50 Proceedings of the Royal Irish Academy. 



Wesenberg Lund states that C. hiruiulinella appears in May in the 

 Danish lakes, and its maximum coincides with the highest temperatures 

 of the water. During September and October it disappears, often very 

 suddenly. The species is present in large quantities in the Baltic lakes. 



Tlie Dinoflagellata, on the whole, play only a secondary part in the arctic 

 lakes. The species C. hirundinella occurs in the lake of Enore in the three- 

 horned form and without seasonal variation. Eichard found three- and 

 partly four-horned individiials. 



C. hirundinella is the chief Dinoflagellate found iu the European alpine 

 lakes, where it occurs in large quantities. The same species occurs also 

 in quantities in the north European lakes, in Iceland and in the Faroes. 

 Kofoid records it from the Illinois Eiver during the summer of 1896, but not 

 in 1898. It was present from June to October, and attained its maximum 

 (19,200) on June 6th. 



Peridinium. 



West records the species F. Willci, P. tahulatum, and P. cindum from 

 Lough Neagh. The species P. tabulatum and P. cincturn alone occur in 

 sufficient quantities for counting purposes. The genus is at no time very 

 abundant ; and from the latter part of September until April it has not been 

 observed in the plankton. In August the maximum, 54,000, is reached, in 

 the bottom-to-surface catch. This occurs quite suddenly, as in July and 

 September there are not more 6,000 individuals present in the catch. 



"West records P. inconspicmim and P. Willei from Ennerdale Water. 

 The former occurs from May to November, with its maximum in August ; 

 the latter occurs from January to July, with its maximum in July. In 

 Wastwater, P. Willei occurs throughout the year, and attains its maximum in 

 July. In Loch Lomond it is absent from November to April, and reaches it 

 maximum in September. In Loch Katrine P. Willei is absent from October 

 to February, and reaches its maximum in July. It is present in Loch Earn 

 and Loch Lubnaig in August. 



In the British lakes Peridinium, although generally present, never occurs 

 in very abundant quantities, and its maximum occurs at different times in 

 the various lakes. 



Wesenberg Lund finds that in the Danish lakes P. tabulatum attained a 

 large maximum in April and a smaller one in October. P. cincturn is also 

 present in the Danish lakes. According to Apstein, P. tabulafimi appears in 

 April in the Ploner Sea and Dobersdorfen Sea, and reaches its maximum in 

 July. It disappears towards the end of November. 



In the arctic lakes the Dinoflagellata are unimportant, but P. Willei^ 

 P. cincturn, and P. umbonatum have been recorded. 



