THE LICHENS OF SOUTH LANCASHIRE. 101 
place, or where, as is often the case, there is normally sufficient moisture to 
produce a vigorous growth of mosses and other aquatic vegetation. The 
lichens of the dunes, therefore, exist in habitats of a character intermediate 
in varying degrees between the two extremes mentioned ; and since, for the 
most part, lichens are of pronounced xerophilous tendencies, by far the greater 
proportion occurs on the sides of the dunes rather than in the * pans" or 
* slacks”? where moister conditions prevail. 
On the outer Marram-clad growing dunes, or on older dunes which 
have been partially denuded and are being remoulded by the wind, lichens 
are not present at all. On those dunes, however, where the surface is slightly 
fixed by the growth of Marram grass (Ammophila arenaria, Link), and a few 
other species of phanerogams and some bryophytes, lichens soon put in an 
appearance. It is on this type of dune, where the ground is bare and open 
and the degree of surface fixation but slight, that Collemodium turgidum, Nyl., 
Leptogium scotinum, Fr., and Collema ceranoides, Nyl., occur. These species 
spring into prominence during the cool, damp weather of autumn and winter, 
when the sand is moist, and the moisture helps to keep the surface firm. In 
places where the sand-grains have become somewhat cemented together by 
the growth and decay of Nostoc and small mosses, such as Ceratodon pur- 
pureus, Barbula spp., and Bryum spp., several inconspicuous rare species of 
Lecideæ have been found incrusting the decaying bryophytes, e. g. Bacidia 
arceutina var. brevispora, Lecidea pleiospora, А. L. Sm., and (rarely) Bia- 
torella campestris, Th. Fr. 
With the accumulation of humus due to the decay of Marram grass and 
other vegetation, some of the Cladoniw soon appear, particularly С. pyxidata, 
Fr., C. chlorophwa, Floerke, and C. fimbriata, Fr., and these rapidly spread and 
form a crust over decaying moss and other humus. These Cladoniæ play no 
small part in fixing the surface and forming a soil-layer. On these open dry 
dunes several species of Peltigera also appear at an early stage. These com- 
prise Peltigera horizontalis, Hoffm. forma, P. canina, Hoffm., P. rufescens (very 
sparingly), P. spuria, Leight. (occasionally), and P. polydactyla, Hoffm. ; 
and the first two often form fairly large circular patches, with the thallus 
appressed to the ground. Another lichen which is prominent in the same 
association is Parmelia physodes, Ach., which starts on small partially-buried 
dead twigs of Salix repens, and often spreads in grey patches, some inches in 
diameter, on to the surrounding soil. Of common occurrence, also, is Lecidea 
uliginosa, Ach., the thallus of which forms a brownish film on dead Marram- 
grass leaves, and other vegetable débris, particularly in the shade of tufts 
of grass, or on the shady side of dunes. In the winter months Bilimbia 
sphwroides, Koerb., recognizable by its pale yellowish or dirty-white apothecia, 
is frequently met with on the barer places on this type of dune. 
As vegetation increases, and a closed plant-association is formed, lichens 
tend to disappear ; but on parts of the inland dry dunes where the conditions 
