388 DESMIDE^E. 



1. Pediastrum tetras Ralfs, 



Plate LXXXVI. Fig. 17. 



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

 Divisions, truncate, emarginate. 



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

 rocarpella tetracarpa Bory de St. Vincent, Diet. 

 Classique d'Hist. Xat. 1825. Heterocarpella poly- 

 morpha Kiitzing, in part, Linn^ea, 1833, t. xix. fig. 82. 

 Stauridium hicuspidatum, Stauridium crux melitensis 

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

 Pediastrum tetras Ralfs, in Annals, vol. xiv. p. 469. 

 pi. xii. fig. 4. 



Hah. Bai-mouth and Penzance : Mr. Ralfs. Beckley 

 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 LXXXVI. Fig. 18. 



Char. Frond constituted of seven cells disposed in a cii^cle, con- 

 taining in the centre one or two other cells which are emar- 

 ginate. Divisions of the marginal cells slightly dentate^ 



Pediastrum heptactis Ralfs, in loc. cit. 



Hah. 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 3Iicrasterias 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. 



