142 DESMID8 OF THE UNITED STATES. 



seiniorbicular : end view triangular, angles obtusely rounded, 

 si<l straight or moderately convex. 



Diameter 40-60 //.. 

 A common species. 



St. sociatum, Wolle. Plate XLV, figs. 22, 23. 



Separated from the last two, mainly by the arrangement of 

 the spines which are sociated, or twinned. 



Diameter without -pine- 2830 //., with spines 40-45 ft. 



This species has some resemblance to St. <ji inimttum, Nord, 

 but the twinned spines are smaller, and the number twice as 

 large. 



St. tridentiferum, Wolle. Plate XLV, figs. 9, 10. 



Very small, smooth, sinus ample ; semicells elliptic, ends 

 mure or less convex, angles furnished with three, firm, diverg- 

 ing aenlei ; end view triangular, sides retuse, angles with three 



large aenlei. 



Diameter 1 72' > />., without spines ; with them 25-28 //. 

 Sluggish waters, Pennsylvania. 



St. cruciatum, Wolle. Plate XL\'. figs. 11-13. 



Small, smooth, front view cruciform, lobes short linear, ends 

 rounded; sinuses wide, rectangular; end view three or four 

 lohed, slightly tapering, end- rounded and furnished with a 

 number of more or less diverging setae, which are as long as 



the lobes. 



Diameter 25 /<., without setae. 

 Ponds, Northampton Co., Pa. 



St. Cerberus, Bailey. Plate XLV, tig*. 7, 8. 



"Cells small, deeply constricted, semicells three-lobed; 



lobes with four teeth, two of which project upwards and two 

 downwards, at each truncate angle." 

 Diameter 25-30 n. 



This species is reported by Bailey, from lakes in Florida. 



St. Hystrix, Ralfs. Plate XLV, figs. 14-16. 



Semicells in front view subquadrate, extremities somewhat 

 rounded, end margins nearly straight, furnished with a few 

 scattered, subulate, acute spines, chiefly confined to the lateral 



