184 BRITISH DESMIDIE^. 



the free margin is deeply bipartite, and the lobes are again divided or truncato- 

 emarginate. The endochrome is bluish-green, with minute scattered granules. 

 Interstices between the cells hyaline. 



Kiitzing unites this and the two preceding species under Ehrenberg's name, 

 Rotula ; Meneghini keeps it distinct. Typal specimens of this species differ 

 from P. Tetras and P. Heptactis, not only in the number of the cells, but also 

 in the deeply divided and tapermg lobes of the outer cells. Specimens how- 

 ever of the variety /3, gathered in Sussex by Mr. Jenner, render it doubtful 

 whether Kiitzing' s A-iew is not the correct one ; for whilst these specimens have 

 the same number and disposition of cells as P. biradiatum, the lobes of their 

 marginal cells are merely slightly emarginate like those of P. Heptactis. 



Length of cell in the outer circle ^wos ^^ ^"^ '^^^^ 5 breadth ytTX' 



Length of /3. -^^Vo 5 breadth ^^Vo- 



Tab. XXXI. fig. 3. a. frond with endochrome ; b. empty frond : fig. 4. o, b. 

 fronds of /3. with endochrome ; c. empty frond. 



** Lobes of the segments entire. 



t Connecting substance coloured. 



4. P. Selencea (Kiitz.) ; cells crescent-shaped, arranged in one or 

 more circles around one or two central ones ; connecting medium 

 coloured. 



Selencea orbicidar'is, Nitzsch, Kiitzing, Synopsis Diato7n. inLinnceal833,Tp. 604. 

 Micrasterias Selencea, Kiitzing, Synop. Diatom, p. 604. f. 92 (1833). 

 Micrasterias Boryana, Ehrenberg, Infusor. t. 11. f. 5. e (1838). 

 Pediastrum Napoleonis, Ralfs, Annals of Nat. Hist. v. 14. p. 470. t. 12. f . 6 



(1844) ; Trans, of Bot. Soc. of Edinburgh, v. 2. p. 158. t. 17 ; not of 



Meneghini. 

 Pediastrum Selencea, Kiitzing, Phycologia Germanica, p. 143? (1845). 

 Pediastrum elegans, Hassall, British Freshtoater Algce, p. 389 (1845). 

 Pediastrum lunare, Hassall, British Freshwater Algce, t. 92. f. 3 (1845). 



Barmouth, North Wales, J. R. 



Germany, Kiitzing, Ehrenberg. 



Pediastrum Selencea, like the other species of this genus, varies in the 

 number of cells. Kiitzing' s figures represent three states, ha-sang respectively 

 one, two, and three circles, romid a central cell, all the cells being lunate ; the 

 number of cells consisting of 5 in the first circle, 10 in the second, and 15 in 

 the third. 



The following description applies to British specimens only. 



Frond minute, with two angular cells in the centre, and six crescent-shaped 

 ones arranged in a circle round them. The cells are not approximate, or 

 separated by hyaline interstices, as in other species ; but are connected by a 

 coloured portion, which indeed may be said to constitute the frond, the cells 

 being imbedded in it. It will be seen that the British specimens differ from 

 those figured by Kiitzing by having two cells in the centre mstead of one, and 

 six in the circle instead of five. I have never seen specimens with more than 



