Mr. .). Ralfs on (fie British Dcsniidicfc. 159 



a mucro at each extremity. Mgh. /. c. p. 228. Euastrum, Bailey, 

 Amer. Bacil. pi. 1. fig. I'i ? 



Shallow pools. Weston Bogs near Southami)ton ; Uackham Com- 

 mon near Pulborough, Sussex, Mr. Jcniier ; DolgcUey and Penzance. 



Fronds very minute, smooth, deeply constricted in the middle ; 

 segments externally lunate, about twice as broad as long, their 

 angles with a mucro wliieh is generally curved outwards. The 

 end view is elliptic with a mucro at each extremity. 



The front view of this species bears a considerable resemblance 

 to that of Staurasti-um mucronatum, but the end view is very 

 different. 



Plate XII. fig. 2. S. Incus: a, front view; b, frond dividing; c, end 

 view. 



16. S.} octocorne. Fronds smooth, compressed ; segments broader 

 than long, with four angles, each terminating in a spine ; end view 

 subelliptic, with a spine at each extremity. Arthrodesmus octo- 

 corni.^, Ehr. Infus. p. 152. 



Boggy pools near Dolgelley : rare. 



Fronds minute, deeply constricted in the middle ; segments 

 broader than long, having four angles, each of them terminated 

 by a slender spine, the intervals between them concave. The 

 spines diverge from each other. Endochrome pale. 



The newly-formed segments at first have only two spines, and 

 in this state somewhat resemble those of Staurasti-um Incus, of 

 which indeed this plant may eventually prove a variety. But S. 

 Incus has only two spines on each segment, and its end is not 

 concave but truncate. 



Meneghini refers the Arthi-odesmus octocornis, Ehr., to Micras- 

 terias ; but if the plant above described be identical with Ehren- 

 berg's, of which I have little doubt, it cannot be placed in a ge- 

 nus distinguished by its deeply lobed and incised fronds, and I 

 therefore presume the Micrasterias octocornis, Mgh., must be a 

 different plant from the Arthrodesmus octocornis of Ehrenberg*. 



The characters of Sfaurastrum are not strictly applicable, but 

 I have placed this plant in that genus because it cannot be sepa- 

 rated from S. Incus. Further examination has indeed led me to 

 doubt whether the two preceding and allied species do not equally 

 require removal, but as their description is already in the press, 

 I must here content myself by stating the change in my views 

 respecting them. 



• " Arthrodesmus octocornis. Corpusculis viridibus, leviter compressis qua- 

 drangulis binis singulis quadricorniibus," Ehr. 



" Micrasterias octocornis, cellulis inciso-radiatis, radiis quatuor, attenuatis, 

 acutis, divergentibus ; e latere elongato-ellipticis, medio compressis, super- 

 ficie laevi." — Mgh. /. c. p. 21 G. 



