388 DESMIDE^!. 



1. PEDIASTRUM TETRAS Rolfs. 

 Plate LXXXVI. Fig. 17. 



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

 Divisions, truncate, emarginate, 



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

 rocarpella tetracarpa Bory de St. Vincent, Diet, 

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

 morpha Kiitzing, in part, Linna3a, 1&33, t. xix, fig. 82. 

 Stauridium bicuspidatum, 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. 



Hob. Barmonth 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 genusw 



2. PEDIASTRUM SIMPLEX Hass. 

 Plate LXXXVI. Fig. 18. 



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



Hob. 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. 



