60 Proceedings of the Royal Irhh Academij. 



lakes. It is also the most important species of Cyclops in the north European 

 region. In the Baltic lakes, C. stremms is common ; but the chief form is 

 C. oithonoides, while C. strenuus is here mainly a winter and deep-water form. 

 C. strenmis is also the commonest form occurring in the Alpine lakes, more 

 especially in the high ones. 



This species of Cyclops far outnumbers all others occurring in Lough 

 Neagh. In fact, the other species are somewhat rare, Cyclops viridis being 

 present in small numbers. It gives, as will be referred to in another part of 

 the paper, an arctic feature to the plankton of the lake. 



Diaptomus gracilis Sars.—Is the most abundant Copepod in the plankton 

 of Lough Neagh. It is present throughout the year, and, like Cyclops strenuus, 

 has two maxima, one in May (4,903 in all the catches), and one in October 

 (5,336 in all the catches). The numbers remain high until February. 



Wesenberg Lund finds this species in several of the Danish lakes ; but 

 there is no strongly marked sexual period. Females with eggs are, however, 

 most frequent in May ; and in September to October there is another sexual 

 period which is evenless marked than in spring. Murray records D. gracilis 

 as general in Scotland, where it is perennial in some of the lochs. In certain 

 lochs at high elevations it has a seasonal limitation. It is the commonest 

 species in Scotland, as in Europe generally. D. gracilis is one of the main 

 forms of the large lakes in the arctic district designated by Wesenberg Lund. 

 It is also the chief form of the north European lakes, the Baltic lakes, and 

 the central European alpine lakes. The distribution of the genus Diaptomus 

 has been worked out recently in very great detail. A further reference to 

 this will be found on page 77. 



CLADOCERA. 

 Daphnia hyalina Leydig. 



D. hyalina var. lacustris occurs in Lough Neagh in large quantities 

 during certain months of the year. It appears for the first time in April, 

 and increases slowly at first. In June it suddenly reaches its maximum 

 (6690 in all the catches). It then decreases slowly, and disappears completely 

 in January. 



Wesenberg Lund finds this variety, which he terms subspecies, in Viborgso 

 and Haldso only. In Viborgso, it has a great maximum during the summer 

 and a second great maximum towards winter, after which it disappears 

 completely. In Haldso, the species has a great maximum in the early 

 summer and a smaller maximum towards winter, throughout which it is 

 present. 



