204 



again they appear larger, colorless, three to five-septate with thin 

 septa and spore- wall. It is likely that further studies of the group 

 will separate it into several genera ; at present I have retained them 

 in the one genus, since only a few representatives occur in the terri- 

 tory. Even if all the undoubted and doubtful Omphalarias are 

 grouped together they do not form a large genus. The difficulty of 

 studying the group is increased since the higher Omphalarias are 

 frequently or quite constantly sterile. 



The species occur in the temperate and arctic zones ; they grow 

 upon rocks in much the same localities as the Lichinas. 



PLATE 67. 



OMPHALARIA UMBETLA Tuck. 



1. Natural size. 



2. Terminal portion of thallus with apothecia, magnified. 



3. Section of apothecium. 



4. Longitudinal radial section of the thallus. 



5. Spore-sac and paraphyses. 



6. Spores. 



7. Algae and haustoria. 



8. Chain of Nostoc. 



5. POLYCHIDIUM Ach. ; S. F. Gray, Brit. PI. I : 401. 1821. 



This interesting genus is represented by only one species, P. 

 muscicolum, quite generally included under Leptogium, from which 

 it is, however, essentially different ; the casual observer is at once 

 struck by its resemblance to Ephebe pubescens ; its histology is, how- 

 ever, quite unlike. 



The thallus is minute, typically fruticose, consisting of cylindri- 

 cal branches of a dark color, in fact almost identical with Ephebe 

 as far as external appearances are concerned ; usually, however, the 

 lobes are shorter. 



Upon examining carefully prepared sections the following struc- 

 ture is revealed : there is an outer tissue consisting of one layer, 

 which is typically cortical, very closely resembling the epidermal layer 

 of a leaf in higher plants ; the entire interior is occupied by a hyphal 

 tissue and the algae ; the algae {Rivularia nit i da) are most numer- 

 ous toward the outer surface ; the central tissue is, however, never 

 totally devoid of algae ; since the thallus is typically fruticose the 



