Dakin and Latarche — The Plankton of Lough Neagh. 39 



breaking up of the ice in December and lasting through January. No maxima 

 occurred, however, at the time in those lakes which did not freeze. Lough 

 Neagh did not freeze during the year of our visits ; and as will be seen from 

 the tables, the numbers present in the winter months are not far removed 

 from a minimum. The usual dates for maxima given by other observers 

 agree with that for Lough Neagh — July-August. These dates are recorded 

 by Lemmerman, Wesenberg Lund, and the Messrs. West. In Loch Lomond, 

 just as in Lough Neagh, the maximum occurs in June-August, and is 

 associated, just as with us, with Tabellaria. 



There is, however, considerable difference in the two types of lake. Our 

 curves for Asterionella agree almost exactly with those of Messrs. West for 

 Loch Lomond. In Windermere, on the other hand, there are two maxima 

 recorded for Asterionella, one in May-June, and another in November. 

 Ennerdale Water is again very different, and this is a Desmid lake. Form 

 variations are recorded elsewhere (p. 68). 



A. gracillima and A. formosa both occur in Illinois Eiver, the latter being 

 very much less abundant than the former. 



A. gracillima attains its maximum about the end of April, there being no 

 autunmal maximum. It practically disappears from the plankton in July 

 until January. 



A. formosa was recorded by Kofoid in November-December, and from 

 February to April. The maximum occurred towards the end of March. He 

 remarks that " this species is distinguished with difficulty from A. gracillima 

 and may include only old, and in our planktons often heavily incrusted, 

 individuals, or it may be only a low- temperature variety of the species above 

 named, which, in the grand total of all our collections, outnumbers it ten 

 thousand to one." We are inclined to agree with this statement, as our 

 observations of these species in Lough Neagh point to the same conclusions. 



Melosira. 



Melosira italica subspec. stiburctica 0. Miiller. 

 islandiea „ helvetica 0. Miiller. 

 crenulata Kiitz. 

 varians Ag. bottom species. 

 arenaria Moore. 



The above species have been recorded by us from Lough Neagh. The three 

 last are also recorded by West with the addition of M. granulata (Erent) Ealfs. 

 We have considered all the species together in our counts. The most common 

 appear to be the three first-named above. 



