OSTENFELD: THE PHYTOPLANKTON OF VICTORIA NYANZA. 177 



tween them. When I studied the Victoria Nyanza samples collected by- 

 Professor A. Borgert, I could not identify the specimens observed with 

 Midler's species as they seemed to form continuous series from one 

 species to another. I therefore asked Professor Miiller to examine my 

 slides, and this he did with his usual readiness, and placed at my disposal 

 the results of his examination, permitting me to publish them in my 

 paper. From his notes therein I quote the following sentences, which 

 show that my suggestion on the absence of distinct limits between the 

 species was quite correct : — - 



" Die im Plankton des Victoria Nyanza enthaltenen Surirellen sind 

 . . . mannigfacher gestaltet und in grosserer Anzahl vorhanden, als 

 diejenigen des Nyassa-und Malombasees. In den letzgenannten Seen 

 sind die drei Arten S. Nyassce, S. Malombce und S. Engleri ziemlich 

 scarf begrenzt ; im Victoria Nyanza dagegen finden sich vorwiegend 

 Ubergangsforinen, d. h. die typischen Arten kommen nur selten und 

 meistens in veranderten Grossen vor, an ihrer Stelle aber sind gleitende 

 Ubergange zwischen den drei Arten vorhanden" (I. c. p. 340-341). 



He then gives a number of measurements of the observed specimens, 

 studying the limits of their variation, and concludes : — " es diirfte daher 

 liber den Zusammenhang von S. Nyassce und S. Malombce kaum ein 

 Zweifel bestehen" (I. c, p. 341); and further : — " Alle diese Abwei- 

 chungen bilden eine Reihe, deren Endglider S. Nyassce und S. Malombce 

 sind. Ebenso deutlich ist der Zusammenhang von S. Nyassce und S. 

 Engleri, var. constricta 1 ' (I. c, p. 342). 



In his own paper on Nyassa Diatoms part 1 (1903), 0. Miiller has illus- 

 trated the new species of Surirella with beautiful drawings ; we find there 

 the typical forms of his species ; but the intermediate stages — such as 

 they occur in Victoria Nyanza — have not been illustrated in my former 

 paper. Therefore I take the opportunity to fill out this lacuna in repro- 

 ducing a series of microphotographs taken by Mr. A. Hesselbo under my 

 direction. When one wishes to demonstrate a series of transition stages, 

 it is always better to use the camera than to rely upon drawings which 

 can be spoken of as influenced by the author's own opinion. 



All the microphotographs have been taken at the same magnification 

 (X 300) and are fully comparable. Fig. 1 represents a typical S. nyassae 

 O. M1U1., Figs. 9-10, typical 8. malombae O. Mull., and Fig. 13, typical 

 S. Ewileri O. Mull, var. constricta O. Mull. I trust that the figures from 

 1 to 10, Plate 1, show at once how continuous the transition from »S'. 

 nyassae to S. malombae is; further that in Fig. 2, 3, and 4 we find trans- 

 itions through Fig. 11 and 12 to Fig. 13, that is from S. nyassae to S. 



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