PEDIASTRUM. 387 



1. HOLOCYSTIS OSCITANS HaSS. 



Plate XC. Fig. 4. 



Char. Frond large, smooth. Segments dilated below, con- 

 tracted above, and again dilated into a large triangular 

 lobe, lohich is somewhat rounded on its distal side. Angles 

 spinous. 



Micrasterias 9 oscitans Ralfs, MSS. ; Jenner, in Flora of 

 Tunbridge Wells. 



Hab. On Ashdown Forest, near Duddleswell : Mr. 

 Jenner. 



Of this species I noticed several fine specimens mixed up 

 with other Desmide(B sent me by Mr. Sidebotham. 



64. PEDIASTRUM Meyen. 



Char. Fronds compressed, circular, or star-like, each com- 

 posed of several cells, arranged in one or more united con- 

 centric circles. Marginal cells bipartite. 



Micrasterias Ehrenberg. 



Derivation. From TrsStoi^, a bond or fetter, andacrrpov, a star. 



The DesmidecB are a beautiful tribe of productions ; and 

 none of them more so than the subjects of the present genus, 

 which cannot be confounded with any other. Distinct, how- 

 ever, as the genus Pediastrum is, considerable difficulty is ex- 

 perienced in the determination of the species, owing to there 

 being no one character which can be depended upon ab- 

 solutely in their definition ; the size of the fronds, number 

 of cells in each, and the degree of incision of the marginal 

 cells varying considerably in each : there is, however, a cer- 

 tain mean in these particulars which enables the observer to 

 determine in most cases the species or variety to which each 

 frond which presents itself to him really belongs. 



Meneghini has noticed the occurrrence of transparent 

 vesicles in the cells of the frond, the number of these varying 

 according to the species : he has therefore availed himself of 

 these vesicles in their determination. 



c c 2 



