132 DESMIUS OF THE UNITED STATES. 



Var. ELLIPTICUM. Fig. 13. This form is more elliptic in front 

 view than the others. Plate XLIX, figs. 41, 42. A form 

 differing from that of Plate L, and more strictly elliptical, 

 semi-cells in front view slightly contracted near the ends. 

 Diameter 33-35 //. 



Pond, St. Anthony Park, Minneapolis, Minnesota. 



ST. ORBICULAEE, (Ehrb.), Ealfs. Plate L, figs. 9, 10. 



Orbicular, constriction narrow linear; semi-cells not ellip- 

 tic, but semiorbicular ; end view triangular, angles rounded, 

 sides somewhat concave ; smooth. 



Diameter 30-45 yu. 



Plate XLIX, figs. 36-38 not so distinctly orbicular, some- 

 times approaching the form of a Turk's cap, sometimes the 

 sides are more concave than figured and the sinuses between 

 the semi-cells are not always so closely linear. It is a common 

 species, and often involved in a mucous envelope. 



ST. ANGULATUM, West,, J. E. M. S., Nov., 1888. Plate XLIII, 



figs. 5. 6. 



Semi-cells smooth, rhomboid, with a faint indication of an 

 obscure mucro: end view triangular, with concave sides. 

 Length 76-78 p. ; breadth 60 p. ; sinus 17-18 p. 

 Pond near Amherst, Mass. Seen sparingly. 



ST. VESICULATUM, Wolle. Plate XII, figs. 6, 7. 



Small, smooth, about one-half longer than broad, constric- 

 tion deep, sinuses acute angled, much ampliated; semi-cells 

 subpyramidal, not as long as broad ; base wide, inferior 

 angles round, sides convex and inclining to a rounded apex: 

 end view triangular, angles rather broadly rounded, sides 

 slightly convex or straight. 

 Diameter 31 yw. ; length 45 ju. 



Differs from St. cordatum, F. Gay ; in front view, sinuses 

 are not narrow linear, but much ampliated, thereby giving 

 the cell a more elevated appearance. Green's Lake, New 

 Jersey. 



ST. BACILLAEE, Breb. Plate XLVIII, figs. 5, 6. 



Cells small, deeply constricted, sinus a wide, acute angle ; 

 semi-cell narrow lanceolate, base convex, ends inflated, 

 head-like, in vertical view 3-4-5 angled; angles radiately 

 produced with ends enlarged and rounded. 

 Diameter 25-30 n. 

 Pond, Minnesota. 



