470 Mr. J. Ralfs on the British Desmidiese. 



by a gelatinous matrix, which forms colourless interstices. The 

 seven external cells are bipartite, and their segments terminate 

 by a broad, shallow notch with acute corners ; in other respects 

 the cells are variable ; the figure of the central one especially 

 differs in almost every specimen, being sometimes angular and 

 entire, and sometimes deeply divided with rounded segments. 



The colouring matter is very pale. Meneghini describes a so- 

 litary hyaline vesicle at the base of each cell. 



Plate XII. fig. 5. P. hepiactis. 



** Marginal cells bipartite, their segments entire, and either acute or 



elongated. 



3. P. Napoleonis, Mgh. Six bipartite cells in a circle, and two 

 angular ones in its centre. Menegh. "I. c. p. 212. Micrasterias 

 Napoleonis, Kutz. /. c. p. 602 ; Ehr. Infus. p. 156. Helierella Na- 

 poleonis, Turp. (1820). 



In freshwater pools : rare. Dolgelley. 



Fronds minute, with two angular cells in the centre, and six 

 bipartite oues arranged in a circle round them. The form of the 

 external cells varies, as in most of the species of this genus ; they 

 are generally somewhat crescent-shaped with elongated and acute 

 rays ; the colour of the interstices is mostly pale green ; the en- 

 dochrome in the cells is minutely dotted. According to Mene- 

 ghini, hyaline vesicles, if present, are solitary in each cell. 



This species may be distinguished from all states of the follow- 

 ing one by having two cells in the centre and six in the circle. 



Plate XII. fig. 6. P. Napoleonis. 



4. P. Boryanum, Mgh. Central cell solitary, surrounded by one 

 circle of five cells, and generally by a second of ten cells ; occa- 

 sionally by a third and even by a fourth circle. Menegh. /. c. p. 210. 

 Micrasterias Boryi, Ktz. I.e. p. 603. Micrasterias Boryana, Ehr. 

 Infus. p. 157. tab. 11. fig. 5 ; Pritch. Infus. p. 193. fig. 5 ; Bailey, 

 Amer. Bacil. fig. 20. Micras. tricyclia, Ehr. Infus. p. 158. tab. 11, 

 fig. 8. Helierella Boryana, Turp. (1820). 



In freshwater pools, &c, not uncommon. Cheshunt, Herts, Mr. 

 Hassall ; Weston Bogs near Southampton ; Dorking, Surrey ; Stor- 

 rington Common, Brambletye near East Grimstead, and Tunbridge 

 Wells, Sussex, Mr. Jenner ; Dolgelley, Barmouth, and Penzance. 



This species, which is by far the most common, is a very va- 

 riable plant. There is always a single cell in the centre, which 

 may be surrounded by one, two, three or more circles. In the 

 most common form there are two circles, and in their common 

 centre a single cell which is pentagonal and has one face slightly 

 concave ; the inner circle consists of five hexagonal cells slightly 

 concave on the outer face ; the second or outer circle consists of 



