Mr. J. Ralfs on the British Desmidiese. 469 



All the above characters, it must be allowed, are more or less 

 uncertain ; still upon them we must for the most part depend in 

 discriminating between nearly allied species. 



Meneghini adopts as a specific character the number and po- 

 sition of hyaline vesicles in each cell ; I regret that I have not 

 paid sufficient attention to this point to enable me to determine 

 its value. 



Professor Bailey suggests that the form of the cells may be a 

 more certain character, but I believe that in this respect also they 

 are variable ; for even in the same species the segments of the 

 marginal cells are in some specimens much elongated, and in 

 others very short. 



The segments are in some species more or less emarginate, 

 and in others entire ; as this character appears to be constant, I 

 have taken advantage of it to divide the genus into two sections. 



Under Micrasterias I have stated my reasons for agreeing with 

 Meneghini and retaining Meyen's name for this genus, which 

 Ehrenberg has supposed to be the Micrasterias of Agardh. 



* Segments of the outer cells emarginate. 



1. P. Tetras. Cells four, separated by colourless interstices which 

 form a cross ; segments truncato- emarginate. Micrasterias Tetras , 

 Ehr. Infus. p. 155. tab. 11. fig. 4 ; Pritch. Infus. p. 192. 



In boggy pools : rare. Barmouth and Penzance, J. R. ; Beckley 

 Furnace near Battle, Sussex, Mr. Jenner. 



Fronds extremely minute, composed of four cells, which form 

 a star-like figure ; the cells are somewhat triangular, and con- 

 nected together by a hyaline matrix which forms colourless inter- 

 stices in the figure of a cross ; the free margin is bipartite ; the 

 segments terminated by a broad, shallow notch with acute corners. 

 The colouring matter is pale green. 



Meneghini gives this species as a synonym of Pediastrum hep- 

 tactis, but I cannot suppose it to be the young of that plant. At 

 Penzance I have never gathered the P. heptactis, and at Bar- 

 mouth, where I found them intermixed, I have not seen any in- 

 termediate forms. 



Plate XII. fig. 4. P. Tetras. 



2. P. heptactis, Mgh. Frond constituted of seven cells disposed in 

 a circle and one in the centre ; segments of the cells emarginate. 

 Menegh. /. c. p. 211. Micrasterias heptactis, Ehr. Infus. p. 156. 

 tab. 11. fig. 4 ; Pritch. Infus. p. 193. fig. 114. Micrasterias reni- 

 carpa, Kutz. Syn. Diatom, in Linnsea 1833, p. 603. 



In boggy pools : very rare. Barmouth, J.R.; Sussex, Mr. Jenner. 



Fronds very minute, composed of eight cells, one in the centre 



and seven in a circle round it ; the cells are connected together 



