52 Proceedings of the Royal Irish Academy. 



of investigations, divided lakes into Chroococcus and Dinobryon lakes. The 

 characteristic conditions are stated by him to be as follovrs : — 





Chroococcus lake. Dinobryon lake. 



Chroococeaceae, 



. common. 



rare. 



Dinobryon, 



. none or rare, 



numerous 



Chydorus, 



. pelagic. 



littoral. 



Plankton in general, 



. rich. 



poor. 



"Water, . 



. muddy, 



clear. 



One of the first objectors to this theory was Zacharias. Apstein states 

 that if there is much Clathrocystis, for example, in a lake there is little 

 Dinobryon. Eeighard, however, for St. Clair, showed that the features were 

 not as Apstein's theory would have made them. He found 



Clathrocytis, . . common. 

 Dinobryon, . . common. 



Plankton, . . . poor. 



"We find Chroococeaceae common, Dinobryon common during certain 

 months, and plankton rich. The theor}' does not seem to hold good, and only 

 expresses conditions seen perhaps in the lakes examined by Apstein. 



Dinobryon is recorded for Greenland and Lapland often under the ice. 

 It occurs in the alpine lakes, and is recorded by Kofoid for the Illinois Eiver. 

 Kofoid considers all the species as growth varieties of the one species. 



Pedias train. 



The genus Pediastrum is represented in our plankton chiefly by the 

 species P. Boryanum, P. Boryamun var. longicorne, and P. duplex var. clathratum, 

 P. dvfphx var. retindatuni Lagerh. Pediastrum is a fairly frequent constituent 

 of the plankton. It is absent only in December. The two species were 

 studied separately, but the varieties were counted together. 



P. Boryanum (Turp.) Menegh. has two maxima, one in April, when it 

 reaches 37,200 in all the catches, and another slightly larger in August, 

 numbering .39,000. In July it diminishes to 1500. After the August 

 maximimi it diminishes until December, when none were found in any of the 

 catches. From February onwards it is present in fair quantities. 



P. duplex Meyen (= P. pertusum Kiitz) has a similar seasonal variation to 

 P, Bo^ryanum, but is usually somewhat less abundant. After the early 

 maximum in April it diminished until the early part of July, when none were 

 found at all. In August it reaches a higher maximum than P. Bo-yanum, 

 viz., 45,500 in all the catches. Then it diminishes suddenly and is absent 

 altogether in December and January. 



