132 GUIDE TO THE STUDY OF LICHENS. 



colorless to brown, constricted at the middle, 84/u 

 X 40^. 



7. Arthothelium. 



This group is usually associated with Arthonia. 

 It also resembles Mycoporum in many respects. 



The thallus is rudimentary, often indistinctly areo- 

 late, grayish to dark in color. The algae are Ghroo- 

 lepus. The apothecia are comparatively large, stel- 

 late. The disk is dark. The entire thecium and 

 hypothecium may be dark ; again it is found to be 

 colorless. 



The spores resemble those of Mycoporum, but dif- 

 fer in that they are colorless and not constricted at 

 the middle. Each spore-sac bears eight spores, which 

 is uncommon with lichens bearing such large spores. 



The Arthothelias are southern in their range. They 

 occur upon trees, most frequently upon hickories. 



1. Arthothelium spectahile. Thallus rudimentary, 

 areolate. Apothecia numerous, comparatively large, 

 irregular, stellate. Disk black. Spores large, multi- 

 locular, colorless, 78/a X 4:0/*. 



V. PHYSCIACE^. 



The representatives of this family are characterized 

 by the two-celled spores in which the protoplasmic con- 

 tents are joined by a slender thread, a characteristic 

 not noticeable in spores of any other lichen-group. 



The thallus varies from typically crustose to foliose 

 and fruticose, and, as is natural to suppose, it is varia- 

 ble in size, though it never attains any very great 

 dimensions. The foliose forms are often mistaken 



