RRITISH DESMIDIE.E. 185 



one circle. Notwithstanding these differences, I beheve that our plant is cor- 

 rectly referred to this species. It agrees both in the crescent-shaped cells and 

 in the coloured connecting medium ; characters which distinguish it from all 

 others. 



Tab. XXXI. fig. 5. frond with endochrome. 



ft Inlei'slices of i he frond hyaline. 



5. P. simplex (Meyen) ; frond composed of four cells with or without 

 one or two central ones ; lobes of marginal cells ovate, tapering to 

 a point. 



a. Marginal cells truncate at the base and forming a circle, its 

 centre vacant or occupied by one or two cells. 



Pediastrum simplex, Meyen, Nov. Act. Acad. Nat. Cur. v. 14. t. 42. f. 1-3 



(1828). Kiitzing, Phycoloyia Germanica, p. 142. 

 Micrasterias simjjiex, Kiitzing, Si/iwpsis Diatom, in Linncea 1833, p. G02. 

 Micrasterias Coronula, Ehrenberg, lufasor. p. 156. t. 11. f. 2 (1838). 

 Pediastrum Napoleonis, Hassall, Brit. Freshwater Alyae, t. 92. f. 10, 1 1 



(1845). 



13. crvciatum', cells angular at the base, connected in a cruciform 

 manner without a central space. 



Pediastrum cruciatum, Kiitzing, Phycoloyia Go-mauica, p. 142 (1845) ; in 

 lit. cum icone. 



Near Aberdeen, Mr. P. Grant. 

 Germany, Kiitziny, Ehrenbery . 



In all the specimens that I have seen of this species, the number of cells was 

 invariably four ; but, according to Ehrenberg and other authors, it occasionally 

 has one or two other cells in the centre. I have observed two forms : in one 

 of them the cells were truncated at the base and the frond had a central square 

 vacancy ; in the other the bases of the cells extended into angles, and, as these 

 exactly fitted each other, the interstices or joinings formed a cross. The latter 

 state I beheve to be the Pediastrum cruciatum of Kiitzing, although the figures 

 which he sent me represent smaller fronds. 



At the base of each cell in /3. there is generally a large vesicle, which how- 

 ever is sometimes absent. At first sight this variety appears to be distinct, 

 but I have seen intermediate specimens, and, in all, the lobes of the cells are 

 ovate at the base and taper into a cuspidate point. 



Length of cell including lobes -x^^o ^^ ^" i^^^^ ' breadth xwn 5 length of 

 /3- tbVb- ; breadth from -^^ to ■^^. 



Tab. XXXIV. fig. 15. a. frond with endochrome ; b. empty frond ; c. frond 

 of /3 ; d. P. cruciatum from a drawing by Kiitzing. 



6. P. pertusum (Kiitz.) ; inner cells leaving hyaline intervals resem- 

 bling foramina ; outer cells rectangular, notch triangular, as broad 

 as the cell. 



Micrasterias Boryaau, Ehr. Infnsor. i. \\. i. b. b, d, i (1838). 



