592 A intuit of the South African Museum. 



parallel (Fig. 40, c), and from these again to specimens with slightly 

 retuse sides (Fig. 40, </), which, in their turn, grade over to P. meso- 

 lepta. As the figures show the apices are also very variable, as regards 

 their actual shape. In none of the individuals were such short striae 

 encountered as West figures for his P. bicapitata. 



It would seem most satisfactory to group these diffei-ents forms as 

 follows : 



Navicula (Pinnularia) interrupta, W. Smith.* 



(1) Forma genuina. — Sides of valves straight and parallel; striae 

 interrupted at the centre of the valve; apices markedly capitate. 

 (Smith, PI. XIX, fig. 184.) 



(2) Forma biceps, Cleve. — Sides of valves straight and parallel ; 

 striae not interrupted at the centre of the valve ; apices markedly capi- 

 tate. (Van Heurck, PI. VI, fig. 14.) (Syn. : P. interrupta, var. /;>, 

 W. Smith ; P. biceps, Gregory.) 



(3) Forma bicapitata. — Sides of valves slightly retuse, but valve 

 linear: striae not interrupted at the middle of the valve; apices 

 markedly capitate. (Syn.: P. bicapitata, Lngerstedt, op. cit., PI. I, 

 fig. 5.) 



(4) Forma sub-bicapitata. — Sides of valves slightly retuse; striae 

 interrupted at the middle of the valve; apices subcapitate. (Fig. 

 nostr. 40, d.) 



(5) Forma subcapitata. — Sides of valves straight and parallel ; striae 

 interrupted at the centre of the valve ; ends not very markedly capi- 

 tate (Fig. nostr. 40, c). (Syn. : P. subcapitata, Greg. ; P. gracillinia, 

 Pritchard, var. subcapitata, Eabenh.) 



(6) Forma braunii. — Sides of valves more or less markedly convex ; 

 striae interrupted at the middle ; ends more or less capitate. (Syn. : 

 P. braunii, Grun.) (Fig. nostr. 40, a, b.) 



(7) Forma westii. — Sides of valves slightly convex; striae not 

 interrupted at the middle, but short ; ends slightly capitate. (G. S. 

 West, loc. cit., PI. Ill, fig. 13.) 



Perhaps one should also include P. mesolepta amongst these forms 

 (cf. Cleve, p. 76), but for the present I prefer to keep it distinct. It 

 should be added that all the specimens encountered in my material 

 agreed with one another in the nature of the striation ; the dimensions 

 were: Long., 51-66 jx: lat., 9-12 /a; striae, 10-12 in 10 p. 



* G. S. West (loc. cit., p. 79) also includes P. microstanron, Ehrenb., in this 

 group of similar species, but as I am not personally familiar with it, I have not 

 dealt with it here. 



