Mr. J. Ralfs on the British Desmidieae. 159 



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

 Amer. Bacil. pi. 1. fig. 12 .? 



Shallow pools. Weston Bogs near Southampton ; Rackham Com- 

 mon near Pulborough, Sussex, Mr. Jenner ; Dolgelley 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 which 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 Staurastrum mucronatum, but the end view is very 

 different. 



Plate XII. fig. 2. S. Incvs : 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" 

 cornis, 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 Staurastrum Incus j 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 Arthrodesmus 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 Staurastrum 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 quadricornubus," Ehr. 



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

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

 ficie Isevi." — Mgh. I. c. p. 216. 



