GENERA AND SPECIES. 137 



thecia numerous, small to medium. Disk flattened or 

 slightly convex, darker orange than thallus, margin 

 scarcely raised. Spores typical, 10/x X 5/x,. 



Placodium murorum has been classed by authors 

 with Lecanora as well as ParmeUa. It occurs upon 

 walls, rocks, bricks, tiles, mortar, etc. 



7. Placodium elegans. Thallus distinctly foliose, 

 radial growth, forked branching, lobes of uniform 

 width, convex above, somewhat concave below, ends 

 of branches turning down ; orange above, dark-gray 

 beneath with black rhizoids. Apothecia medium to 

 large, cup-shaped. Disk orange. Spores typical, 

 10/x X 8/x. 



This species as well as other species of Placodium 

 has been classed with ParmeUa. P. elegans is said 

 to occur on granitic rock on the highest mountains. It 

 has been collected on Orizaba at an elevation of 14,850 

 feet. Agassiz brought it from the summit of the Jung- 

 frau. 



3. Pyxine. 



Only two species of this genus occur in the United 

 States, one of which is probably not a Pyxine. P. 

 sorediata is nearly always sterile. As represented by 

 the only two species, the thallus is foliose, quite 

 large, thin and closely adnate to the substratum, branch- 

 ing quite distinct. Color of the thallus ash-gray, re- 

 sembling that of the Pkyscias, to which this group is 

 evidently closely related.^ 



The apothecia are discoid. Disk dark, likewise the 



1 UnlesB otherwise stated, the algsB of any lichen-genus or lichen- 

 *pecie» are Protococcus (Cystococcus humicola of Nageli). 



