KEYS TO THE STUDY OF LICHENS. 77 



I. ARTIFICAL KEY TO THE MORE IMPORTANT GENERA 

 OF LICHENS OCCURRING IN THE UNITED STATES. 



Thallus Crcstose. 



Stipes present with terminal apothecia, plants small. 

 Stipes and apothecia black. 

 Apothecia somewhat flattened (discoid) . . Cyphelium. 



Apothecia globose (spherical) Calicium. 



Stipes light colored. 

 Apothecia small, brownish, bark-lichens . . Coniocybe. 

 Apothecia large, earth-lichens Bceomyces. 



Stipes wanting, apothecia more or less immersed. 

 Apothecia distinctly linear. 



Length of apothecia usually exceeding four times their 



width ; black or white Graphis. 



Length of apothecia usually not exceeding four times 

 their width ; black. 



On decaying wood Xi/lographa. 



On bark of trees Opegrapha. 



Apothecia not linear, irregular. 



Oval or oblong Hazslinskya. 



Irregular, not oval or oblong. 

 Disk distinctly visible, black. 



Apothecia small Arthonia. 



Apothecia comparatively large . . . Arthothelium 

 Disk indistinct, mostly immersed . . . Mycoporum. 



Apothecia regular in outline. 

 Immersed in thallus or substratum ; globose. 

 Not in colonies. 



Disk light colored Pertusaria. 



Disk black or dark. 



Occurring on rocks Verrucaria. 



Occurring on bark 

 Apothecia almost entirely covered 



by substratum Pyrenula. 



Apothecia more elevated, urn- 

 shaped Gyrostomum. 



Apothecia crateriform Thelolrema. 



In colonies Trypethelium. 



Immersed in thallus ; not globose. 



Disk not projecting above surface of tliallus. 



Thallus light-gray Urreolaria. 



Thallus dark Acarospora. 



Disk projecting above the green thallus . . Acolium. 



