GENERAL CHARACTERS OF BRITISH LICHENS. 43 



of the thallus, in the same genus, soredia occur in their usual 

 form, frequently of a bright yellow colour. These cyphellae 

 are originally globular or wart-like, and become urceolate, 

 or cup-shaped, only on expanding. With age the soredia 

 sometimes fall out, leaving the cavity empty. In other 

 species gonidia are developed on the surface of the thallus 

 in the form of granules or very minute wartlets, producing 

 the condition termed furfur aceous ; or in the form of 

 folioles or squamules, constituting that called squamulose. 

 The former condition occurs in Thyscia furfuracea, and 

 frequently in Parmelia saxatilis ; the latter in many Cla- 

 donice. On the surface of the thallus of some crustaceous 

 species, — as Lecanora joallescens, vdfi.parella, and L. rimosa, 

 — they are developed in the form of minute, solid, cylin- 

 drical, or cone-like bodies, arranged perpendicularly to its 

 surface, and so closely aggregated as to form a compact 

 tissue, apparently composed of a multitude of small columns. 

 Such a condition is termed isidioid, and is the basis of the 

 old genus Isidium, which is thus found to be an abnormal 

 state of the thallus of certain crustaceous species. The 

 isidioid thallus resembles the tartareous in being usually 

 pale or whitish, cretaceous and friable, and in possessing a 

 considerable amount of colorific and mineral matter; the 



