54 DI->MI1)S OF THE UXITKD STATES. 



D. CONSTKH n M. Bail. Plate XIV, fig. 2. 



Cells with moderately deep constrictions, which separate 

 four equal, gently 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 /v. 



This species bears some similarity to D. nodonum in its 

 wavy margins, but the undulations are less prominent, and 

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

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

 sections of D. nodosum. 



D. (PLEUROTAENIUM) BREVE, Wood. 



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

 but not undulate in the middle; slightly attenuated towards 

 the ends; apex truncate and somewhat rounded; cytioderin 

 very thick, densely minutely granulate ; margins either 

 straight or shortly undulate. 



Diameter 20-24 p. 



Dr. Wood remarks, "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 XIV, figs. 11, 12, and Plate XV, fig. 20. 



Semi-cells with four prominent nodes separated by con- 

 strictions ; end view six creuate, formed by whorls of tuber- 

 cles ; 8-10 times longer than broad. 



Diameter 45-55 /^. in center ; ends about half as thick. 



Frequent in ponds, Pennsylvania, New Jersey, Mu-~:i- 

 chusetts. 



D. REPANDUM, Wolle. Plate XIV, fig. 1. 



Smooth or punctate, about twenty times longer than broad, 

 margins repaud 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- 

 lated. No suture. 

 Diameter 25 p. 



Sparsely found in Pennsylvania and New Jersey. 



