14 Mr. J. Ralfs on the British Desniidiese. 



have not observed that the filaments are twisted, as in Desmidium 

 and Didi/moprium. At the junction of the joints there are on 

 each margin one or two minute glands or processes which are 

 scarcely discernible in the front view, and do not interfere with 

 the close junction of the joints. The transverse view is linear or 

 oblong, and the processes, one or two at each side, are much more 

 evident than in the front view. 



^ This genus differs from Desmidium^ Didymoprium and Glao- 

 prium in its flat filaments (which are not twisted), in the deep di- 

 vision of the joints into segments, and especially in the presence 

 of the minute gland-like processes at the junction of the joints. 

 From the Odontella of Ehrenberg it may be known by the joints 

 being united along their entire breadth, whereas in Odontella they 

 are connected only by the elongated angles which inclose a small 

 vacant space between them. 



On account of its deeply constricted joints, this genus forms a 

 connecting link between the three preceding genera and Stau- 

 rasti'um. 



In Sphcerozosma, as in the other genera with deeply constricted 

 cells, the segments are frequently unequal during the growth of 

 the plant, and they become in like manner equal when it ap- 

 proaches maturity and its joints no longer divide. 



1 . >S^. unidentata. Joints as broad as long, divided into two segments 

 by a linear notch on each side ; junction-glands stalked, oblique, 

 solitary at the centre of each margin. Odontella unidentata, Ehr. 

 Infus. p. 159 ; Pritch. Infus. p. 191. Desmidium compressum, An- 

 nals of Nat. Hist. vol. ix. p. 253. Isthmia vertebrata, Menegh. /. c. 

 p. 205 ? 



This plant has been gathered at Rotherfield and near Tun- 

 bridge Wells by Mr. Jenner since the publication of my former 

 notice of it. I then considered it an un described species of Des- 

 midium, I have since been favoured by Mr. Berkeley with the 

 following extract from Ehrenberg^ s observations on the Odon- 

 tella unidentata : — " This species is surprising by reason of its 

 evolution. There are always two great and two small joints 

 alternately turned to one another, and the processes are found 

 alternately between the small and large joints. This reminds one 

 of Scenedesmus convergens and the FMastra" This extract leaves 

 no doubt that Ehrenberg^s Odontella unidentata is identical with 

 the plant I described under the name of Desmidium compressum. 

 He correctly points out the affinity between the joints in this 

 plant and the frond in Euastrum. As in both genera he consi- 

 ders the segments of the cell to be distinct joints, he has de- 

 scribed the process or junction-gland in the present plant as oc- 

 curring at every alternate dissepiment. But his description of the 



