GENERAL CHARACTERS OF BRITISH LICHENS. 33 



it is typically horizontal or vertical, according to the inti- 

 macy of its adhesion to its base of support, and the di- 

 rection and mode of development of its constituent cells. 

 The subdivisions or modifications of the horizontal, or typi- 

 cally adherent, thallus, are the crustaceous and the folia- 

 ceous, or frondose. The crustaceous thallus forms a mere 

 crust or coating, of varying thickness, on its base of sup- 

 port. When comparatively thick, dense, and hard, it is de- 

 nominated tartareous, as in many Lecanoras ; this form is 

 frequently whitish or pale- coloured, chalk-like, friable, and 

 rough or warted on its surface. As constituent elements, 

 it often contains a considerable amount of mineral matter, 

 such as the carbonate and oxalate of lime, besides colo- 

 rific principles capable of yielding valuable purple pigments ; 

 hence many Lichens having this form of thallus are used in 

 the arts. Species with a tartareous thallus most frequently 

 grow on rocks, to which they adhere by their whole under 

 surface so intimately as to be inseparable therefrom ; there 

 appears to be a corrosion or disintegration, and subsequent 

 firm union by and between the Lichen and the surface of 

 the rock, which is sometimes the hardest and purest quartz. 

 Prom the nature of its habitat, the tartareous Lichen is 

 one of the most common forms of its family, growing on 



