48 Proceedings of the Royal Irish Academy. 



the Contiaent, too, that a great development of M. aeruginosa accompauied 

 by "water-bloom" phenomenon was followed by the death of the fish. This 

 was probably due to the decomposition gases. 



Microcystis is absent from the atctic lakes. It is very rich indeed in 

 the Central European lakes, while it occurs and forms water-bloom in Alpine 

 lakes. M. aeruginosa appears to have a very wide range, and occurs in the 

 lakes of Ceylon. 



Clathrocystis aeruginosa (Kiitz.) Henfr. is present in the Illinois plankton, 

 where it is predominantly a midsummer species. It attains its maximum in 

 August and September (108,000). It is not very abundant, Microcystis 

 ichthyolaie being much more frequent. Other species may have been counted 

 with the latter owing to the lack of striking characteristic differences. M. 

 ichthyolabe attains a maximum in August-September or September-October, 

 and is present throughout the year. 



Chroococcus. 



C. linineticus Lemm. is the only species of the genus which is at all 

 common in Lough Neagh, though G. minimus also occurs. Most species of 

 Chroococcus are not found in the typical plankton of large lakes. It can 

 hardly be called common in Lough Neagh ; 3600 colonies was about the 

 largest of the bottom to surface catches in August. It was absent altogether 

 or else present in very small quantities indeed in April, May, January, and 

 February. Messrs. "West say that it was very abundant in Lough Neagh. 

 Their catches probably represented the maximum time of occurrence, 

 July or August, though it was certainly never abundant in our year of 

 examination. 



C. limneticus occurs in the Danish lake plankton, but is never abundant 

 and appears to reach its maximum during the winter. Wesenberg Lund 

 adds that if this is the normal it differs from all the other Cyanophyceac 

 which reach maximum usually at higher temperatures. Our largest 

 numbers seem to occur in summer, though a very large catch was made in 

 February. (February catch abnormal — new net.) 



Chroococcaceae seem to be very abundant in some of the African lakes. 

 Chroococcus hirgiclus has been recorded from arctic lakes in Eussian Lapland. 



DINOFLAGELIATA. 



Ceratium. 

 The genus Ceratium is represented in our plankton by the species C. 

 hirundinella, and perhaps no other fresh-water protozoan or protophyton has 

 been so much discussed as this organism. Whilst Ceratium is represented by 



