Dakin and Latako'HK — The Plankton of Lough Neayh. 51 



P. WiUfi occurs iu the plankton of the nortli European lakes and also 

 other species. It seems to be absent from the Baltic lakes, its place being 

 taken by P. cinctitm, while other species also occur. They rarely attain a large 

 maximum. In the central Eiiropean alpine lakes P. tabHlatinn is recorded as 

 common. 



We see that although Peridinium is widely spread, it is nowhere very 

 abundant. 



P. tahiiliitiim is recorded from the Illinois Eiver by Kofoid. It is per- 

 ennial, although rare, during the cold months. It attains its maximum about 

 July or August. It plays quite an insignificant part in the plankton of this 



DINOBRYON. 



This genus is represented in the Lough Neagh plankton almost entirely by 

 D. cylindricum var. dirergens (Imhof) Lemm. Binobri/on profuberaiis and 

 D. sertnlnria var. thyrsoideiun also occur, but in such small quantities that it 

 is not worth while counting them separately. As a matter of fact, there 

 seems at present to be great confusion amongst specialists as to the different 

 species, and temporal variations require to be worked out. In Lough Neagh 

 Dinobryon is not a very abundant constituent of the plankton until July is 

 reached, when it attains a maximum of 462,000 colonies (in all the catches of 

 the day combined). It is absent altogether from September to March, and 

 does not begin to increase very much until June. 



Messrs. West record species of Dinobryon from Ennerdale Water, 

 Wastwater, Loch Lomond, Loch Katrine, and Loch Lubnaig, but they are 

 exceedingly rare, and occur only during a few of the summer months, and 

 then only in very small quantities. The genus appears to be more common 

 in Loch Lomond and Loch Lubnaig in September and August respectively. 



The Danish lakes are poor in Dinobryon, though a great spruicj maximum 

 has been known to occur in May in certain takes. Apstein found great 

 quantities of Dinobryon in the Baltic lakes. Two species predominate, 

 D. divergens and D. stipitatum. He found the genus common in one lake in 

 April, and it increased steadily until June was reached. There was a great 

 recession in July, and only a few were present in September. In another 

 lake, however, the numbers were greatest in June and July, when ten 

 individuals were present in every cc. of water, or a colony of twenty 

 individuals to every 2 cc. 



Lauterborn finds a maximum in May, and even finds what may be termed 

 a Dinobryon plankton. His numbers diminished during June, July, and 

 August, but there was a second maximum in October. Apstein, as the result 



E.I. A, PROC, VOL. XXX., SECT, B, [-Ef ] 



