466 Mr. J. Ralfs on the British Desmidiese. 



segment has been noticed. If, as is not unlikely, these projec- 

 tions should be discovered to exist in all the true species of this 

 genus, it will be more decidedly separated from Cosmarium than 

 at present, when, it must be confessed, the diagnostic marks be- 

 tween these genera are not so strong and satisfactory as could be 

 wished. These central projections were first pointed out to me 

 by Mr. Dalrymple in Xanthidium furcatum, under which species 

 a more detailed description will be found. 



I have little doubt that there are more British species of Xan- 

 thidium than I shall describe here, and, indeed, believe that I 

 have more than once gathered X. hirsutum ; but as my attention 

 was not particularly directed to the Desmidiem at the time, I neg- 

 lected to examine it with sufficient care to be able to state my 

 observations with any confidence in this paper*. 



1. X. furcatum, Ehr. Spines marginal and scattered, stout, termi- 

 nated by three or more acute, diverging points. Ehr. Infus. p. 146. 

 tab. 10. fig. 25 ; Pritch. Infus. p. 188. fig. 110 ; Menegh. Synop. 

 Desmid. p. 224. Xanthidium, no. 2, Bailey, /. c. p. 291. pi. 1. 

 fig. 15. 



In boggy pools. Weston Bogs near Southampton, Piltdown Com- 

 mon near Uckfield, Rackham Common near Pulborough, and Ash- 

 down Forest, Sussex ; near Tunbridge Wells, Mr. Jenner ; Dolgelley 

 and near Carmarthen. 



This species forms cloud-like masses at the bottom of shallow 

 pools, and on being detached immediately rises to the surface. 

 Its fronds are visible to the naked eye, being dispersed, like mi- 

 nute glittering dots, through the cloud-like substance which con- 

 nects them. 



The fronds are deeply constricted ; their segments reniform or 

 suborbicular ; in the newly formed segment the spines are either 

 wholly wanting or only minute conical projections, which first 

 elongate and finally form stout spines more or less divided at the 

 apex into diverging points. At first the spines are confined to 

 the margin, but as the plant approaches maturity they are spa- 

 ringly scattered over the surface. 



Plate XII. fig. 1. Xanthidium furcatum : b, with newly-formed seg- 

 ment; c, lateral view. 



2. X. fasciculatum, Ehr. Spines simple, acute, few, always in pairs 

 and confined to the margin. Ehr. Infus. p. 146. tab. 10. fig. 24 a ; 



* In Bailey's 'American Bacillaria,' " Euastrum, no. 11," pi. 1. fig. 13. 

 represents a species of Xanthidium with six pair of marginal spines on each 

 segment, and apparently the central projections described above. The oc- 

 currence of these projections on this plant is interesting both because it sup- 

 plies another example, and also because their presence in a species with 

 marginal spines only is another argument against the separation of the spe- 

 cies with marginal spines from those in which they are scattered. 



