54 DESMIDS OF THE UXITKD STATES. 



D. coxsTEicTUM. Bail. Plate XIV, fig. 2. 



Cells with moderately deep con.strictions, which separate 

 four equal, geutly curving prominences on each semi cell; 

 end view entire. Apex furnished with a few conspicuous 

 teeth of which four are ordinarily in view. Plant 10-12 

 times longer than broad. 



Diameter 40-50 i-i. 



This species bears some similarity to D. nodosum in its 

 wavy margins, but the undulations are less prominent, and 

 the transverse sections are quite distinct ; in the one they are 

 circiilar, in the other undulate ; compare Plate XI, fig. 11, 

 sections of D. nodosum. 



D. (PLEUEOTAEXIUil i BEEVE, Wood. 



Eobust, 4-8 times longer than broad, distinctly constricted, 

 but not undulate in the middle; slightly attenuated towards 

 the ends : apex truncate and somewhat rounded ; cytioderm 

 very thick, densely minutely granulate ; margins either 

 straight or shortly undulate. 



Diameter 20-24 n. 



Dr. Wood remarlis, "This species was sent to me by Dr. 

 Billings, who obtained it near Washington, D. C. The mar- 

 gins are sometimes straightish, but in other fronds there are 

 three or more distinct, short undulations or rounded projec- 

 tions in each half margin." 



The shape of the semi-cell is very near Plate X, fig. 12, but 

 only about half the size, not so undulate and not spinous. 



D. NODOSUM, Bail. Plate XIT. %s. 11. 12, and Plate XT. fig. 20. 

 Semi-cells with four prominent nodes separated by con- 

 strictions : end view six crenate, formed by whorls of tuber- 

 cles : S-10 times longer than broad. 



Diameter 45-5.5 yu. in center ; ends about half as thick. 



Frequent in ponds, Pennsylvania, Xew Jersey, Massa- 

 chusetts. 



D. EEPAXDUM, Wolle. Plate XIY, fig. 1. 



.Smooth or punctate, about twenty times longer than broad, 

 margins repand or undulate from the base of the semi-cell to 

 near the end: only slight variation in diameter from base to 

 end ; apices truncately rounded, and sometimes slightly di- 

 lateil. Xo suture. 

 Diameter 25 ij.. 



Sparsely found in Pennsylvania and Xew Jersev. 



