Dr. G. C. Wallich on Desmidiacea from Lower Bengal. 195 



cornua, and the uncinate processes given off from the extremities 

 of the segments, at once distinguish this genus from all others. 



1. O. uncinatum, n. s. Joints quadrangular in front view, deeply 

 constricted. Segments furnished with two obliquely placed 

 cornua on their outer surfaces, and with a recurved claw at 

 each of their extremities. 



The joints in this form are somewhat more turgid than in 

 Spheerozosma or Leuronema. The constriction is so deep as to 

 leave little more than a third of the entire breadth of the joint 

 as a connecting isthmus. The segments are closely approximate. 

 The cornua are situated obliquely to each other, at the outer 

 thirds of the lateral surfaces ; they are cylindrical, half as long 

 as the segments are broad, and slightly capitate. Between the 

 cornua and the claw-like processes a row of minute teeth pre- 

 sents itself, whilst the median line of the segments, on their 

 outer aspects, is bordered by two rows of minute puncta. These 

 are, however, rarely visible, except in such joints as happen to 

 be empty. The whole of the cornua (as seen in the front view) 

 placed on the same side of the median line and same extremity 

 of the segments, overlap the adjoining joint in the same direc- 

 tion. The two sets of cornua, therefore, as seen in this view, 

 face in opposite directions, the upper in one, the lower in an- 

 other. These cornua are not mere tubercular solid excrescences, 

 as in Spheerozosma, but are formed by an extension of the cell- 

 walls, like the fingers of a glove, and are accordingly hollow for 

 a certain distance, the tenacity of the filament being so far in- 

 creased by their instrumentality that it is constantly seen to 

 divide rather at the segments than at the joints. 



In the young state, the uncinate processes are sometimes im- 

 perfectly developed, and might induce the belief that the form 

 exhibiting them in this state was a variety. 



The marginal view is linear, and faintly sinuate or incised, 

 from the outline of the cornua being observable, and the con- 

 striction, which in this aspect is just discernible. In the end 

 view, the alternating and divergent character of the cornua is 

 very remarkable. This last feature affords a close point of 

 resemblance, it may be observed, to the structure and appa- 

 rent function of the obliquely arranged spines and cornua in 

 some species of the genus Biddulphia. 



Length of joint -0008" to -0012" ; breadth of ditto -0012" 

 •0020". 



Lower Bengal, 1855. 



Plate VIII. fig. 7. Front view of a portion of a filament. Fig. 8. Mar- 

 ginal view of filaiMcnt. Fig. 9. Transverse view. Fig. 10. End view. 

 Fig. 11. A joint undergoing division. 



