134 DESMIDS OF THE UNITED STATES. 



ST. DEJECTUM, Breb. Plate LI, figs. 7-11 and 17-21. 



Semi-cells smooth elliptic, or extrorsely Innate with sides 

 convex and ends nearly straight, concave, or convex ; each 

 angle furnished with a longer or shorter aculeus, or awn ; 

 these are sometimes horizontal, sometimes they converge, but 

 more frequently they diverge, directed obliquely upward. 

 End view triangular, or occasionally, four lobed. 

 Figs. 17-21 represent the more usual forms. 



Var, MUCRONATUAI, Ralfs. Fig. 8, front view and two end views. 



Var. CONVERGENS, Wolle. Figs. 7, 9, 10, 11, are unusual forms 

 collected near Minneapolis, Minn. ; aculei are stout and often 

 stand nearly at right angles with the sides ; they are also of 

 unusual size. 



Diameter of the various forms 25-38 yw. without the awns. 

 A common species. 



ST. MEGACANTHUM, Lund. Plate LXII, figs. 10, 11, 12. 



Cells about as long as wide (without aculei) profoundly 

 constricted; sinus acute-angled, or subrectangular ; semi- 

 cells triangular fusiform ; sides somewhat convex, ends sub- 

 truncate or lightly convex, angles each terminating in a 

 strong and long aculeus; viewed from apex triangular, or 

 rarely four lobed ; sides retuse ; angles produced into a long 

 and firm aculeus ; membrane finely punctate. 



Diameter 50-57 p. ; length about 50 p. ; aculei 15-18 //. 

 Pennsylvania, New Jersey; rather rare. 



ST. BREVISPINA, Breb. Plate LI, figs. 1, 2; and Plate LXIV, 



figs. 2, 3. 



Semi-cells smooth, turgid-elliptic, minutely mucronale: 

 end view triangular with sides usually so sinuate as to pro- 

 duce a three lobed appearance; each lobe terminated bv ;i 

 short mucro. 



Diameter 45-48 f*. Smaller forms occur also. 



The cells of this species vary in size and somewhat in form 

 from elliptic to subreniform, turgid. 

 Neither very rare nor common. 



Var. INERME, Wille. Plate LI, figs. 3, 4; Plate XLVI, figs. !). 1 0. 



Somewhat larger than the true form, but otherwise reminds 



one of it, notwithstanding the absence of the mucros. The 



