192 Dr. G. C. Wallich on Desmidiacece from Lower Bengal. 



detracting somewhat from the otherwise straight character of 

 the margins. 



Lower Bengal, 1855. 



Length of joint -0008"; breadth -0009". 



Plate VII. fig. 7. Portion of a filament. Fig. 8. End view of same. 

 Fig. 9. Sporangial state of same. 

 A. Baileyi, var. quadrangulatum. Filament linear. Joints in 



front and end view quadrangular. Uniting cushions circular. 



Lower Bengal, 1855. 



Length of joints from '0008" to -0012"; breadth the same. 



Plate VII. fig. 5. Front view of portion of filament. Fig. 6. End view 

 of same. 



4. Sphcerozosma, Corda. 



Filament compressed. Margins incised, sinuate, or angular. 

 Joints constricted, and united to each other by minute tubercles. 



I have found it necessary to modify the definition of this 

 genus as given by Mr. Ralfs, in order to admit the several varie- 

 ties which cannot be termed " plane." None of the filamentous 

 forms of Lower Bengal exhibit greater diversity of outline and 

 size. So great, indeed, is this diversity in some specimens, that, 

 were it not for the gradual transition recognizable from one to 

 the other, we should have to elevate them into distinct species. 

 As it is, I consider the constant aspect and position of the con- 

 necting tubercles in the three varieties of S. eoccavatum figured 

 by me a sufficient ground for accepting them as varieties only. 

 The deviations from the typical form of S. vertebratum, which 

 occur with the former species abundantly, although great as 

 regards size, are not otherwise of sufficient consequence to de- 

 mand notice in this place. 

 S. excavatum. Joints constricted. Lobes rounded, abruptly 



truncate or- angular at their extremities, with two sessile 



tubercles on each margin at their junction with each other. 



In the three varieties, the tubercles are situated about a third 

 of the length of the lobes apart. 

 Var. ^. Lobes abruptly truncate, their truncate surfaces flattened, 



with a minute tubercle at each angle, and four or more tuber- 

 cles on each front view of the joints. (PI. VII. fig. 15.) 

 Var. 7. Lobes broadly lanceolate, constriction angular ; extremities 



acute, with one tubercle at their apices. (PI. VII. fig. 16.) 

 Var. 8. Outline as in typical S. excavatum, but with numerous 



tubercles arranged irregularly over the surface of the joints. 



(PL VII. fig. 17.) 



Plate VII. fig. 15. Portion of filament of Spharozosma excavatum, var. /3, 

 front view. Fig. 16. Ditto of ditto, var. y, front view. Fig. 17. Single 

 joint of ditto, var. , front view. Fig. 18. Single joint of ditto, ditto. 

 Typical form. 



