350 DESMIDEJE. 



Hob. Jack's Wood spring ; Cross in Hand, &c. : Mr. 

 Jenner. Dolgelly and Penzance : Mr. Ralfs. 



The fronds are rather large, smooth, deeply constricted in 

 the centre ; the segments, broader than long, are in close ap- 

 proximation with each other, and when taken together form 

 an orbicular frond in their front view, in which aspect the 

 species might be taken for a Cosmarium, from which genus 

 it differs in the triangular transverse form of the segments. 



T. orbicularis forms a distinct link between Cosmarium on 

 the one side and Desmidium on the other. It is distinguished 

 from the following species by its suborbicular frond and by 

 its blunt angles being destitute of processes. 



2. TRIGONOCYSTIS MUCRONATA Hass. 

 Plate LXXXIV. Fig. 8. 



Char. Fronds smooth, deeply constricted in the front view, seg- 

 ments cither transversely elliptical or lunate. End view 

 three-lobed. Lobes inflated, blunt, mucronate, a single 

 mucro terminating each lobe, 

 a In the front view the segments are transversely elliptic 



and the mucro straight. 



ft Segments, in front view, transversely lunate, outer mar- 

 gin straight, mucro curved outwards. 



7 Segments, in front view, transversely lunate, outer margin 

 curved, mucro curved inwards. 



Staurastrum mucronatum Ralfs, in Annals, vol. xv. p. 152. 

 pi. x. figs. 5, 6. ; Jenner, in loc. cit. p. 192. 



Hob. On Ashdown Forest ; Piltdown Common ; Fisher's 

 Castle, and near Cross in Hand : Mr. Jenner. Dol- 

 gelly and Penzance : Mr. Ralfs. 



This, although a variable species, would appear to be well 

 characterised by the lobed form of the segments viewed end- 

 ways and the presence of the spine-like teeth. 



Mr. Ralfs and Mr. Jenner have observed the occurrence 



