74 Proceedings of the Royal Irish Academy. 



Lough Neagh. For example, the number of species each present in large 

 quantity may be taken as about fifty. Against this the number of species 

 that could be easily counted in our fresh-water catches was only about 

 thirty-five. This feature was very striking. Most of our catches under the 

 microscope consisted of enormous quantities of a few species. This rendered 

 the enumeration of some of the more rare organisms quite impossible. If 

 the catch was diluted enough to thin out the individuals, none of the rare 

 species would be perhaps on a counting-plate. If, on the other hand, the 

 dilution was arranged to bring a considerable number of the rarer species 

 on the plate, they were completely obscured by thousands of individuals of 

 perhaps one or two species. These two differences, 1st, the quantity, and 

 2nd, the constitution, were perhaps the most marked ones observed. 



The constitution of the marine phytoplankton is almost entirely of species 

 of Diatomaceae and Peridineae. In the composition of the Lough Neagh 

 phytoplankton the Diatoms, Desmids, and Myxophyceae play a great part. 

 The Peridineae are represented by Ceratium liirundinella, which often occurs in 

 great quantities, and Peridinium. The Myxophyeeae, which are amongst the 

 most conspicuous forms in the Lough Neagh plankton, are represented by 

 Oscillatoria, Gomphosphaeria, Anabaena, as well as some other less frequent 

 species. 



From the observations on phytoplankton of other fresh-water lake-areas, 

 we notice that the arctic plankton is characterized by the absence of the 

 Myxophyeeae, the presence of large quantities of Tabellaria, Melosira, 

 Asterionella, and the presence of Fragillaria, Stephanodiscus, etc. ; a few 

 Dinoflagellates also occur, and species of Dinobryon. 



The central European lakes and especially the Baltic lakes, the best 

 known so far as their plankton is concerned, are characterized by the 

 abundance of Myoxphyceae and by Fragillaria crotonensis, Asterionella and 

 Melosira amongst the diatoms. Tabellaria is somewhat rare. 



Our phytoplankton is characterized by the presence, first and foremost, of 

 Tabellaria fenestrata, which is present all the year round, and attains 

 34,000,000 colonies in one catch ; then 



Oscillatoria AghardJiii (Gomont). 



Gomphosphaeria Naegeliana (Unger) Lemm. 



Anahaena flos-aqxiae (Lyng.) Bret. 



Melosira sp. 



Asterionella. 



Fragilaria crotonensis. 



Ceratium. 



Thus the pliytoplankton of Lough Neagh contains in great abundance the 



