OSTENFELD: THE PIIYTOPLANKTON OF VICTORIA NYANZA. 179 



Melosiua Agassizii, sp. nov. (pi. 2, fig. 18-22). Ex affinitate M. granu- 

 latae. Theca cylindrica, rohusta ; disci circulares, arde connati, margine 

 denticulato ; diacus vulvae terminalis dentibus longis ina&jualibus marginali- 

 hus jjraeditus ; pseudo-tailcus (sensu de 0. MuUer) distinctus ; pars cylin- 

 drica valvae ut visa aspectu cingulato rectangulata cum lateribus duohus 

 incurvatis, granidis (i^oris) rotundis in serielms curvatis (in valva 

 terminali rectis), 10-12 in 10 /x praedita ; sulcus distinctus in lumine 

 cellulae prominens. Diametrum cellulae {12-)2 4:— i:2 fj., altitudo pjartis 

 cylindricae valvae 9-14 /x.. 



This species is larger and coarser than M. nyassensis, and is, when 

 seen in side view, easily recognized by the more or less inwardly curved 

 sides of the cylindrical parts of valves, the inwardly prominent sulcus, 

 the round pores, and the long marginal teeth of the terminal valve. The 

 description given by 0. Mliller in my paper is as follows : " Die zweite 

 Art gehort offeubar zum Formenkreise von Melosira granulata, denu sie 

 besitzt die, diesen Art eigenthtimlichen, langen Dornen an den Endhalf- 

 ten des Fudens. Sie stimmt indessen nicht mit M. granulata selbst 

 iiberein. Die Zellwand ist starker als bei M. granulata, die inneren 

 Mantellinien sind nicht gerade, sondern nach dem Zelllumen zu konkav. 

 Die Porenreihen verlaufen in stark gekr mmten Linien, 10-12 auf 10 

 ix; in den Endhalften des Pervalvarachse parallel, 14-15 auf 10 (i. 

 Poren kreisrund. Sulcus eine Hohlkehle. Der Durchmesser schwaukt 

 von 12-30 jjL die Hohen der Zellhalften messen 9-15, 5 jx." 



It will be seen that this description agrees very well with my draw- 

 ings and my diagnosis. Only my specimens are somewhat larger ; I have 

 found the diameter varying from 24 /u, to 42 fi, while MliUer in his 

 material found 12 /a to 30 /a. Now my specimens were collected at a 

 time when the species has probably its maximum, and perhaps there has 

 been a formation of auxospores before this maximum. This would ex- 

 plain the differences in diameter ; but it is only a supposition, as I have 

 not succeeded in finding any trace of such auxospores. 



There is another matter with regard to this species which may have 

 some interest. Undoubtedly it was in full and active development in 

 February when taken ; this is evident from the many cell-divisions 

 observed. Although the preservation of the cell-contents has not been 

 entirely satisfactory, some points of interest have been made out, as I 

 hope the accompanying drawings will show. Plate 2, fig. 19 shows 

 two cells of a normal filament, not in cell-division ; through the preser- 

 vation the plasma has withdrawn from the inner side of the cell-wall ; 

 but the nucleus is distinct and the aggregated chromatophores are also 



