128 DESMIDS OF THE UNITED STATES. 



St. pyg.m.eu.m, Breb. Plate XLII, figs. 14-21. 



Small, sinuses wide, subrectangular ; semicells cuneiform or 

 broadly elliptic, often alternately attached, angles more or less 

 obtuse, and sometimes spinous; membrane granularly rough. 



Diameter 1 625 ;x. 



Variable in size and form. They may be separated into 

 three divisions. 



Forma, genuina, Breb., tigs. lV-15. \f 



Forma, TRUNCATA, Wolle, figs, lh-1!>. 



Forma, bhomboides, Wolle, figs. 2<>, 21. 



Found in nearly all standing or quiet waters, sometimes in 

 large numbers. 



St. A.LTERNAN8, Breb. Plate XLI, figs. 26-28. 



Semicell granulate, elliptic or oblong, two or three times as 

 long as wide, and from their twisted position, unequal in front 

 view ; end view with three obtuse and rounded angles, form- 

 ing -lmrt rays, alternating with those of the other semicell. 



Diameter 20-28 y. Frequent. 



St. dilatatdm, Ehrb. Plate LII, figs. 32, 33. 



Small, granulate; semicells fusiform, equal ; end view with 

 three, four or five short, broad, truncate rays, granules ar- 

 ranged in transverse line-. 

 I diameter 4<> //. Frequent. 



Distinguished from the preceding, mainly, by the semicells 

 not being twisted. 



St. silatatum, Nord. Plate XLII, figs. 22, 2:;. 



( lells in front view fully twice as wide as long, deeply con- 

 stricted; sinus with rounded base, outwardly enlarged; semi- 

 cells sublanceolate, end- straight, lightly undulate; end view 

 five lobed, margins smooth. 



Diameter 30 //.; length 13-16//. 

 Ponds, Florida. 



St. cyrtcm brum, Breb. Plate XLII, figs. 30, 31. 



Cells, rough with minute granules; semicells, fronl view, 

 subcuneate, top broadly convex, superior angles produced into 



short, -tout, curved processes; end view triangular: each 



