jDakin and Latarche — The Plankton of Lough Neagh. 43 



followed throughout the year in our counts. The maximum numbers occurred 

 in April, the total for all catches being 35,000 on the 13th April. This number 

 may be compared with the maximum numbers of the following : — 



Asterionella, 



2,722,500 



Tabellaria, . . . . . 78,810,000 



Fragilaria, 819,000 



Stephanodiscus, .... 103,500 



Coscinodiscus, .... 1,540,000 



The spring maximum of the Surirella extends from February onwards 

 until May. A great depression occurred in June and July, and was then 

 followed by high numbers again in late July and August. After this the 

 numbers diminished considerably, though the genus was always present. 



Surirella hiserinia is recorded as rare in Ennerdale Water, and present in 

 December, January, March, May, and June. In Wastwater it was present 

 chiefly in February. In Loch Lomond, April was the month for S. hiseriata, 

 and later August. *S'. rohusta var. splendida also attained its maximum in the 

 months of March and April. On the whole, Surirella rohusta var. sple.idida 

 appears to be the most common form in the British lake plankton. 



Surirella hiseriata is recorded for the Danish lakes, and lakes of Central 

 Europe and the Alps. Kofoid records only the S. ovalis and S. spiralis from 

 the Illinois Eiver. 



Synedra. 



Synedra Bcvaliensis Lemm. — This species, which is now recorded for the 

 first time from Lough Neagh, has been the most interesting species of Synedra 

 in the plankton. In the first place, other species occurred so very rarely that 

 they did not come out in the counts at all. This species was unrecorded for 

 all the months from the first catches made in February to end of May. 

 On June 10th it first appeared, and then was present only in the bottom 

 water of the lake. The numbers steadily increased until August, when a 

 maximum was reached quite suddenly — 37,500 colonies were present in the 

 top 20 feet catch alone ! 30,000 colonies were present in the bottom 20 feet. 

 Thus, on this date, more colonies were present in the surface catches. This 

 sudden maximum was followed by just as sudden a fall, and whilst it was 

 recorded for the surface catches in October, the number was only 300. After 

 this date it disappeared once more. In no other case, except Dinobryon and 

 Peridinium, were such a sudden appearance and maximum observed in the 

 phytoplaukton of the lake. 



Synedra Bevaliensis occurs in lakes in west and south-west Ireland 

 (Messrs. West) ; but this is not recorded for Scotland and England. 



R.I.A. rKOC, VOL. XXX., SECT. B. [Gr\ 



