60 BRITISH DESMIDIE/E. 



dissUiens, but the segments are still more widely separated on the inner side, 

 and consequently so far thrown back as to cause the attached joints to form a 

 zigzag. The elliptic sporangium lies between the cells, which remain perma- 

 nently attached to it. 



Kiitzing has established for this plant a distinct genus {Bamhusina). The 

 characters which he relies upon are the absence of a mucous sheath, the cask- 

 like joints, and their union, not by a thickened rim as in Bidymoprium Gre- 

 villii, but in the manner of the Zygnemata. To these characters, if he had 

 been acquainted with the reproductive state of the two plants, he would 

 doubtless have added the different position of their sporangia. 



Even if the presence of a sheath could be supposed to have a generic value, 

 it would not apply to the present question ; for, as Mr. Jenner has pointed out 

 to me, a mucous covering does exist, and may occasionally be detected as a 

 narrow border to the filament. The agreement also between the plants in 

 their angles and the regular twisting of their filaments appear to me to forbid 

 their separation. 



Kiitzing suggests* that this plant may be identical vnth the Gymnozyga 

 moniliformis, Ehr. (Bericht d. Berl. Ac. Novb. 1840). I am unable to de- 

 termine this point, as I have seen no other notice of Ehrenberg's plant. 



This plant was known to M. de Brcbisson before it was described in the 

 * Annals of Natural History,' and indeed under the name of JDesmidium bam- 

 businum was included by him in a list of Desmidiese inserted in a treatise on 

 the microscope by M. Chevalier in 1839, but I have retained the present 

 name, it being the first under which it was either described or figured. 



Length of joint g-L of an inch ; breadth of the joint including teeth xwn:' 

 Length of sporangium -g\-^, breadth yiVb"* 



Tab. III. a, b. portions of mature filaments ; c. portion of a filament with 

 jouits dividing ; d, e, f. portions of filaments vdthout endochrome to show the 

 longitudinal markings ; g,h. transverse views ; i, k. portion of filaments conju- 

 gating ; /, m. sporangia with the empty fronds attached. 



3. DESMIDIUM, Ag. 



Filament fragile, elongated, triangular or quadrangular, regularly 

 twisted ; joints bidentate at the angles. 



The filament is fragile, of a pale green colour and slightly opake. 

 When dried, the British species usually acquire a yellowish appear- 

 ance, and adhere to paper or talc less firmly than plants belonging to 

 allied genera. They are regularly twisted, but being triangular or 

 quadrangular, two of the bidentate angles of each joint are always 

 visible at the margins. The endochrome is divided into linear por- 

 tions by a pale transverse line between the angles. 



* Pliycologia Gcrmanica, p. 110. 



