OSTENFELD : THE PI1YTOPLANKTON OF VICTORIA NYANZA. 175 



abundant than M. nyassensis. Melosira nyassensis and the forms of 

 Surirellae are less abundant in February than later in the year, and 

 Nitzschia nyassensis is very rare. 



In the samples from Shirati I have found a few species of the peculiar 

 form of Geratium hirundineUa which inhabits Victoria Nyanza. It has 

 also been recorded from April, October, and November, but at all times 

 very sparsely. It would be quite strange if this organism, which in 

 temperate lakes plays so dominant a part in the composition of the plank- 

 ton and which always has a great maximum of development, should not 

 behave in the same manner in Victoria Nyanza, but hitherto we have no 

 indications as to this point. 



If we pass over to the Green Algae we find that several species have 

 been found in the February samples, but that all are very rare, and it 

 must be added that some of the individuals met with were dead, espe- 

 cially among the species of Staurastrum. Also the Blue-green Algae are 

 very unimportant in the samples, only Microcystis aeruginosa and Lynghya 

 Lagerheimii reaching to -(-. Of these Microcystis occurs in colonies of 

 a peculiar kind, the mucus-envelope being unusually firm (a resting 

 stage 1) ; Lynghya Lagerheimii is a tycholimuetic species carried out in 

 the water from the shore and bottom by the waves. 



The table shows further that there is very little difference between the 

 two samples. 



3. Remarks on Some op the Observed Species. 

 A. Peridiniales- 



Geratium Mrundi?ieUa (0. F. MM.) Schrank, var. brachyceras (v. 

 Dad.) Ostf., I. c, p. 345 ; G. brachyceros E. v. Daday, Plankton-Tiere 

 aus dem Victoria Nyanza, in Zool. Jahrb. Abt. f. System., 1907,45, 

 p. 251, fig. A; G. hirundineUa, G. S. West, /. c, p. 189,- pi. 9, fig. 4. 



The form of Ceratium which occurs in Victoria Nyanza is, as above 

 mentioned, a very aberrant one. E. v. Daday has described it as a new 

 species closely related to G. hirundineUa and has given a rather rough 

 drawing of it. At about the same time G. S. West recorded it as G. 

 Urundinella and figured it very well. His description contains the 

 main points in which it differs from typical G. hirundineUa. He says 

 that "the few specimens observed differed very much from any others 

 which have come under my notice." The more important differences 

 from G. hirundineUa are : (1) the short and clumsy horns, (2) the very 

 much reduced second autapical horn and its place close to the first one, 



