38 Proceedings of the Royal Irish Academy. 



March (agreeiug with Lough Lomond), the number of colonies in the bottom 

 and surface catch was 55,000, so that it could not be said to be rare even at 

 that time. 



Tahdlaria fenestrata has been recorcled by some observers to occur in chains 

 in winter and stars in summer (Bally for the Zurichsee). For form-variation 

 in Lough Neagh see p. 68. 



Tahellaria fenestrata is one of the characteristic diatoms of the Arctic 

 lakes. It is somewhat rare in Central European lakes. It turns up 

 again, however, in the Alpine lakes of Switzerland, and has been recorded 

 as occurring in enormous quantities, just as in Lough Neagh, in 

 Zurich See by Schroeter. It has also been noticed in the brackish water of 

 the North Sea. Schroeter makes an observation of some interest on the 

 quantity of plankton in the Zurich See when mostly Tahellaria. He estimates 

 that at this period for a depth of 8 metres, 1 square metre of surface water 

 corresponded to about 1000 cc. of plankton. It would have required a goods 

 train with seven waggons to have removed the dry plankton, while the silica 

 present would have been sufficient to have made a quartz block 2'25 metres 

 square with a weight of 303'8 cwts. 



1'. fenestrata Kiitz occurs in the waters of the Illinois Eiver, but the variety 

 asterionelloides, common in Lough Neagh, is not recorded by Kofoid. The 

 species was only found on one occasion in the Illinois Eiver. 



Asterionella. 



Asterionella gracillima and A. formosa both occur in the lake. They have 

 been united, as it is very probable that they are only variations of one and 

 the same species. A. gracillima was by far the most frequent form, A. formosa 

 being present in an almost negligible quantity. 



Asterionella is extremely common in the plankton of Lough Neagh, and 

 is present all the year round. The minimum with us occurs during 

 early March. A maximum occurs during the summer months from May to 

 August, and the numbers' fall gradually to March. The maximum number 

 obtained in a bottom-to-surface haul was practically 2,000,000 ; the minimum 

 was 20,000. The individuals occur at all depths, but more commonly in the 

 surface waters. 



The maximum of the diatoms seems to be largely influenced by temperature, 

 probably with light and other unknown factors aiding. Usually the chief 

 maxima occur at temperatures below 15° or 16° C, and in the Irish Sea diatoms 

 are rare in the summer months. Asterionella seems capable of having a 

 maximum at any temperature. Thus Wesenberg Lund records a curious 

 maximum which took place just before the freezing or immediately after the 



