182 ' GUIDE TO THE STUDY OF LICHENS. 



long, margin bearing short small branchlets ; dark 

 above, lighter beneath. Apothecia numerous, on short 

 thalloid branches, margin somewhat crenulate, dark. 

 Disk dark. Spores colorless, two-celled, small, 



9. Collema pUcatile. Thallus comparatively small, 

 lobes narrowed, ascending, margin somewhat crenu- 

 late ; nearly black above, somewhat less dark beneath. 

 Apothecia small, on short thalloid branches, margin 

 somewhat raised. Disk nearly black. Spores multi- 

 locular, oblong, irregular outline, constricted at the 

 septa, 24/t X 6/x. 



10. Collema crispum. Thallus medium size, com- 

 paratively thick, margin thickened and indistinctly 

 crenulate ; dark above, less dark beneath. Apothecia 

 numerous, marginal, cup-shaped, margin crenulate and 

 warty. Disk dark-brown. Spores multilocular, color- 

 less, somewhat constricted at septa, 27yw, X 12/x. 



11. Collema Jlaccidum, Thallus large, lobes large, 

 only slightly ascending, more or less folded and warty ; 

 dark-blue, lighter beneath. Apothecia small, upon 

 surface of thallus, margin entire and raised above the 

 reddish-brown disk. Spores colorless, four to six- 

 celled, 23/x X 6/x. 



2. Leptogium. 



The thallus resembles the thallus of Collema. It is 

 somewhat larger, thicker, and more rigid. The essen- 

 tial difference is made apparent upon a microscopical 

 examination. In Leptogium there is a cortical struct- 

 ure consisting of a single row of cells. This cortical 

 tissue is more highly developed in the apothecium, 



