers. JInundations occasionally occur. The 
whole district is under pastoral cultivation, and the fields are 
separated by shallow ditches, or rhines. 
(1). AQUATIC FORMATION. 
Amongst the vegetation of the marshy land, on the margins 
of the rhines, we find the following molluscan association : 
Agriolimax agrestis, Linné. 
Vitrea crystallina, Miller. 
