NOSTOC'. 291 



threads moniliform, in the interior substance very lax, in the 

 external stratum much more curved than in Nostoc commune, 

 smaller in diameter, they scarcely attain three millimetres ; 

 but the larger spherical globules, twice as thick, abound, and 

 sometimes form tracts more or less long, no smaller globule 

 occurring between. In adult specimens the threads are vari- 

 cose wherever they occur." Meneghini. 



6. NOSTOC VERRUCOSUM Vauch. 

 Plate LXXV. Fig. 1. 



Char. Frond attached, large, subglobose, in the beginning solid, 

 externally subcoriaceous, within gelatinous, at length hol- 

 low, vesicceform, verrucose, brownish green ; when dried 

 ceruginous. Filaments dense, slender, almost cylindrical. 



Nostoc verrucosum Vauch., Hist, des Conf. Tremellafluvi- 

 atilis Dillw., Muse. ; Harv. in Hook. Br. Fl. p. 400. ; 

 also in Manual, p. 185. ; Meneghini, in loc. cit. ; Thuret, 

 in Annales des Sciences Naturelles. 



Hab. On stones in Alpine streams ; Aberdeen : Dr. 

 Dickie. 



This species may be distinguished from most others of the 

 genus by the nature of the filaments, which are almost cylin- 

 drical. The species with which it is most likely to be con- 

 founded is Nostoc pruiniforme, the filaments of which resemble 

 it closely, but are considerably larger ; in other respects the 

 two species are widely different, being estranged in form, 

 habit and consistence. 



7. NOSTOC PRUINIFORME Ag. 

 Plate LXXVI. Figs. 3, 4. 



Char. Frond unattached, solitary, globose, smooth, olivaceous, 

 gelatino-coriaceous, within watery. Filaments somewhat 

 thick, almost cylindrical. 



Ag. Disp. p. 45. ; Berk. Gl. p. 48. t. 19. f. 2. ; Lyngb. 



u 2 



