144 DESMIDS OF THE UNITED STATES. 



tracted into a colorless arm about three times as lone as 

 the breadth of the body, diverging, apices tricuspidate, 

 margins rough with minute, firm, perpendicular, irregularly 

 placed aculei ; end view triradiate. 



Spread of the arm 58-63 //. 



Pond, Brown's Mills, N. J. 



The vertical spines, like the thorns of a rose, give this plant 

 a distinctive character. 



St. quaternium, Wolle. Plate LI I. figs. 17. 18, 19. 



Small, smooth, quadrangular in from view ; deeply con- 

 stricted ; sinus acute-angled, much ampliated ; semicells l- 

 long, side- rounded, end truncate, each angle furnished with 

 four firm aculei : end view triangular, sides concave, angles 

 broadly rounded and furnished with four aculei, two on the 

 margin, somewhal separated, and two within the margin, one 



on each side of the cell, and projecting between the other two. 



By turning the cells two -mallei- aculei may be detected near 

 the margins of the sides. 



Diameter of bodv 25 "..; including aculei tO-50 u. 



Ponds, near Malaga, N. J., and near Wilkesharre, Pa. 



St. FORFICULATUM, Lund. Plate LI. figs. 16-19. 



Cells rather broader than long; deeply constricted ; sinus 

 wide; semicells subtrapezoid, end truncately rounded; base 

 convex, sides somewhat produced and divided into two large 

 diverging mucros; margins ornate with large emarginate 

 verruca?, or more or less conspicuous prominences; in vertical 

 aspect three, or more rarely, tour or five angled; angles pro- 

 duced and divided, and sides furnished with spines as in front 

 view. 



Diameter including aculei 7080 fx. 



Bamber Pond, New Jersey. 



The form of the semicells is more elliptic than trapezoid, 

 but in other points it is a good representative of Landell's 

 Swedish plant. 



St. monticulosum, Breb. Var. bifarium, \<>rd. Plate LI, figs. 

 24-26. 

 Semicells with a forked spine at each lateral angle, and at 

 the end, two twinned, stout, acute, or furcate prominences ; 



