GENERAL CHARACTERS OF BRITISH LICHENS. 35 



stumps of dead trees, or in any glen, in damp, shady fissures 

 of rocks. It must however be distinguished from a pulve- 

 rulent condition of the surface of the thallus, which occurs 

 in many foliaceous species. The crustaceous thallus may 

 be effuse, that is, of no determinate shape, and with no 

 visible limits or border, which is gradually lost on the sur- 

 face of the base of support ; or uniform, that is, of definite 

 shape and with a distinct margin. It is denominated effi- 

 gurate when its circumference assumes a radiate or irregu- 

 larly stellate appearance ; and squamulose or imbricate when 

 its peripheral segments, or lacinm, assume a foliaceous cha- 

 racter, and are arranged in a scale-like or house-tile-like 

 manner. 



The foliaceous or frondose thallus is a flattened, leaf-like 

 expansion, whose texture or consistence, according to its 

 thickness and the arrangement of its cellular tissue, is 

 membranaceous, coriaceous, or cartilaginous, and whose 

 margins are divided into segments, which, according to 

 their size, are denominated lobes or lacinice, the former 

 being typically broadish and rounded, the latter narrow and 

 linear. Foliaceous Lichens include the largest and most 

 brilliantly coloured species, such as those of the genera 

 Parmelia, Peltigera, and Sticta. Their geographical dis- 



