68 Proceedings of the Royal Irish Academy. 



Tabellaria fenestrata. 

 In the Danish lakes this appears in stellate form in the summer months 

 June, July, and in chains for the grqater part of the year. Schroeter also 

 found stars in summer, hut chains in winter. In Lough ISTeagh Tabellaria 

 fenestrata is common all the year round, and is always found in stars. A few 

 zigzag chains may sometimes he found in the cooler months ; but they are rare 

 and often end in a star. We have made tables illustrating the number of 

 cells in the colonies, and find rather an interesting variation. Taking the 

 average number of individuals in a colony from several counts, we have the 

 following table :— 



Average Nuniber of Individuals in the Colonies of Tabellaria. 



1911— Feb. 23, 4-85 ; March, 3-8 ; April, 3 ; May, 57 ; June, 5-3 ; 



July, 5-5; Aug., 6-6; Sept., 4-3 ; Oct., 7-2 ; Nov., 8-1 ; 



Dec, 8-3. 1912— Jan., 7-6; Feb., 6-1. 



The maximum occurs between June and September. The largest number 



of individuals in colonies occurs just after this maxinmm. The smallest number 



of individuals in colonies occurs in March and April just after the minimal 



catches were made. 



The variation in size is extremely slight, as the following figures show : — 



1911— Feb. 23, -336 ; 



June, "338 ; 



Oct., -322 ; 

 1912— Jan., -31 ; 



These figures show that the smallest occur in the winter and when the 

 largest number of individuals are found in the colonies. 



Asterionella gracillima. 



In the case of Asterionella we have also tabulated the number of 

 individuals in stars or colonies. 



The variation is as follows : — 

 1911— Feb. 23, 5-45 ; Mar., 3-3 ; April, 4-4; May, 57; June, 5'6; 

 July, 4-6; Aug., 5-4; Sept., 5-8 ; Oct., 8-0; Nov., 67 ; 

 Dec, 6-3. 1912— Jan., 7-05 ; Feb., 5-9. 



Thus in both Tabellaria and Asterionella, for some reason,the largest colonies 

 occur in the months October to January. Another similarity is that this 

 maximum number of individuals in colonies occurs in both forms after the 

 great maximum of colonies in the plankton. The smallest numbers in the 



March 5, 



•337; 



April 13, -326 ; 



May, •338: 



July, 



•348; 



Aug., -328 ; 



Sept., -333, 



Nov., 



•306; 



Dec, '306. 





Feb. 3, 



•307. 







