Fink: Some Common Types oF LicHEN ForMATIONS 417 
‘ These formations are commonly best developed on hillsides 
where the plants are washed by lime-impregnated water which 
flows down the slope during rains. As to structural adaptations 
to an exposed and usually dry environment, the plants all have 
small thalli and are closely adnate. The thallus of Biatora mus- 
corum (Sw.) Tuck. is very small and devoid of cortex. That of 
the Urceolaria is somewhat larger and has a pseudo-cortex above. 
The other larger thalli furnish good protection to the algal cells 
within, that of Béatora decipiens (Ehrh.) Fr. having a very heavy 
cellular cortex above, that of Exdocarpon being provided on all 
sides with a well-developed cortex while that of the Hefpia is 
cellular throughout. These cellular areas of the larger thalli 
serve of course not only for protection against excessive evapora- 
tion, but also for support. It must be said that none of the thalli 
of this formation are really large, those of the three last considered 
averaging from 3-6 mm. in diameter. Indeed a large lichen 
thallus found in such a formation would needs be considered as 
an accident in distribution. 
Closely related to the formations of calcareous earth and usu- 
ally occurring with or near them, are those of the calcareous 
pebbles or horizontally-disposed calcareous rocks. Taking the 
name partly from a lichen which I have thus far never failed to 
find whenever the formation is well developed and giving a list of 
species commonly found in such formations, we have the fol- 
lowing : 
THE LECANORA CALCAREA CONTORTA FORMATIONS OF EXPOSED 
HorizontaL LimMEsTONE SURFACES (OR OF Limy PEBBLES) 
Placodium vitellinum aurellum Ach. 
Lecanora calcarea contorta Fr. 
Lecanora privigna (Ach). Nyl. 
Endocarpon pusillum Hedw. 
Verrucaria muralis Ach. 
These formations are less constant as to floral elements than 
those of calcareous earth and when well developed usually contain 
several variable elements not given in the above list. The lichens 
composing such formations have small thalli, closely adnate or 
even more or less strictly hypolithic. All except the hypolithic 
