Mr. J. Ralfs on the British Desmidiese. 467 



Pritch. Infus. p. 187. Cosmarium aculeatum, Menegh. /. c. p. 218. 

 Euastrum, no. 10, Bailey, Amer. Bacil. p. 296. pi. 1. fig. 10. 



/3. polygonum, Ehr. Infus. tab. 10. fig. 24 b. 



In peat bogs near Dolgelley, J. R. ; near Tunbridge Wells, Mr. 

 Jenner. 



Fronds deeply constricted; segments transversely elliptic or 

 reniform. In a. each segment has four angles and eight spines 

 in four pairs. The pair situated at each side angle converges 

 with the similar one of the other segment, but those placed at 

 the angles at the end generally diverge from each other. In each 

 pair the spines usually have the same direction, and hence it ap- 

 pears single until carefully examined. 



The variety j3. is a smaller plant, scarcely angular, and its spines 

 are more numerous, but are also in pairs and confined to the 

 margin. This state much resembles Xanthidium aculeatum, but 

 in that species the spines are not in pairs, and are more or less 

 scattered. 



In both varieties, though the spines seem to be in pairs and to 

 arise from the same point, they really arise on opposite sides of 

 the marginal line, as may be ascertained by an end view. 



I regret that I omitted to observe whether this species has any 

 projecting process on its surface, such as I have noticed in X.fur- 

 catum and aculeatum. 



Endochrome dark green, with large scattered starch granules. 



Plate XII. fig. 3. X. fasciculatum : b, with newly-formed segment ; c, la- 

 teral view ; d, end view ; e, /3. polygonum. 



3. X. aculeatum, Ehr. Spines simple, marginal and scattered. Ehr. 

 Infus. p. 147. tab. 10. fig. 23; Pritch. Infus. p. 187. fig. 109; 

 Menegh. /. c. p. 224. 



Weston Bogs near Southampton, and near Tunbridge Wells, Mr. 

 Jenner; Penzance. 



Fronds deeply constricted, so as to form a linear notch on each 

 side. The spines are simple and acute ; most of them are mar- 

 ginal, but somewhat scattered. 



In the centre of each segment, on both surfaces, is a projection 

 similar to that described under X. furcatum, but less evidently 

 dentated. 



Xanthidium aculeatum has a considerable resemblance to the 

 variety polygonum of the preceding species, but the spines are 

 not in pairs, and, although sometimes confined to the margin, 

 are irregular and not in a single line on each side of it. 



Plate XII. fig. 2. X. aculeatum : b, with newly-formed segment; c, la- 

 teral view ; d, end view. 



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