No. 10.] FRESH- WATER ALG.E. 21 



2. Heterocysts terminal, and spores contiguous with 



them ; spores long and cylindrical . . . Cylindrospermum* 

 Heterocysts not terminal 3 



3. Filaments aggregated without order Anabcena* 



Filaments aggregated in bundles of plate-like masses 



Aphanizomenon 



Description of Genera. 



Aphanizomenon Morren. Trichomes a little attenuated 

 towards the apex, glued together parallelly in dense fascicles. 

 Cells nearly cylindrical, light blue or nearly colorless, and 

 slightly granular. Thallus somewhat membranaceous, free- 

 swimming, blue-green, or light pure blue, or at length olive; 

 spores solitary, smooth, cylindrical, elongated, round at the 

 ends, pale blue or olive. 



Nostoc Vauch. Filaments necklace-shaped, enclosed in 

 a more or less distinct gelatinous envelope. The cells are 

 spherical or elliptical, and more or less closely connected, with 

 heterocysts rarely terminal. The filaments are clustered to 

 form thalli, usually surrounded by a membrane, which is some- 

 times colorless, sometimes dark blue-green, dark brown, light 

 yellow, or, most often, olivaceous. 



N. minutissimus Kiitz. (?), Fig. 211. 



N. sp. (?), Fig. 18. 



N. rupestre Kiitz., Figs. 16, 17. 



N. comminiitum Kiitz., Fig. 19. 



[N. commune Vauch. ; N. microscopicum Carm. ; N. par- 

 melioides Kiitz. ; N. pruniforme Ag.] 



Cylindrospermum Kiitz. Filaments sheathless, single or 

 glued together in an indefinite gelatinous stratum ; occasionally 

 a number enclosed in a tegument. Cells spherical, oblong, 

 elliptical, or compressed. Heterocysts single, on the ends of 

 the filaments; spores next the heterocysts very long and 

 cylindrical. 



[C. majus Kiitz.] 



Anabaena Bory ( Sphaerozyga Ag. ; Trichormus Allman ; 

 Dolichospermum Thwaites). Filaments similar to those of 

 Nostoc, only nearly straight ; rarely provided with a* sheath ; 



