235 



Professor Wittrook suggests that the genus Anabaena should be 

 divided into four sab-genera, under the following designations : 



SUB-GEN. 1. Trichormus. Spores globose, or subglobose, hetero- 

 cysts intercalated, distant from the spores. 



SUB-GEN. 2. Dolichospermum. Spores subellipsoid or sub- 

 cylindrical, heterocysts as in Trichormus. 



SUB-GEN. 3. Sphaerozyga. Spores subellipsoid or subcylindrical, 

 heterocysts intercalated, proximate to the spores. 



SUB-GEN. 4. Cylindrospermum. Spores subellipsoid or cylindrical, 

 heterocysts for the most part terminal and proximate to the spores. 



Anabaena flos-aquae. Kutz. Tab. Phyc. i., t. 94. 



Free swimming, membranaceous, blue-green. Trichomes more 

 or less curved, often circinate, joints sphserical, or from mutual 

 pressure elliptic or quadrate. Heterocysts intercalated, elliptical. 

 Spores globose. 



SIZE. Cells -0045--006 nmi ; heterocysts -012--014 mm. 

 long ; spores 'OOS-'Ol mm. 



Rabh. Alg. Eur. ii., 182. Kirch. Alg. Schl. 235. 



Nostoc ftos aquce, Lyngb. Hydro, t. 68, f. D. 



Trichormus incurvus, Allm. Ann. Nat. Hist, xi., 163. 



var. circinalis. Kirch. Alg. Sclil. 235. 



Trichomes more circinate, and joints rather larger. 

 SIZE. Cells -007--01 mm. ; spores -012- -014 mm. diam. 

 Anabaena circinalis, Rabh. Alg. Eur. ii., 183. 

 Anabaena spiralis, Thompson Ann. Nat. Hist. (1846) v., 

 p. 81. 



Trichormus spiralis, Ralfs Ann Nat. Hist. (1850) p. 328. 



In ponds, moor pools, &c. 



Plate XCIII. Jig. 1. Trichomes X 40Q diam.; b Trichormus spiralis 

 after Ralfs ; c var. circinalis, trichomes x 400. 



From the description and figure given by Ealfs, under the name of 

 Trichormus spiralis, we have no donbt that Thompson's Alga, which 

 himself termed Anabaena spiralis, was the variety circinalis of this 

 species, although Hassall's figure and interpretation is entirely different. 

 The account which Thompson gives of his " Alga which colours Bally- 

 drain Lake," is as follows : 



" On visiting the lake (July 15) to-day I found that the whole body of 

 water was tinged with a dull faintly glaucous hue. On going out in a 

 boat to ascertain the cause of this appearance, I saw that the w r ater was 

 everywhere filled with extremely minute particles, which might be com- 

 pared to the motes in a sunbeam. To the unassisted eye they seemed 

 as delicate as the finest human hair, and of a spiral form. On enquiry 

 I learned that the appearance had been observed only for the last four 

 or five years, and for about three months in each year. One of my 

 friends had looked upon its approach with dread, as it interfered so 

 much with his angling, that during the period of its continuance this 

 spot had to be abandoned." Subsequently the plant was observed as 



2 L 



