42 Proceedings of the Royal Irish Academy. 



Unlike Coscinodiscus, however, it occurs in some considerable quantity in 

 the Danish lakes ; and "Wesenberg Lund states that it seems to be of much 

 more importance there than in lakes abroad. It has also been recorded from 

 the Swiss lakes and the Central European lakes, but does not appear to be 

 common. 



The maximum occurs iu spring, the highest numbers being found in April. 

 It remains common untU August, and then the number falls rapidlv, so that 

 practically none were present in September, and extremely few from that 

 time onward until February. 



Stephanodiscus astraco. seems also to be an Arctic plankton diatom. This 

 species is not recorded by Kofoid from the IllinoLs Eiver. 



Cymatopleura. 



The genus Cymatopleura is represented by the species elliptica and sok/i. 

 The species elliptic^ is far the most common, and for this form curves have 

 been made for the year. "Wesenberg Lund records Cymatopleura elliptica as 

 a bottom diatom, occurring, however, as a typical plankton organism. Perhaps 

 the shallowness of Lough Neagh has something to do with the relative 

 abimdance of Cymatopleura as a plankton organism. 



Cymatopleura elliptica occurs all the year round in the plankton, and the 

 range of variation is not very high. The maximum occurs in July and August, 

 at the same time as the small second maximum of Fragilaria. The numbers 

 are also high in February and March, although not as high as the July and 

 August maxima. After September there is a great decrease, and the number 

 then remains about constant and small until the spring. 



Cymatopleura does not occur in the English or Scottish lakes investigated 

 by Messrs. West, but has been recorded by them for Lough Neagh and the 

 Orkneys and Shetlands. 



A species of Cymatopleura occiu's in Victoria Nyanza. C. dliptita occurs 

 in Eui-opean and Alpine lakes. Kofoid records C. soka from Illinois Eiver, 

 where it occurs in small numbers during the colder months, with also some 

 isolated individuals in the summer months. 



We have observed it as isolated specimens in April. May, June, and 

 October. 



Surirella. 



SurireUa hiseriata is the most common species in Lough Xeagh plankton ; 

 though S. ovalis Breb., and S. rohvMa Ehrenb., also occur, the latter often in 

 considerable quantity. 



This genus is fairly common in the plankton, but is inferior in numbers 

 to most of the algae which were present in such quantity as to be easily 



