The 'Freshwater Algae of Africa. 



59] 



forma biceps differs from /'. interrupta proper in the fact thai the 

 striae are not interrupted at the centre of the valve. According to 

 these different authorities, therefore, /'. interrupta may have straight 

 or slightly ret use sides, may or may not have the striae interrupted at 

 the centre of the valve, and may apparently also vary somewhat in the 

 dimensions and shape of the capitate ends. More recently, G. S. West 

 (Algae Yan Yean Reservoir, Journ. Linn. Soc , Bot,, xxxix, 1909 

 pp. 78. 79. PI. Ill, fig. 13) has referred still another form to P. bicapi- 



Fig. 40. — a-d. Navicula interrupta, W. Smith, diverse forms, cf. the text. 

 e. N. pupula, Kuetz., var. subcapitata, Hustedt (the very fine and nnmeroiis 

 radiating striae are not shown). /. N. borealis, Ehrenb., f. rectangularis, Carl- 

 son, g. Cymbella pusilla, Grun. h. Eunotiabicapitata,'Grira.iL. h'. Endof valve 

 of same, strongly enlarged, a-f x 1000; g x 1800; h x 500; h' x 2700. 



tata, in which the sides are slightly convex, the ends larger and not so 

 clearly demarcated, and the striae appreciably shorter. 



In the material from the Cape Peninsula the Diatom, above referred 

 to P. interrupta, invariably had the striae interrupted at the centre of 

 the valve (Fig. 40, a-d), Out the shape of the latter was subject to very 

 considerable variation. In many cases the valves were lanceolate, 

 tapering gradually from the middle towards the ends, which were more 

 or less markedly capitate (Fig. 40, a, b) ; this form is certainly very 

 like P. bran nil. although not cpaite so tumid as Van Heurck (PI. VI, 

 fig. 21) figures it. From such a form all transitions are found to 

 specimens in which the sides are almost absolutely straight and 



