40 Proceedings of the Royal Irish A cademy. 



Melosira occurs throughout the year, but differs from Asterionella and 

 Tabellaria in the great variation in quantity. Thus, for example, it vanishes 

 almost completely from September until February, that is, during the 

 winter months. Our maximum occurs 'in late February and early March, and 

 continues more or less towards the end of May. There is, however, only one 

 maximum, and no other maximum appears in October or September as 

 recorded by Apstein and Wesenberg Lund. In the same months, from June 

 onwards, Melosira is remarkably scarce with us. 



In the English lakes, Messrs. West record Melosira as completely absent 

 from Ennerdale Water. It is very rare in Wastwater. On the other hand, 

 the figures for Windermere agree almost exactly with those for Lough Neagh, 

 there being one maximum in April. In Loch Lomond Melosira is abundant 

 and dominant in June. At this period, moreover, a Melosira plankton is present, 

 and this never occurs in Lough Neagh. Melosira is one of the Central 

 European forms, and occurs chiefly in shallow and warm lakes. In no other 

 case do the observations on times of maxima show such remarkable variation 

 according to district. Thus the maximum has been recorded in November, 

 December, September, October, June, April, February, July, and May. 

 Melosira has often been noted as forming the food of Crustacea — Bosmina, 

 Daphnia, and Diaptomus. 



Kofoid records M. gramilata var. s^iinom and J/, variaiis from the Illinois 

 Eiver, the former being very much more abundant. M. varians is perennial, and 

 exhibits two well-defined maxima, a vernal one in April-May and an autumnal 

 one September- October. In July and August, 1894, 1895, and 1896 it was 

 almost absent, also in December, February in 1896-97 and 1898-99. 



Fragilaria. 



Fragilaria crotonensis is the only species that we have met with in the 

 plankton of Lough Neagh. This form was present throughout the year 

 except February, 1911. It was present, however, in considerable quantity in 

 February, 1910. The maximum occurred in March-April, and then the 

 numbers went down during May, June, and the first half of July. The 

 numbers were again up in August. The early maximum agrees with that 

 found by other workers, but occurred much earlier than is usually the case 

 (May onwards, Wesenberg Lund for the Danish Lakes, and July onwards, 

 Lemmerman for Ploner Sea). If the increase in July and August can be 

 called a second maximum, it is earlier again than the second maximum 

 recognized on the Continent in September. Obviously the Lough Neagh 

 conditions are very different from those observed in the Baltic and Danish 

 lakes. 



