BRITISH DESMIDIEiC. 183 



Length of cell Yrrr of an inch ; breadth ^^Vr- 



Tab. XXXI. fig. 1. a, b. fronds with endochrome ; e. distorted frond; 

 d. empty frond. 



2. P. Heptactis (Ehr.) ; frond constituted of seven cells disposed in a 

 circle round another in its centre j cells bipartite with emarginate 

 lobes. 



Micmsterias heptactis, Ehr. Abh. d. Berl. Ak. (1833), p. 300 ; Infus. p. 156. 



t. 11. f. 4. b, e, d. 

 Euastrum hexagonum, Corda, Almanack de Carlsbad 1835, p. 122. f. 31. 

 Pediastrum heptactis, Meneghiui, Synop. Desmid. in Linncea 1840, p. 211. 



Ralfs, Annals of Nat. Hist. v. 14. p. 469. t. 12. f. 5 ; Trans, of Bat. 



Soc. of Edinburgh, v. 2. p. 157. t. 17. 

 Pediastrum simplex, HassaW, Brit. Freshwater Algce, p. 388 (1845) ; not of 



other authors. 



Barmouth and Penzance, /. R. Beckley Furnace, Sussex, and near South- 

 ampton, Mr. Jenner. Rochdale, Mr. Coates. Near Aberdeen, Mr. P. Grant. 



Germany, Ehrenberg and Corda. Mexico, Bailey. 



Frond very minute, composed of eight cells, one in the centre and seven in 

 a circle round it ; the cells connected by a gelatinous matrix, which forms 

 colourless interstices. The seven external cells are bipartite, their lobes end- 

 ing in a broad shallow notch with acute comers ; 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 veiy pale. 



Length of cell 2To^ ^i an inch ; breadth g-rVo • 



Tab. XXXI. fig. 2. a, b. fronds with endochrome ; c. distorted frond ; 

 d. empty frond. 



3. P. biradiatum (Meyen) ; outer circle generally of eleven bipartite 

 cells, with bifid or emarginate lobes. 



/3. Lobes of outer cells truncato-emarginate. 



Pediastrum. biradiatum, Meyen, Nov. Act. Acad. Nat. Cur. v. 14. f. 21, 22 



(1828). Meueghini, Synop. Desmid. in LirmcBa 1840, p. 211. 

 Micrasterias Rotida, Ehr. Infus. p. 158. t. 11. f. 7 (1838). 

 Pediastrum Rotula, Kiitzmg, Phycologia Germanica, p. 143 (1845). 



a. Penzance, J. R. Ambleside, Mr. Sidebotham. Rochdale, Mr. Coates. 

 13. Beckley Furnace, Sussex, and near Southampton, Mr. Jenner. 



Germany, Meyen, Ehrenberg, Kutzing. New Jersey, Bailey. 



Frond minute, generally composed of 1 6 or 17 cells arranged in two con- 

 centric circles, with a solitary cell in the centre ; this central cell is said to be 

 often wanting. I have never found it in British specimens, nor indeed even a 

 vacancy in its place. The five angular cells forming the inner circle are often 

 quadrilateral, and the exterior of each has a linear notch. The outer circle 

 consists of eleven cells, which are either quadrilateral or somewhat cuneate ; 



