4:4 Proceedings of the Royal Irish Academy. 



Very few diatoms have a maximum in the height of the Hummer, as this 

 species of Synedra appears to possess. Another interesting feature is the marked 

 origin from the bottom of the lake and the gradual ascent to the surface 

 waters. The Synedra species recorded by Wesenberg Lund for the Danish 

 lakes attain their maxima in April and May. 



In all cases Synedra Revaliensis occurred with us in the form of star-shaped 

 or radiating colonies.. 



Kofoid does not record this species from the waters of the Illinois Elver. 



Pleurosigma AV. Sm. Gyrosiyma Hassall. 



PUiwo&vjma attenuahom Kutz occurs in the plankton of Lough Neagli in 

 small numbers. The highest numbers occur in July and August, viz. 16,500 

 in the combined catches. It diminishes during September and October, and 

 is absent altogether during November, December, and January. 



Messrs. West do not record Pleurosigma in their paper on the seasonal 

 variation in the British lakes examined by them. They have observed it in 

 the plankton from the Orkneys and Shetlands and from west and south-west 

 Ireland. 



Neither Wesenberg Lund nor Apstein makes any mention of Pleurosigma. 



CYANOPHYCEAE. 



The Cyauophyceae play a great part in the plankton of Lough Neagh, and 

 in this respect v/e find Lough Neagh agreeing with the Danish lakes. Just 

 as in the Danish lakes, we have few species but large numbers of individuals ; 

 and at times the phenomenon known as " water-bloom," or the" breaking of 

 the meres," is prodrrced by rapid reproduction on calm days, when the 

 organisms can concentrate in the upper inch of surface-water. 



Oscillatoria. 



Oscillatoria Agardhii Gomont. is the most frequent of the Oscillatoria in 

 Lough Neagh. This is not recorded for Lough Neagh by Messrs. West, though 

 they have noted its presence in other lakes in Ireland, and in England and 

 Scotland. The dominant species is 0. riobescens De Candolle. 



The Oyanophyceae are characteristically summer plankton forms, though 

 sometimes a maximum occurs in the winter months. Oscillatoria Agardhii 

 reaches its maximum with us in May. It was present in large quantities, 

 however, in March, April, May, June, and July ; and with Anabaena was 

 responsible for " water-bloom " on March 30th. This date is early for 

 water-bloom, and the phenomenon was not nearly so well marked as in late 

 summer. 



