. Dr. G. C. Waliich on Desmidiacca from Loiver Bengal. 275 



*** Sporangia smooth. Fromh yreathj elongated. Joints with 

 slight or no constriction between the segments. 

 15. Tetmemouus. Frond straight, constricted at the centre, and 

 notched at the extremities. 



IC. Penium. Frond straight, witli very faint or no constriction. 



17. DociDiuji. Frond greatly elongated, straight, constricted at 



centre. Ends truncate. 



18. Closterium. Frond cresccntic or arcuate. Without a definite 



constriction. 



**** Fronds composite or arranged in clusters. Sporangia 

 unknovm. 



19. ScENEDESMUS. Cclls oblong or fusiform, placed side by side in 



one or more rows. 



20. Ankistrodesmus. Cells aggregated into faggot-like bundles. 



***** Cells arranged on the same plane, around a common 

 centre. 



21. Pediastrum. Cells arranged in a stellate or radiate series. 



The outer series mucronate, dentate, or bldcntate. 



8. Holocystis, Hass. 



Frond lenticular. Segments horizontally lobatc. 



1. //. oscitans, llalfs. Frond divided by an angular constriction 

 into two horizontal bilobatc segments. Lobes fusiform or 

 subfusiform. 



Micrasterias oscilans, llalfs, British Dcsmid. 1845. 



Eunstrinn pinnatifidum, Kiitz. 1845. 



Euastrum, No. 7, Bailey, Amer. Bacil. pi. 1. lig. 25, 1841. 



As every gradation between the form presenting the lobes 

 fusiform and inflated, and that in which the margin of the ter- 

 minal lobe is straight or even hollowed, is constantly to be met 

 with, 1 have ventured to set aside the unimportant distinctions 

 derived only from size or colour, and to unite, under one head, 

 Micrasterias oscitans, Ralfs, and M. pinnalifida, Kiitz. The in- 

 termediate varieties occur in abundance in Lower ]3engal ; and 

 in no instance have I detected evidence of their being immature 

 states of two distinct species, as suggested by Elirenberg and 

 ]\Ieneghini; on the contrary, I have found proof of their being 

 true varieties of the same in the fact of their dividing. 



The two figures of Holocystis oscitans in Hassall's ' Freshwater 

 Alg?e' (pi. 90. figs. 3 & 4) evidently depict two very distinct 

 species, neither of which accords witli the characters of Micras- 

 terias oscitans, Balfs. Fig. 3, which is referred to in the expla- 

 natory index to the plates, with a (picry as to its being the youn"* 

 state of M. Crux-melitemis, is not alluded to in the letter-press, 



