342 Mr. J . Ralfs on the Nostochineae. 



Aphanizomenon cyaneum differs from A. Flos-aqua by its stra- 

 tum not appearing flocculose, by its paler inconspicuous filaments, 

 which do not cohere in laminae, and would often escape detection 

 under the microscope but for the presence of the sporangium, 

 which has a far broader hyaline covering in this than in the latter 

 species. 



Plate IX. fig. 7. Filament highly magnified. 



3. A. incurvum (Morren) . " Filaments articulated, cohering together 

 in flat laminae, laciniated at the apex ; articulations two to eight 

 times longer than broad." Trichodesmium Flos-aquae, Ehrenb. in 

 Poggend. Annal. 1830, p. 168 (according to Kiitzing). Aphani- 

 zomenon incurvum, Morren in Memoir (1837) ; Thompson in An- 

 nals of Nat. Hist. vol. v. p. 82 ; Harvey, Manual of Brit. Algae, 

 p. 145 ; Hassall, Brit. Freshwater Algae, p. 280. t. 76. fig. 6. Lim- 

 nochlide Flos-aquce y. Harveyana, Kiitzing, Species Algarum, 

 p. 286 (1849) ; Tabulae Phycologicae, t. 91. fig. 2. 

 Ballydrain Lake, Mr. W. Thompson ; Lough Neagh, Mr. D. Moore. 

 Belgium, Morren ; Germany, Kiitzing. 



I regret that I am unable to afford any satisfactory informa- 

 tion respecting this plant, for although Mr. Thompson has sup- 

 plied me with specimens, they are unfortunately preserved upon 

 paper, and could not be removed in a condition fit for examination. 

 Respecting the Ballydrain species, one would suppose there could 

 be no reasonable doubt as Dr. Morren has determined it. Still 

 it is very possible that his Aphanizomenon incurvum has been 

 rightly supposed by Kiitzing to be merely the Flos-aquce, for 

 nothing in the descriptions renders the opinion improbable. The 

 Dublin plant at least is decidedly the A. Flos-aqua. Mr. Thomp- 

 son's A. incurvum however differs materially from all my speci- 

 mens of the A. Flos-aquce both in colour and in the form of the 

 particles, which are circular and dot-like, very unlike the larger 

 and lobed flakes of the other. Mr. Hassan" s figure represents 

 the filaments as tapering — a character, indeed, amply sufficient 

 to distinguish the A. incurvum from the preceding species ; but 

 unfortunately, if the filaments really possess this character, 

 neither Dr. Morren nor Mr. Hassall have noticed it in their de- 

 scriptions, and so the matter is still in doubt. 



Since the above was written, I have received from Mr. Moore 

 some specimens better suited for examination. Although not 

 from the same station as Mr. Thompson's plant, they exactly 

 agree with it in appearance ; the specks are unlike any other spe- 

 cies that I have seen, and in form as well as in size and scattered 

 habit resemble the small dots made by house-flies. Under the 

 microscope the filaments appear parallel ; they do not closely 

 cohere as in A. Flos-aquce, but are rather held together by the 



