Plate IV; Figures 16 to 19; Fig. 18 Magnified 500 Diam- 

 eters ; the others iooo Diameters. 



Fig. 16. Nostoc rupcstrc Kiitz. 



Fig. 17. " "... 



Fig. 18. " sp. (?) 



Fig. 19. comminutum Kiitz. 



