98 CONTRIBUTIONS FROM THE NATIONAL HERBARIUM. 
KEY TO THE SPECIES. 
Thallus whitish or ashy. 
Thallus inclined to rounded forms. 
Apothecia frequently concentrically arranged; spores 
22 to 39 » long and 11 to 16 » wide................. 3. R. caleareum 
concentricum. 
Apothecia not concentrically arranged; spores 12 to 22 
uw long and 4 to 9 » wide .........2..............-. la. R. alboatrum 
saxicolum. 
Thallus not inclined to rounded forms, but widely spread 
and thin; spores as in the last above.................... 1. R. alboatrum. 
Thallus not whitish or ashy (except sometimes in R. ignobile). 
Thallus from greenish to bright yellow................... 5. RK. geographi- 
cum. 
Thallus from dark ashy to brownish black. 
Apothecia immersed. 
Spores always becoming muriform, rather large... 2a. R. petraeum 
grande. 
Spores rarely muriform, smaller.................. 4. R. ignobile. 
Apothecia not always immersed. 
Apothecia immersed or adnate. 
Spores 8 in each ascus............-....----- 2. R. petraeum. 
Spores | or 2 in each ascus................... 2b. Rk. petraeum 
montagnaet. 
Apothecia adnate or somewhat immersed......... 2c. R. petraeum ob- 
scuratum. 
1. Rhizocarpon alboatrum (Hoffm.) Th. Fr. Nov. Act. Reg. Soc. Sci. Ups. III. 
3: 337. 1861. 
Lichen alboater Hoffm. Enum. Lich. Icon, 30. 1784. 
Thallus ashy-gray, whitish or white, commonly widely spread over the substratum 
as a continuous crust but sometimes scattered or rarely disappearing, at first smooth 
but becoming chinky and verrucose-areolate and even rarely mealy; apothecia rather 
small, 0.3 to 1 mm. in diameter, adnate or immersed, the disk dull black or more or 
less white-pruinose, flat or becoming convex, the exciple black and commonly dis- 
appearing; hypothecium blackish brown; hymenium pale or pale brownish below 
and darker above; paraphyses simple or rarely branched, commonly enlarged and 
brownish toward the apex; asci clavate; spores brown, 4-celled and becoming muri- 
form, 12 to 22 » long and 4 to 9 » wide. . 
Quite common in the northwestern and southwestern portions of the State. On 
trees, especially elms. 
Generally distributed throughout North America. Found also in Europe, Asia, 
and Africa. 
Buellia alboatra of the preliminary reports. 
la. Rhizocarpon alboatrum saxicola (Fr.) Fink. 
Lecidea alboatra saxicola Fr. Lich. Eur. 337. 1831. 
Thallus thicker and more inclined to rounded forms upon the substratum. 
Collected at Mankato and at Rainy Lake City. On limestone and igneous rocks. 
Widely distributed in North America, but not yet reported from arctic regions. 
Known also in Europe. 
Buellia alboatra saxicola of the preliminary reports. 
