BRITISH DESMIDIE^. 121 



and Brebisson, the rule established in such cases scarcely permits its 

 exchange for the more appropriate term. 



Sporangia of several species have been gathered. They are gene- 

 rally spinous, but differ in other respects. 



A little care will distinguish Staurastrum from the other genera in 

 this family, although some of its species appear at first sight to ap- 

 proach forms which belong to them. It differs from Desmidium in 

 never forming a filament ; and from Arthrodesmus and Cosmarium by 

 its angular shape, or by having the ends elongated into processes. 

 Some species bear a considerable resemblance to species of Xan- 

 thidium, to which genus Ehrenberg refers them ; but in Xanthid'mm 

 the frond, irrespective of the spines, is not angular in the end view, 

 and there is a projection at the centre of each segment in the front 

 view. 



* Frond smooth, or rough with minute puncta-like granules ; end view 

 with the lobes or angles inflated and mucronate or awned. 

 t Frond smooth. 

 1. S. dejectum (Breb.) ; segments smooth, lunate or elliptic; con- 

 stricted portion very short ; end view with inflated awned lobes. 

 a. Segments externally lunate, awns directed outwards. 



Staurasti'um dejectum, Breb., Menegliini, Synop. Desmid. in Linncea 1840, 



p. 227. Brebisson, in lit. cum icone. 

 Staurastrum mucronatum /3, Ralfs, in Annals of Nat. Hist. v. 15. p. 152 



(1845). t. 10. f. 5. 



j3. Segments elliptic, awns parallel. 

 7. Awns converging. 



Staurastrtim mucronatum a. and y, Ralfs, i)i Annals of Nat. Hist. v. 15. 



p. 152. t. 10. f. 5 ; Trans, of Bot. Soc. of Edinburgh, v. 2. p. 139. t. 13. 



Jenner, Ft. of Tunhridge Wells, p. 192. 

 Goniocystis (Trigonocystis) mucronata, Hass. Brit. Alg. p. 350 (1845). 



Common. Dolgelley and Penzance, /. R. Weston Bogs near South- 

 ampton ; Sussex ; Surrey ; and Kent, Mr. Jenner. Kerry, Mr. Andrews. 

 Yate near Bristol, Mr. Broome. Aberdeenshire, Dr. Dickie and Mr. P. 

 Grant. Ambleside, Mr. Sidehotham. Rochdale, Mr. Coates. 



Falaise, Brebisson. West Point, New York, Bailey. 



Frond smooth, deeply constricted at the middle ; segments broader than 

 long, Imiate or elliptic, awned, the awns subulate, varying in length and di- 

 rection. The end view shows three or four mammillate lobes or rays, each 

 terminated by a hair-like mucro or awn. 



Sporangia of this plant are more commonly met with than those of any 

 other species in the family except Hyalotheca dissiliens. Since I first de- 

 tected them at Dolgelley, I have gathered them more or less abundantly every 

 year, both at Dolgelley and at Penzance ; they have also been sent mc by 

 Mr. Jenner, Mr. Broome, Dr. Dickie and Mr. Thwaites. 



