194 Dr. G. C. W allich on Desmidiacea from Lower Bengal. 



somewhat broader than long. Marginal view of joints dumb- 

 bell-shaped, but without any isthmus. 



Length of joint -0006" to -0008"; breadth of ditto -0008" to 

 0012". 



Plate VII. fig. 10. Front view of a portion of filament. Fig. 11. Mar- 

 ginal view of two joints. 



Var. @. This differs from the above in having the segments 

 of the joints separated by a well-defined isthmus. The inter- 

 mediate notch is accordingly somewhat gaping. So long as in 

 filament, the connected surfaces of the joints are slightly in- 

 flated; but this character is transitory, inasmuch as the outer 

 segment of a filament is often found to be entirely free from it. 



It will be observed that the difference between the two forms 

 just described is hardly more than one of degree ; that is, no 

 structural distinction exists. It is probable, therefore, that they 

 may merge one into the other at certain periods of their growth. 

 I may state, however, that the transition, in any one filament, 

 has not been met with by me. 



Length of joint -0008" to -0010"; breadth of ditto -0007" 

 to -0012". 



Both from Lower Bengal, 1855. 



Plate VII. fig. 12. Front view of a portion of a filament. Fig. 13. Mar- 

 ginal view of two joints, showing the perfect dumb-bell shape, due to the 

 presence of the isthmus. Fig. 14. Two of the sporangium -like bodies 

 attached to the filament. 



Var. 7. Filament triangular ; its margins sinuate. Joints deeply 



constricted. Segments 3-lobed. End view triangular. 



This remarkable variety might be regarded as a filamentous 

 Staurasti*um. But the same reason that serves to distinguish 

 the compressed form from Cosmarium must distinguish this one 

 from the genus it so closely resembles. 



I may add that if, by any possibility, the first varieties of this 

 genus could have been merged into Sp/uerozosma, the occurrence 

 of the last-described variety must at once have rendered the 

 junction impossible, except by placing it in a genus by itself, 

 against which course the reasons are equally obvious. 



Length of joint -0008" to -0010"; breadth of ditto -0008" 

 to -0012". 



Lower Bengal, 1855. 



Plate VIII. fig. 12. Front view of a portion of a filament. Fig. 13. End 

 view of a joint. 



6. Onychonema, n. g. 



Filament compressed. Margins inciso- serrate. Joints deeply 

 constricted, and united to each other by diverging subcapitate 

 cornua. 



The remarkable form of the overlapping and alternating 



