GENERA AND SPECIES. 135 



This lichen has been made the basis of the distinct 

 genus {Dimelcena). Its spore-characters closely re- 

 semble those of Buellia. It occurs typically upon 

 granitic rock. 



5. Rinodina chrysomelcena. Thallus rather thick, 

 irregularly areolate, margin indistinctly lobulate ; yel- 

 lowish-green above, black beneath. Apothecia small, 

 margin elevated above the dark flattened disk. Spores 

 dark-brown, connecting thread distinctly visible, 25^ 

 X 11/x. 



6. Rinodina turfacea. Thallus verrucose (warty), 

 more or dess branching, grayish-green. Apothecia 

 from small to medium in size, margin extending slightly 

 above the dark flattened disk. Spores brown, slightly 

 constricted at septum, 25.5/x X 12/x. 



2. Placodium. 



The representatives of this genus are characterized 

 by the orange coloration of the apothecial disk. The 

 thallus varies from typically crustose to distinctly foli- 

 ose, though it never attains any great size ; in color it 

 is usually yellow or orange, rarely gray to dark. The 

 yellow color is due to the deposition of crystals of 

 chrysophanic acid. 



The apothecia are discoid, of medium size, sessile, 

 with a distinct thalline margin. The spores are color- 

 less, with terminal plasmic masses connected by a slen- 

 der plasmic thread. The transverse septum is not dis- 

 tinct, as in Rinodina ; rarely are the spores simple. 



The distribution of the Placodias is much as that of 

 Rinodina, They occur upon rock, also upon bark, fences 



