150 Mr. J. llalfs on the British Desmidiese. 



also a plant which he refers to Arthrodesrmis. This last plant I 

 was at first inclined to keep distinct, but am unable to do so, as 

 one undoubted species [S. tetracerum) has also compressed fronds ; 

 whilst another compressed form added by Meneghini [S. Incus) 

 in its front view so much resembles Staurastrum mucronatum, that 

 I am not sure it may not be a variety of it. 



A little care will distinguish Staurastrum from the other ge- 

 nera in this family, although some of its species appear at first 

 sight to approach forms which belong to them. It differs from 

 JDesmidium in never forming a filament ; the species with com- 

 pressed fronds differ from Cosmarium in the front view by having 

 a process or mucro at each side of the segment. Some species have 

 a considerable resemblance to species oi Xanthidium, but in Xan- 

 thidium the spines are always situated on the convex surface of 

 the segments, whilst in this genus the rays are elongations of 

 their angles. 



I have divided Staurastrum into four sections. 



In the first, the front view is truncate at the ends, and the an- 

 gles of each segment, prolonged in straight processes, diverge from 

 each other. 



In the second, the front view shows the processes, if any, of the 

 one segment, either parallel to or converging with those of the 

 other ; the end view is triangular or quadrangular. 



In the third, the front view also shows the processes, if any, 

 parallel or converging ; but the end view is circular, with five or 

 more marginal rays or lobes. This section also has the central 

 circular portion more distinct from the rays and slightly pro- 

 duced beyond them. 



In the fourth, the processes, if present, are in the front view 

 converging ; but the end view is compressed, and terminated at 

 each extremity by a process or mucro. 



* In the front view the elongated processes of one segment diverge from 

 those of the other. 



1 . S. tetracerum. Fronds rough ; front view with four slender di- 

 verging processes which are entire at the apex ; end view com- 

 pressed, with a process at each extremity. Staurastrum paradoxum, 

 Ehr. Infus. p. 143. tab. 10. %. 14 ; Pritch. Infus. p. 185. figs. 102, 

 103. Micrasterias tricera and tetracera, Ktz. Synop. Diatom, in 

 Linnsea 1833, p. 602. figs. 83, 84 and 85. 



In pools, Dolgelley and Penzance. 



Fronds very minute; front view nearly square; the angles 

 elongated into straight, slender processes, which diverge from 

 each other ; frequently, however, a segment may be so twisted 

 that one of its processes is situated behind its companion, and is 

 not seen until carefully looked for ; in this case the frond ap- 



