GENERA AND SPECIES. 129 



and lowly organized. The difficulty is increased by 

 the fact that many species are generally sterile, that 

 is, they are without spores though apothecia may be 

 present. Tlie thallus is deficient and the algal charac- 

 ters are variable ; in some cases it is evidently Chroo- 

 lepus, in others, Protococcus. Further investigation 

 may make it advisable to subdivide the group upon 

 these algal differences. 



The apothecial characters are also variable. Along 

 with the thallus, the apothecia begin their development 

 below the surface of the substratum, breaking through 

 some time before maturity ; rarely they remain cov- 

 ered over by a thin layer of the thallus and substra- 

 tum. They are small, irregular in outline, never 

 distinctly linear ; sometimes they are stellate or tend 

 to become linear, with radiating projections. The 

 disk and hypothecium are dark, sometimes reddish- 

 brown. 



The spores are characteristic in form ; they are ob- 

 long, either colorless or dark-colored, usually four- 

 celled, the ends blunt, narrowed toward one end caus- 

 ing it to resemble the outline of the sole of a shoe. 



The Arthonias are southern in their range, though 

 less so than Graphis. Tkey occur upon trees. 



1. Arthonia astroidea. Thallus indistinct, mostly 

 hypophloeodal. Apothecia numerous, irregular, stel- 

 late, sometimes somewhat elongated ; only sliglitly 

 raised above the substratum (bark). Disk black. 

 Spores colorless, four-celled, 12/u, X 5^. 



2. Arthonia dispersa. Resembling A. astroidea ; 

 the apothecia are fewer and more scattered. 



