186 Dr. G. C. Wallich on Desmidiacea from Lower Bengal. 



trations merely to outline, every diagnostic purpose being 

 thereby answered, a more definite and clear expression of form 

 being conveyed to the eye, and last, but perhaps not least, a 

 very great deal of useless labour being avoided. 



In like manner, I have refrained from denoting the number 

 of joints engaged in forming one of the dark diagonal bands 

 ranging from angle to angle, as observed in some of the fila- 

 mentous genera, owing to its variable character. Indeed I doubt 

 whether, in the normal state of these forms — that is, during their 

 unrestrained growth in their native element — the twisting to 

 which this appearance is due exists at all. In perfectly still 

 water, it is probable that no twisting occurs ; and in corrobora- 

 tion of this view, I may mention having repeatedly examined 

 filaments under a low power and without compression of any 

 kind, in which no torsion, or but a slight amount of it, was 

 manifest, whilst, under pressure between the glass slide and 

 cover, the same species has exhibited it in a very marked degree. 

 It is probable that, in many cases, the torsion of the filament is 

 engendered under the manipulation of the observer ; and it is 

 certain, at all events, that by modifying the amount of compres- 

 sion employed, the amount of torsion, and, with it, the number 

 of joints engaged in a complete convolution of the filament, may 

 be materially influenced. 



Synopsis of the Filamentous Genera. 

 Sporangia orbicular, or oblong, plane. 



1. Hyalotheca. Filament cylindrical. 



2. Desmidium. Filament compressed, triangular or quadrangular. 



Joints deeply constricted. Connected together by minute 

 projecting cushions at the outer portion of each lobe or 

 extremity. 



3. Aptogonum. Filament triangular or quadrangular. Constriction 



entirely absent. Margins plane or very faintly crenated. 



4. SpHiEROzosMA. Filaments comprcsscd. Margins incised, sinuate 



or angular. Joints constricted. United together by minute 

 lateral tubercles. 



5. Letjronema (n. g.). Filament compressed, plane. Margins 



incised or sinuate. Joints constricted. Without inter- 

 mediate processes. 



6. Onychonema (n. g.). Filament compressed. Margins serrate. 



Joints deeply constricted and united together by over- 

 lapping cornua. 



7. Streptonema (n. g.). Filament triangular, interrupted. Joints 



deeply constricted. Segments 3-lobed. Joints united by 

 cylindrical filaments given off from the base of each lobe. 



