FINK—THE LICHENS OF MINNESOTA. 99 
2. Rhizocarpon petraeum (Wulf.) Koerb. Syst. Lich. 260. 1855. 
Lichen petraeus Wulf. in Jacq. Coll. Bot. 3:4. pl. 6. f. 2a. 1789. 
Thallus varying in color from dark ashy to sea-green and brownish or brownish 
black, smooth or more commonly more or less roughened, chinky, and verrucose- 
areolate, usually widely and irregularly disposed upon the substratum, and continu- 
ous or scattered upon the commonly distinct and black so-called hypothallus, varying 
considerably in thickness, the areoles or verrucz also varying in size in the various 
forms, but small or even minute; apothecia minute or middle-sized, 0.2 to 1.3 mm. 
in diameter, immersed or adnate, the disk black or blackish brown and flat or rarely 
somewhat convex, the exciple of the same color and at first elevated but frequently 
disappearing; hypothecium dark brown; hymenium pale or pale below and com- 
monly brownish above; paraphyses commonly simple but sometimes branched, 
usually enlarged and brownish toward the apex; asci clavate or inflated-clavate; 
spores 4-celled and muriform, brown or hyaline, 15 to 40 » long and 7 to 18 wide. 
Occurring in all portions of the State except the southeastern. On igneous and 
metamorphic rocks. 
Found throughout the northern portions of North America and well southward in 
the mountains. Known in all the grand divisions. 
Buellia petraea of the preliminary reports. 
The species varies greatly in the northern portion of the State, and in the last of 
the preliminary reports a number of subspecies were added on authority of A, Zahl- 
bruckner and T. Hedlund. These forms are variously regarded by Europeans as 
species or subspecies, and the disposition and synonymy are in a hopeless tangle for 
the present. On the whole it seems best after further study to record some of the 
various forms here as subspecies of the above species without any hope of having 
made a final or even a clear disposition of all the difficulties. 
2a. Rhizocarpon petraeum grande (Floerke) Fink. 
Catocarpon grande Floerke, Flora 2: 690. 1828. 
Thallus verrucose-areolate, the areoles swollen, ashy or violaceous brownish, usually 
scattered upon the conspicuous black hypothallus; apothecia immersed between the 
areoles, soon becoming very convex and subglobose; spores rather large. 
Throughout the northeastern portion of the State and as far south as Taylors Falls. 
Habitat as above. 
North American distribution the same as that of the species. Known also in 
Europe. 
Buellia petraea grandis of the preliminary reports. 
2b. Rhizocarpon petraeum montagnaei (Flot.) Fink. 
Rhizocarpon montagnaei Flot.; Koerb. Syst. Lich. 258. 1855, 
Thallus as above, but the spores only one or two in each ascus and of the largest 
size given for the species. 
In all parts of the State where the species is known. Habitat as usual. 
A strictly North American subspecies with same general range as the species, 
Buellia petraea montagnaet of the preliminary reports. 
2c. Rhizocarpon petraeum obscuratum (Ach.) Fink. 
Lecidea petraea obscurata Ach. Lich. Univ. 156. 1810. 
Thallus thin, minutely areolate with flat areoles, ashy or pale brownish, phe hypo- 
thallus commonly poorly developed or absent; apothecia adnate or somewhat im- 
mersed, the exciple thick and persistent around the commonly flat disk; spores per- 
sistently hyaline or only slightly colored. 
Collected at Kettle Falls. On rocks. 
Elsewhere in North America in Newfoundland. Known also in Europe. 
Buellia obscurata of the preliminary reports. 
