134 DESMIDS OF THE UNITED STATES. 



St. dejectum, Breb. Plate LT, figs. 7-11 and 17-21. 



Semi-cells smooth elliptic, or extrorsely lunate 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 upwaid. 

 End view triangular, or occasionally, four lobed. 

 Figs. 17-21 represent the more usual forms. 



Yar. jiucEONATXJir. Ealfs. Fig. 8, front view and two end vie\\>. 



Var. coxvEEGENS. Wolle. Figs. 7, 9, 10. 11, are unusual foiins 



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 yu. without the a'n iis. 



A common species. 



St. JiEGACAXTHUii, Lund. Plate LXIl, 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 filnely punctate. 



Diameter .50-57 fx.; length about 50 /^. ; aculei 15-18 ^i. 

 Pennsylvania, Kew Jersey; rather rare. 



St. beevispixa, Breb. Plate LT, figs. 1, 2 ; and Plate LXIY, 

 figs. 2, 3. 

 Semi-cells smooth, turgid-elliptic, minutely mucronate; 

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

 duce a three lobed appearance ; each lobe terminated by a 

 short mucro. 

 Diameter 45—48 jx. 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. ineejie, Wille. Plate LI, figs. 3, 4; Plate XL VI, figs. 9, 10. 



Somewhat larger than the true form, but otherwise reminds 



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



