THE LICHENS OF SOUTH LANCASHIRE. 93 
surfaces, such as tree-trunks, some species can often be found on horizontal 
surfaces, such as the upper edges of palings, on the top ends of posts, on 
prostrate trunks, or on dead twigs lying on the ground in open situations. 
The species occurring on bark which seem best to tolerate smoke are, princi- 
pally, Lecanora varia, Ach., Parmelia saxatilis, Ach., and P. physodes, 
Ach.; next in order come Parmelia sulcata, Tayl., P. fuliginosa var. late- 
virens, Nyl., and Pertusaria amara, Nyl., but usually poorly developed. 
Owing to the scanty records left by earlier workers, it is impossible to 
estimate the number of corticole lichens which have become extinct in our 
district during the last century ; but it may well be concluded that many of 
the species which are ubiquitous throughout the British Isles in all districts 
unaftected by smoke, must formerly have been present in South Lancashire. 
It is improbable that the larger foliaceous lichens, such as members of the 
Stictaceæ, were ever well represented here, especially in the lowland districts, 
as they require a wetter climate than ours. We have one old record for 
Lobaria pulmonaria, Hoffm. in the Todmorden district, and the same species 
formerly occurred in the Hebden Valley, in the contiguous part of West 
Yorkshire. This, and one or two other old records, given in our Systematic 
List, give a clue to the character of the former corticole Lichen-flora of 
South Lancashire. 
Кирейт Species—The rupestral species have been the next class of 
lichens to suffer from the atmospheric pollution ; and the effects are particu- 
larly noticeable in the hilly districts in the eastern part of South Lancashire. 
The sandstone and gritstone rocks and walls of our upland districts must, 
formerly, have sustained a far richer and more luxuriant lichen-flora than is 
now the case. A comparison between the lichens of the South Lancashire 
hills and those of the hills further north in West Lancashire, shows so great 
a difference as to preclude any possibility of mere geographical situation 
being responsible. Not only the number of species, but also the number of 
individuals, is enormously reduced, and the specimens are usually more 
poorly developed. The reason is, of course, the smoke from the manufacturing 
towns which cluster thickly in ** East Lancashire." Usually, these towns 
are situated in the valley-bottoms, and the smoke rises up and is carried on 
to the surrounding hills and moorlands. 
The effect of this air-pollution, continued through two or three generations, 
has been to cause the disappearance of rupestral lichens from these districts 
to such an extent that only a few of the common species now survive, more 
particularly in sheltered cloughs and on horizontal rocks by the side of 
streams, — Rupestral mosses are similarly affected by the unfavourable con- 
ditions, e. y. several common species of Grimmia and Rhacomitrium, which 
were formerly common in the hillier parts of South Lancashire, are now 
almost extinct. Moreover, it is not only the lichens of rupestral habit which 
are affected, for in some localities the degree of atmospheric impurity is such 
