182 BRITISH DESMIDIE^. 



of Ehrenberg's figures, and not of the plants themselves, his opinions 

 are entitled to less weight than they would have been if founded on 

 his own observation. 



Professor Bailey has suggested, in his paper on the American Des- 

 midieae, that the form of the cells might afford the most certain cha- 

 racter. Kiitzing, who has described nine species in his * Phycologia 

 Germanica,' seems to have arrived at the same conclusion. I have 

 ventured in some instances to differ from Kiitzing's views ; yet I feel 

 persuaded that his definitions of the species in Pediastrum are truer 

 to nature than those of Ehrenberg. 



I have not seen the cells during the process of division ; but I am 

 informed by M. de Brebisson that it takes place at the notch, in the 

 same manner as in other Desmidieae : hence the cells in each circle 

 are connected at their ends, like those of the filamentous genera. I 

 do not however understand in what manner the additional circles are 

 formed, nor why the numbers in each circle are so constant. 



The segments or lobes are in some species more or less emarginate, 

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

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



* Lobes of the outer cells emarginate, or truncate. 



1. P. Tetras (Ehr.) ; cells four, separated by colourless interstices 

 which form a cross ; lobes truncato-emarginate. 



Micrasterias Tetras, Ehr. Infus. p. 155. t. 11. f. 1 (1838). Bailey, Amer. 



Journ. of Science and Arts, v. 41. p. 293. t. 1. f. 19. 

 Pediastrum Tetras, Ralfs, Annals of Nat. Hist. v. 14. p. 469. t. 12. f . 4 



(1844) ; Trans, of Bat. Soc. of Edinburgh, v. 2. p. 157. t. 17. Hassall, 



Brit. Freshwater Alg. p. 388. 



Barmouth ; Dolgelley and Penzance, J. R. Beckley Furnace near Battle, 

 Sussex, Mr. Jenner. Ambleside, Mr. Sideltotham. Near Aberdeen, Mr. P. 

 Grant. 



Germany, Ehrenberg. Maine to Virginia, Bailey. 



Frond extremely minute, composed of four cells, which constitute a star-like 

 figure ; the cells are somewhat triangular, and connected by a hyaline matrix, 

 which forms colourless interstices in the figure of a cross ; the free margin is 

 bilobed ; the lobes terminate in a broad shallow notch, which has acute corners. 

 The colouring matter is pale green. 



Menegbini makes Pediastrum Tetras a synonym of P. Heptactis ; but T 

 have not seen any intermediate forms, and in fact the frond of this species 

 is sometimes as large as that of P. Heptactis, although the latter contains 

 twice as many cells. 



