ERRATIC LICHENS 



259 



bind the mass together. Meyer says that " wanderers " have been noted as 



belonging to Parmelia acetabulum, Platysmaglaucum and Anaptychiaciliaris. 



The most notable instance in Britain of the " erratic " habit is that of 



Parmelia revoluta van concentrica (Fig. 121), first found on Melbury Hill 



Fig. 121. Parmelia revoluta var. concentrica Cromb. a, plant on flint with detached fragment; 

 b, upper surface of three specimens ; c, three specimens as found on chalk downs ; d, specimens 

 in section showing central cavity (S. H., Photo.). 



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