178 BRITISH DESMlblE^E. 



portions of the cell which project into the segments may be mistaken for them. 

 The same circumstance renders it difficult to determine the exact form of the 

 attenuated ends. 



By Brebisson Closterivm subidatum is regarded as a distinct species ; and 

 no doubt extreme states differ considerably, but I am unable to detect any di- 

 stinctive character on which I can rely. 



Closterium acutum differs from C. Cormi in its tapering frond and acute 

 ends ; its fronds are also far more closely united with the sporangium. 



Length of frond -j-yy of an inch ; greatest breadth 2-5V0 '■> length of sporan- 

 gium .pjL. to -^ ; breadth ^^. 



Tab. XXX. fig. 5. a, b. fronds of a ; c. frond of /S ; d. conjugated fronds 

 from a drawing by Brebisson ; e. front view of sporangium ; f. side view of 

 sporangium from a drawing by Brebisson. Tab. XXXIV. fig. 5. sporangium. 



17. SPIROT^NIA, 5r^6. 



Frond simple, elongated, cylindrical or fusiform, straight, entire, not 

 constricted at the middle ; ends rounded ; endochrome spiral. 



The fronds are simple, straight, cylindrical, slightly attenuated at 

 the extremities ; they are quite entire and not constricted at the mid- 

 dle. The endochrome is spiral, as in Zygnema, and, except in fronds 

 about to divide, there is no interruption at the centre. 



The spiral arrangement of the endochrome will distinguish SjnrO' 

 tcenia from every other genus in the family. It also differs from Tet- 

 memorus in the entire extremities, and from that genus as well as 

 from Docidium in the absence of a constriction at the centre. It dif- 

 fers likewise from Closterium by its straight fronds and the absence 

 of a globule near the extremities. 



Spirotcenia affords another example of resemblance to the Conju- 

 gatse. The endochrome is so exactly like that of Zygnema, that, until 

 we saw M. de Brebisson's drawing, both Mr. Jenner and myself had 

 regarded the S. condensata as the young state of a Zygnemu, It 

 however does not form a filament, the cells dividing in the manner of 

 the Desmidieae ; but the division is oblique, as in Scenedesmus ; thus 

 affording a character by which it may be distinguished from Penium, 

 even should the spiral form of the endochrome be absent. 



In Spiroimiia the complete division of the cell is prior to the divi- 

 sion of the gelatinous covering, which thus retains the two newly 

 parted cells together for some time longer ; a fact which convincingly 

 proves that this genus belongs to the Desmidieae. 



