widely different in their general histology ; in the former the algal 

 symbiont forms the general structure, in the latter the fungal symbi- 

 ont gives form to the thallus. 



KEY TO THE GENERA. 



Symbiotic algae SirosipJion i . 



Symbiotic algae not SirosipJion. 



Thallus crustaceous, dark. 2. 



Thallus fruticose, minute. 



Cortical tissue wanting ; spores simple. 



Algae Rivularia. 3. 



Algae Gloeocapsa. 4. 



Cortical tissue present ; spores two-celled, col- 



Ephebe. 

 Lecothecium. 



Lichina. 

 Omphalaria. 



5 . PolycJi idiu m , 



6. 



7- 

 8. 



orless. 

 Thallus foliose. 



Spores simple, colorless. 



Algae bright green ( Cystococcas}. 

 Algae blue-green (Scytonema}. 

 Algae blue-green (Polycoccus}. 

 Spores not simple. 



Lower cortical layer wanting. 



Spores 4 6-cellecl, colorless, acicular. 9. 

 Spores 2 -celled, brown. 10. 



Lower cortical layer present. 



Spores 4-celled, elliptical, colorless, n. Nephromium. 

 Spores 2-4-celled, colorless, acicular. 

 Algae blue-gi een . 12. 



Algae bright green. 13. 



Psoroma, 

 Hep pi a. 

 Pannaria. 



Peltigera . 

 Solorina. 



Stictina. 

 Sticta. 



i. EPHEBE Fr. PI. Homon. 256. 1825. 



This genus, which is represented by two species, only one of 

 which has came to my notice, stands without a parallel in the history 

 of lichen-development. Its general structure and appearance is de- 

 termined by the algal symbiont, which is SirosipJion pidvinatus ; it 

 is therefore very small, dark in color and branching. The thallus 

 consists of the alga mentioned, through the interior of which the 

 hyphae extend. The hyphae, which are so few in number as not to 

 materially modify the appearance and form of the alga, extend in a 

 longitudinal direction between the algal cells, usually terminating at 

 a short distance behind the apex ; it may happen that some of the 



