1904] FINK—A LICHEN SOCIETY 273 
gain possession of any considerable portion of the riprap and is 
still abundantly maintaining itself in the drier places where the rock 
is not disintegrating so rapidly. The second of the list, Bacidia 
inundata, occupies similar but moister surfaces of both wall and 
extensions, its thallus varying considerably according to conditions 
of moisture, being well developed in the moist places where the 
species is usually found, but almost wanting in the dry, exposed 
places. These two species seem to prevail here instead of the better 
developed types of crustose thalli, because the rock disintegrates too 
rapidly for the possibility of extensive establishment of the better 
developed thalli. The better developed crustose thalli are the forms 
that prevail on such hard rocks as the Sioux quartzite, or as we shall 
see shortly, on riprap of similar sandstones where drier and disinte- 
grating more slowly, and are represented in the present society by 
the rare specimens of Lecanora, Placodium, and Acarospora. A 
hasty study of the similar thalli of the Biatora and the Bacidia above 
named scarcely reveals definitely why one should be more xerophytic 
an the other, though the thallus of the former is on the whole more 
closely adnate than that of the latter. However, the Biatora shows 
under the microscope a more pronounced xerophytic adaptation in 
the somewhat tougher, more lecideine condition of the hypothecium 
“iam and in the somewhat better development of the para- 
The cylindrical type of thallus is represented by the Cladonias 
ita single species of Stereocaulon. These lichens thrive best 
when so one here they may have a fair supply of moisture and shade, 
somewhat a lat protected from the wind, and on soil, or on rocks 
Protection oS The conditions as to moisture, shade, and 
the base of a eal fairly well met in the crevices toward 
blocks, and ; 4 northward-facing wall, in the openings between riprap 
block. hs i certain places protected more or less by a projecting 
themselves 2 been Stated, these plants are often able to maintain 
" Spite of disintegration, and when the product of dis- 
Temains in situ are actually invigorated by the process 
Mtegration 
and 
finally come to rest on a sandy soil 
