60 DESMIDS OF THE UNITED STATKs. 



This is one of the largest species of the genus and most 

 common. The end view is nearly a perfect circle ; front view 

 of a semi-cell constitutes about two-thirds of a circle ; a dis- 

 tinct border is always present, and often appears striated. 



Var. MINOR, Nord. Plate XV, fig. 10. Plate LX, tig. 18. 



This form is in all essential points like the last, except in 

 dimensions. 



I have found almost every possible variety of size, mea-ur- 

 iug in diameter from 20 to 40 w. This fact* furnishes a pre- 

 sumptive evidence that the smaller forms are merely unde- 

 veloped conditions; young plants evolved from sporan<,'iums, 

 in accordance with Hofmeister's theory. ST I.NTKOIHVTION, 

 p. 19. 



0. PSEUDOCONNATUS, Nord. Plate XV, fig. 11. Plate LX. ii->. 



10, 11. 



Similar to the two preceding in form and structure, lui in 

 size usually smaller than the typical plant. The distinctive 

 feature is in the arrangement of the chlorophyl ; this is not 

 homogeneous, but divided in each semi-cell, in front view, 

 into two parts, and in end view, into four parts. 

 Marsh pools, Pennsylvania. 



The value of the arrangement of the chlorophyl as a spe- 

 citic character, needs, I think, further corroborative evidence. 



C. CLEVEI, (Lund.), Wolle. (Penium Clevei, Lund.) Plate LXI, 



fig. 27. 



Cell subcylindrical, 2J-3 times longer than broad, some- 

 what constricted in the middle; semi-cells cylindric sub 

 conical, ends distinctly attenuated and rounded at apices ; 

 vertical view a perfect circle. Nuclei large, elliptic single 

 or rarely twinned. Membrane finely punctate, at apices 

 subgrauulate punctate. 



Diameter 40-50 p. Length 115-118 ^. 



Brown's Mills, New Jersey. 



The plant is slightly larger than Lundell's form, but s.> 

 good a counterpart in form and structure, I take ft to be the- 

 same species ; by measurement the constriction is somewhat 

 deeper. The decided constriction makes the natural position 

 of this form, I think, a member of the present genus, not- 

 withstanding the observations made by Lundell in the 

 arrangement of the chlorophyl. 



C. THWAITESII, Kalfs. Plate XV, fig. 19. Plat.- LXI. fig. 28. 



Cells two or three times longer than broad, fusiform in 

 front view; circular in end view; constriction a shallow 



