Dr. G C. Wallich on Desmidiacca from Lower Bengal. 275 



*** Sporangia smooth. Fronds greatly elongated. Joints with 

 slight or no constriction between the segments. 



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



notched at the extremities. 



16. PENIUM. Frond straight, with very faint or no constriction. 



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



centre. Ends truncate. 



18. CL-OSTERIUM. Frond crescentic or arcuate. Without a definite 



constriction. 



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

 unknown. 



19. ScENEDESMtis. Cells ohlong 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 rnucronate, dentate, or bidentate. 



8. Holocystis, Hass. 

 Frond lenticular. Segments horizontally lobate. 



1. H. oscitans, Ralfs. Frond divided by an angular constriction 

 into two horizontal bilobate segments. Lobes fusiform or 

 subfusiform. 



Micrasterias oscitans, Ralfs, British Desmid. 1845. 



Euastrum pinna tifidum, Kiitz. 1845. 



Euastrum, No. 7, Bailey, Amer. Bacil. pi. 1. fig. 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, I 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. pinnatifida, Kiitz. The in- 

 termediate varieties occur in abundance in Lower Bengal ; and 

 in no instance have I detected evidence of their being immature 

 states of two distinct species, as suggested by Ehrenberg and 

 Meneghini ; 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 HassalPs f Freshwater 

 Algse' (pi. 90. figs. 3 & 4) evidently depict two very distinct 

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

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

 natory index to the plates, with a query as to its being the young 

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



