166 DESMIDS OF THE UNITED STATES. 



which is very striltiiig in contrast witli the granular surface, 

 and serrate margins of the others, but by the processes, par- 

 ticularly evident in end view, which do not spring from the 

 upper surface more or less distant within the margin, but from 

 the margin itself. This feature not evident in St. paeu'lofiir- 

 cigerum, Reinsoh, gives it a position f<jr itself. The form, 

 Plate Lir, figs. 8, 9, from Minneapolis, Minn., is Ralfs' St. 

 sphw^tim, now properly transferred to this "old name," has 

 besides the marginal processes some smaller accessory ones 

 within the margin, Plate LXIII, Hg. 34. Plate LXIII, figs. 

 2o, 26, is another variety from the same locality of unusual 

 size and very marked in the wide-spreading forks of the pro- 

 cesses ; Ralfs admits of "sometimes three marginal spines, as 

 seen in end view." 



St. enokme, Ealfs. Plate LII, flg.s. 19-25. 



Cells irregular or quadrate, spinous; end view three or 

 four lobed ; lobes broad, more or less emarg-inate or bifid, and 

 terminated by spines which are either simple or branched. 



Ealfs, the author of this species, says of it, it "is by far 

 the least symmetrical plant in this family, especially in front 

 view, and it is very difficult to trace any division into the 

 cells." 



Some of the forms, as figs. 24, 25, may properly belong to 

 this genus, but the others would be better placed under 

 Polyediium. 



These forms occur frequently in ponds of New Jersey, 

 Pennsylvania, Massachusetts. 



St. leptac'anthum, Nord. Var. tetroctocerum, Wolle. Plate 

 LXII. figs. 29, 30. 

 Cells about as long as broad ; deeply constricted ; semi- 

 cells snborbicnlar, furnished with eight long thin rays, 

 deeply forked, or clawed at the ends ; this whorl is rather 

 below the middle, and another above it with four similar 

 rays ; end view octangular, each angle produced into a long, 

 thin ray, deeply clawed at the end ; between the margin and 

 the center four more similar rays go out ; membrane smooth. 

 Diameter of body 25 /a.; including the processes 75-80 /'. 

 Pond near Malaga, N. .7. 



The only essential distinction between this form, and the 

 typical, Brazil plant, is that it has six rays in the larger whorl, 

 and ours has eight. 



St. Pottsii, Wolle. Plate LXII, figs. 8, 9. 



Small, smooth, sinus gaping ; semi-cells in front view 

 broadly elliptic, furnished on each side with three divergent 



