388 DESMIDE^. 



1. Pediastrum tetras Ralfs. 

 Plate LXXXVL Fig. 17. 



Char. Fronds small, cruciform. Cells four, deeply bipartite. 



Divisions, truncate, emarginate. 

 Micrasterias tetras Ehr., Infus. p. 155. t. xi. f. 1. Hete- 



rocarpella tetracarpa Bory de St. Vincent, Diet. 



Classique d'HIst. Nat. 1825. Heterocarpella poly- 



morpJia Kiitzing, in part, Linnjea, 1833, t. xix. fig. 82. 



Stauridiwn bicuspidatum, Stauridium crux melitensis 



Corda, Almanac de Carlsbad, 1835, pi. iii. figs. 33, 34. 



Pediastrum tetras Ealfs, in Annals, yoL xiv. p. 469. 



pi. xii. fig. 4. 

 Hab. Barmouth and Penzance : Mr. Ralfs. Becklej 



Furnace, near Battle, Sussex : Mr. Jenner. 



A very rare and elegant species, and one of the least va- 

 riable of the genus. 



2. Pediastrum simplex Hass. 



Plate LXXXVL Fig. 18. 



Char. Frond constituted of seven, cells disposed in a circle, con^ 

 taining 171 the centre one or two other cells tvhich are emar- 

 ginate. Divisions of the marginal cells slightly dentate. 



Pediastrum hepdactis Ralfs, in loc. cit. 

 Hab. Barmouth : Mr. Ralfs. Beckley Furnace : Mr. 

 Jenner. 



Distorted fronds of this species frequently occur, in which 

 the central cell is altered in form and displaced in situation. 

 See PI. Lxxxvi. fig. 18. a. Meneghini describes a single 

 hyaline vesicle in each cell. 



Under the name of Micrasterias heptactis, Ehrenberg seems 

 to me to have figured two species, fig. 4. a. t. xi., being dif- 

 ferent from b, c, d, which certainly represent the present 

 plant. 



