Mr. J. Rails on the British Desmidieae. 157 



versely oblong, twice as broad as long, obtuse at the sides and 

 not elongated into processes. In the end view, which is qua- 

 drangular, the sides are concave, and the angles form short, very 

 broad, truncate rays, on which the granules are arranged in 

 transverse lines. 



All the specimens of S. dilatatum which I have examined have 

 agreed with the above description, but Meneghini describes the 

 end view as showing from three to five rays. The broad, trun- 

 cate and entire rays of this plant will always distinguish both its 

 three-rayed state from all the preceding species and its five-rayed 

 variety from S. margaritaceum, 



Plate XI. fig. 5. S. dilatatum : a, front view; h, end view ; c, empty 

 frond. 



*** Front view without diverging processes ; end view circular, with 

 five or more marginal rays or lobes. 



11 . iS^. Arachne. Fronds rough ; segments suborbicular with elon- 

 gated, slender, incurved processes ; end view with five slender 

 rays. 



Boggy pool near Dolgelley : very rare. 



Fronds minute, deeply constricted in the middle; segments 

 about as long as broad, having on each side an elongated process 

 which is hyaline and incurved, and appears as if transversely 

 striated on account of the minute granules. When the frond is 

 viewed obliquely, so that three or four of the long cui'ved pro- 

 cesses are seen at the same time, its resemblance to an insect is 

 considerable. The end view is circular with five slender rays. 



This plant is remarkable for its slender processes, which will 

 easily distinguish it from S. margaritaceum. It cannot be a five- 

 rayed variety of S. gracile, for the rays are longer, more slender, 

 remarkably incurved, and also entire at the extremity. 



Plate XI. fig. 6. S. Arachne : a, front view; b, end view. 



12. S. margaritaceum, Mgh. Fronds rough; front view with short, 

 lateral, converging processes which are entire at the apex ; end 

 view with five or more narrow, short, obtuse rays. Mgh. /. c. 

 p. 227. Pentasterias margaritacea, Ehr. Infus. p. 144. tab. 10. 

 fig. 15 (1838) ; Pritch. Infus. p. 185. fig. 104. 



a. Rays five. 

 j3. Rays six. 

 y. Rays seven. 



In peat pools near Dolgelley, /. i2. ; Ashdown Forest, Sussex, 

 Mr. Jenner. 



Fronds rough with minute granules ; in the front view the 

 segments are convex at the ends and slightly attenuated where 

 they are connected, and on each side is a short, linear, obtuse and 



