214 



Apothecia subterminal, adnate, scutelliform, convex or hooded, 

 occasionally lacerate, emarginate (rarely spinulose), disk dark.' 

 Asci clavate, containing 6-8 spores; paraphyses gelatinous, 

 simple or branched, septate. Spores elHpsoid or ovoid-ellipsoid,' 

 monoblast, hyaline. Spermogones immersed or papilliform. 

 Sterigmata simple. Spermatia cylindrical. 



Observatioxs : The genus is represented in our area by six 

 species. Though the hyphal structure is truly radial, the single 

 stratified layer of gonidial cells links this genus \\dth the dorsi- 

 ventral genera. In the genus Ramalina for instance, the gonidial 

 layer even in the most expanded types adheres to the radial 

 structure, and therefore evidently does not take its position 

 alone because of light exposure. In Cetraria Richardsonii, the 

 most doubtful member of this genus as here Hmited, a common 

 decorticate, concave surface rarely gives the plant a continuous, 

 radial structure. 



The species here included are, however, all separable from 

 such species as Platysma saepincolum (Ehrh.) Nyl., which have 

 a dorsiventral structure throughout, and have been to some 

 degree taxonomically set apart from time to time by many 

 authors. The variety Cetraria islandica suhhihulosa Fr. (tubidosa 

 Schaer.), suggests an interesting evolutionary speculation as to 

 the development of the radial from the dorsiventral types. 



Key to the Species 

 ("Thallus more or less brown 2 



[ Thallus stramineous to yellow 5 



jMargins of branches spinulose , 



[ Alargins of branches entire c 



fLaciniae expanded, soralio-maculate 4 



[Laciniae narrow, esoralio-maculate var. tenuifolia 



^/Margins denticulate, pale var. platyna 



iMargins spinulose, dark islandica 



fLaciniae cartilaginous, branches divaricate Richardsonii 



[Laciniae coriaceous, branches thyrsoid-entangled hiascens 



^fLaciniae narrow-canaliculate, glabrous cucullata 



fLaciniae wide, distinctly lacunose nivalis 



Cetraria islandica (L.) x'\ch. 

 The type species of the genus. 



Synony:my: Lichen islandicus Linn., Spec. Plant. 2: 1145. 

 1753- 



