162 BRITISH DESMIDIEiE. 



The striated species, besides the central suture, frequently have 

 other transverse lines that divide the segments themselves into two 

 or more portions. Brebisson considers that their number is constant 

 in the same species, and that they may be used in framing a specific 

 character ; but my experience leads me to agree with Meneghini, that 

 it is generally unsafe to place much reliance upon them : nevertheless 

 it must be admitted that in many species no other suture than the 

 central one is ever present, except during the process of division. 



Closterium may be distinguished from all the other genera of the 

 Desmidieas by its elongated, curved, entire, and attenuated fronds. 



I have divided Closterium into sections answering to the genera of 

 Kiitzing and Brebisson, according to a list of the species sent me by 

 the latter ; but I must notice that one or two species will probably be 

 found to agree just as well with some other section as with that in 

 which it is placed. 



In the first section the fronds are commonly either semilunate or 

 crescent-shaped, and have the extremities blunt or but moderately 

 elongated ; the terminal globules are situated so near the ends that 

 the endochrome nearly fills the cell ; the extremities appear to have 

 a slight notch or depression (the opening of Ehrenberg) ; the spo- 

 rangia are orbicular, and are situated between the conjugating fronds 

 and but slightly connected with them. 



The second section contains but one species, which by Brebisson 

 is still retained in Closterium; the form however of its frond agrees 

 with that of Kiitzing's Stauroceras, whilst its sporangium is unlike 

 that of any other species. The fronds are inflated at the middle and 

 elongated at each end into a short curved beak, and there is no notch 

 apparent at the ends. The sporangium, which is placed between 

 the conjugating cells, is geminate, consisting of two nearly orbicular 

 portions, flattened at their junction, where they often separate. 



The plants in the third section are in general curved only at their 

 extremities, which taper considerably, and frequently form a slender 

 beak. The margins of the frond, exchisive of the curved extremities, 

 are in most cases nearly similar ; the moving granules are more or less 

 distant from the extremities, which have no distinct terminal notch ; 

 and the segments are permanently attached to the sporangium, vhich 

 is cruciform or quadrate. 



