CLOSTERIUM. 43 



One of our most common species, often forming gelatinous 

 floating masses on small ponds and pools. I have it from 

 many States, and probably it exists in every one. 



The entire chlorophyl of this species often retracts itself 

 from the cell-wall and breaks up into a number of oval or 

 globular forms, every one of which acquires a firm envelope. 

 Plate IX, fig. 17, represents one of these forms. 



C. PEELONGUM, I Breb. \ Delph. Plate VI, figs. 15, 16. 



Cells somewhat linear, cylindrical, elongate, about thirty 

 times longer than broad, slightly curved, apices obtusely 

 rounded, sometimes slightly recurved ; cytioderm striated. 

 Diameter 20-21 //. 



Ponds, Pennsylvania and Minnesota. 



C. SUBOOSTATUM, Nord. Plate VI^ fig. 11. 



Cells usually four to five times longer than broad ; back 

 rather high arched, below nearly straight, may be either 

 slightly concave or convex ; apices obtusely rounded. Mem- 

 brane usually yellowish brown with 9-12 longitudinal costae. 



Diameter 50-60 yu. 



C. NASUTUM, Nord. Plate VII, fig. 12. 



Large, fusiform, only slightly curved, about five times as 

 long as wide, gradually tapering from the middle to near the 

 ends, then suddenly contracting, making the sides parallel, 

 to the truncate or rounded apex. Cytioderm smooth. Speci- 

 mens of this species from (Jypress Swamp, Florida, appear 

 perfectly straight and somewhat thicker and longer than the 

 forms hitherto noted, apical vacuole distinct. Chlorophyl 

 arranged in five or six fillets ; centrally separated, length 

 seven times more than the breadth. Coll. A. C. Stokes. 

 Diameter 75 jj.. more or less. 



Ponds, Berkshire Mountains, Massachusetts. 

 The only counterpart to this form, hitherto found, is from 

 Brazil, and is represented in Witt, and Nord's Algce ExsiocatcB 

 No. 366. 



O. TUEGIDUM, Ehrb. Plate VII, fig. 15 ; Plate VI, figs. 3, 4. 



Dorsum convex, lower margin somewhat concave, inclin- 

 ing upward at the rounded ends ; upper margin with a de- 

 pression near each extremity; empty cells reddish brown, 

 striae numerous, fine but distinct. 

 Diameter 64-75 //. 



Wet, marshy places, Pennyslvania. 



