PEDIASTRUM. 387 



1. HOLOCYSTIS OSCITANS Ilass. 



Plate XC. Fig. 4. 



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

 tracted above, and again dilated into a large triangular 

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

 spiyious. 



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

 Tunbridge Wells. 



Hab. On Ashdown Forest, near Duddleswell : Mr. 

 Jenner. 



mixed up 

 with other 



Jenner. 



Of this species I noticed several fine specimens 

 ith other Desmidece sent me by Mr. Sidebotham. 



64. PEDIASTRUM Mcyeii. 



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

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

 centric circles. JSIarginal cells bipartite. 

 Micrasterias Ehrenberg. 

 Derivation. From ttsSiov, a bond ox fetter, and aarpov, a star. 



The Desmidca 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. 



]\Icncghini has noticed the occurrrence of transparent 

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

 according to the species : he has therefore availed himself of 

 these vesicles in their determination, 

 c c 2 



