152 



The apothecia are disk-like, rarely immersed in the thallus ; the disk 

 is usually flattened or concave, sometimes convex, especially in Pla- 

 c odium ; the apothecia usually vary from small to medium. A yel- 

 low color predominates in the thallus and apothecia of the genera 

 Placodiuni and Theloschistes due to a deposition of crystals of some 

 lichen acid in and upon the hyphal cell-walls. Rinodina is also 

 more or less impregnated with this coloring material. 



The algal symbiont is Cystococcus humicola Nag. The fungal 

 symbionts are doubtless derived from one, or, at most, a few ances- 

 tral forms of the Patellariaceae (plate 5). 



KEY TO THE GENERA. 



Thallus crustaceous, areolate, margin often foliose and lobed. 



Spores brown ; thallus not bright yellow. i. Rinodina. 



Spores colorless ; thallus yellow to orange. 2. Placodiitm. 

 Thallus foliose, grey, not yellow. 



Hypothecium dark. 3. Pvxine. 



Hypothecium colorless. 4. Phvscia. 



Thallus foliose to fruticose, yellow. 5. Theloschistes. 



i. RINODINA Ach. ; Gray's Brit. PI. p. 448. 1821. 



This genus doubtless forms the lowest group of the Physciaceae ; 

 it is distinguished from Placodhim by the uniformly dark-colored 

 spores and the absence of any orange coloration in the thallus ; the 

 thallus is grey or greenish in most species ; in R. orctiia, however, 

 it is a pale or dirty lemon or sulphur color, and in R. chrysomelaena 

 there is a marked yellowish tinge. 



The thallus is usually recognized as typically crustaceous. There 

 is, however, a tendency toward the foliose type, especially near the 

 margin, where lobation is sometimes quite distinct ( R. oreina.} 

 Sometimes there is a warty structure as in R. chrysomelaena. Usu- 

 ally the surface is crustaceous and more or less distinctly areolate ; 

 it is always closely adnate and attached to the substratum by means 

 of numerous black rhizoids. The black hypothallus of authors 

 is not sufficiently understood ; in many cases it seems to be the 

 remnant of some lichen (usually a related species) over which the 

 Rinodina has spread. (Syntrophy of Minks. See parasitic lichens.) 

 Different species of the Phvsciaceae are frequently in close prox- 

 imity, especially among the Rinodinas and Placodiums ; there 

 seems to be an inherent tendency to form symbiotic associations. 



