152 Mr. J. Ralfs on the British BesmidiesEf. 



Fronds minute, smooth, scarcely constricted in the middle, 

 truncate at the ends ; each segment has three elongated, straight 

 processes, which diverge from those of the other segment ; fre- 

 quently in the front view only four processes are visible, as one of 

 those at each end is concealed behind another. The end view is 

 triradiate, the rays hyaline, stout at their base and gradually 

 tapering. In this view, as the frond is generally twisted, the rays 

 of the lower segment may be faintly seen between those of the 

 nearer one. The processes are really deeply trifid at the extre- 

 mity, though in certain positions they appear bifid, in which case 

 the diverging points are forked like the tail of a swallow. 



This species, even in the ft*ont view, may always be known 

 from the two preceding ones by its smooth and divided processes. 



Plate X. fig. 3. S. hifidum: a, front view; h, frond dividing; c, end 

 view ; d, newly-formed segment. 



** Front view without diverging processes ; end view triangular or 

 quadrangular. 



4. S. orbiculare, Mgh. Fronds smooth: front view suborbicular ; 

 end view bluntly triangular. Mgh. I. c. p. 225. Desmidium orbi- 

 culare, Ehr. Infus. p. 141. tab. 10. fig. 9 ; Pritch. Infus. p. 183. 



In pools and slow streams. Weston Bogs near Southampton ; 

 Rackham Common, Sussex, and Jack's Wood Spring near Tun- 

 bridge Wells, Mr, Jenner ; Dolgelley and Penzance. 



Fronds rather large, smooth, deeply constricted in the middle ; 

 the segments, broader than long, are generally in close approxi- 

 mation for their entire breadth, and hence by their union form a 

 suborbicular frond; their sides are rounded and have no processes. 

 The end view is triangular, with sides straight or slightly concave 

 and angles rounded. 



Staurastrum orbiculare may be recognised by its smooth frond, 

 angles very blunt and destitute of processes, and the orbicular 

 form of its front view. 



Plate X. fig. 4. S. orbiculare : a, front view ; b, end view ; c, frond di- 

 viding. 



5. S. mucronatum. Fronds smooth ; end view three-lobed; lobes in- 

 flated, blunt, mucronate. 

 a. In the front view the segments are transversely elHptic, and the 



mucro straight. 

 /3. Segments lunate, mucro curved outwards. 

 y. Mucro curved inwards. 



In shallow pools, not uncommon. Weston Bogs near Southamp- 

 ton ; Ashdown Forest, Sussex, Mr. Jenner ; Dolgelley and Penzance. 



Fronds smooth, deeply constricted in the middle ; segments 

 broader than long, rounded at the sides, which are furnished with 



