124 BIOLOGY OF THE RICHMOND RIVER, 



This is var. ji Kiitzing, Bac, T.30, f.68. It is quite unlike a, 

 but the '''•forma " recorded above is quite certainly intermediate. 

 Var. convexa, which is the characteristic Cyclotella of the river 

 here, has a sharply defined exceedingly smooth area (generally in 

 diameter about half that of the cell) in the centre of the face of 

 the valve, and this part alone is undulate (but the undulation is 

 often slight and sometimes absent). The marginal portion of the 

 valve is convex, and in large forms of 30 /x and over, the stri^ 

 appear to be the edges of radiating corrugations. In a tilted 

 cell, these show as crenulations at the edge. There is no definite 

 rim. 



Var. QUADRATA, n.var. (PI. v., f.21, 22). 



Valvae sine striis, punctis autem intra margines notatae. Cellulae 

 a latere cingulato paene quadratic, lateribus rectis. Tota facies 

 valvae levissime undulata. 



Diam. 8-9; crass. 7 /x. 



Var. quadrata is a form produced from var. convexa by long- 

 continued mitosis. The striate marginal area has been gradually 

 whittled away concentrically, and the frustule now consists 

 merely of the central smooth area of the original cell, in which 

 the undulation occurs. Hence in this form the undulation in 

 girdle-view runs right across the valve instead of being confined 

 to the central portion. The marginal striae are represented by 

 faint puncta within the rim itself. The diam. of the valve also 

 is exactly the breadth of the smooth central area in the smaller 

 sizes of var. convexa. 



Var. BREViSTRiATA, n.var. (PI. v., f.23). 



Forma area centrali glabra prae diametro valvae latissima, area 

 marginali striata angustissima, striis brevissimis. 



Diam. 12-14, arese centr. 10 /x. Striae in ambitu circ. 30. 



This would seem to be an intermediate form in which the 

 marginal striate area is not yet quite deleted, or else one in which 

 it is gradually developing again by growth. The striae end 

 abruptly at the edge of the smooth central area, which is rela- 

 tively very wide. With central area of 10 /x broad, the valve in 

 var. convexa would have a diameter of 20-24 /x. 



