PENIUM. 35 



obtusely rounded ends. Arrangement of chlorophyl usually 

 somewhat like the last, but not separated in the middle. 

 Diameter 55-80 /*. 



Not so common as the preceding. 



P. OBLONGUM, D. By. Plate V, fig. 17. 



Cells oblong cylindrical, 3-4 times as long as wide, not 

 constricted in the middle, slightly tapering towards the 

 ends ; apices flatly rounded. Zygospore spherical, not un- 

 like Plate V, fig. 7. 



I find this form mostly in fresh spring water, in gelatinous 

 gatherings. 



P. MARGARITACEUM, Breb. Plate V, figs. 5, 6, 11. 



Cylindrical, usually 8-9 times longer than broad, rarely, 

 only twice as long ; not at all, or scarcely constricted in the 

 middle ; ends rounded, ornate with pearly granules arranged 

 in longitudinal series which often give a denticulate appear- 

 ance to the margin. 



Diameter 24-28 yu. ; length ordinarily about 225 v. I have 

 had specimens measuring 375 /u. 



Prevails over a wide range. I have it from Vermont, Con- 

 necticut, and almost every State southward to Florida, also 

 from the west to Minnesota. 



It varies somewhat in size, but is easily recognized by the 

 arrangement of the granules. 



P. SPIROSTRIOLATUM, Barker. Plate X, fig. 17. 



Large, elongated ; single cells somewhat swollen in the 

 center and tapering slightly towards the rotundo-truncate 

 ends; the cell walls possessing a number of superficial, con- 

 spicuous, rather coarse striae, running in a spiral direction ; 

 these somewhat interrupted at a number of annular rib-like 

 projections varying in number; these projections most nu- 

 merous towards the ends. 



Mr. Turner, of England, who reports this form from Min- 

 nesota remarks, "As I do not know of the publication of aiiy 

 measurements or authentic figure of this species, I may pos- 

 sibly be in error in referring these American forms to it : 

 the figures therefore must speak for themselves." 



Diameter of cells 23-31 /<. ; length of double cell as figured 

 227-260 /u. 



Another form, Plate X, fig. 19, which may for the present 

 be referred to this species, I found frequent in the Tocoi 



