STAURASTRUM. 155 



center of the semi-cells, aculeated; 7-10 aculei to a side, 

 end view. 



Diameter 27-30 /J. 



Not infrequent in pools and ponds, New Jersey, Pennsyl- 

 vania and other States. 



ST. PECTEN, Perty. Plate LVI, figs. 35, 36. 



This looks like and probably is a depauperated variety of 

 the last, St. echinatum. 

 Diameter 20 f*. 



Denmark Pond, Passaic County, N. J. 



ST. HIRSUTUM, (Ehrb.), Breb. Plate LVI, figs. 19-21. 



About as long as broad, more or less densely covered with 

 thin, short, even, hair-like spines ; semi-cells elliptic or sub- 

 semiorbicular; end view triangular, angles obtusely rounded, 

 sides straight or moderately convex. 

 Diameter 40-60 p. 



A common species. 



ST. SOCIATUM, Wolle. Plate LVI, figs. 22, 23. 



Separated from the last two, mainly by the arrangement 

 of the spines which are sociated, or twinned. 



Diameter without spines 28-30 /*., with spines 40-45 n. 



This species has some resemblance to St. geminatum, Nord., 

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

 large. 



ST. XIPHIDIOPHORUM, Wolle. Plate XLVIII, figs. "21, 22, and 



Plate XLIX, fig. 19. 



Small, one and one- half to two times as long as broad, 

 deeply constricted, sinus narrow, widening irregularly out- 

 wardly ; semi-cells transversely oblong, with lateral margins 

 notched ; the end margins drawn out into a sort of one-sided, 

 hastate, poignard-like spines or slender points, usually nine 

 in number ; membrane smooth ; with several verrucae ; end 

 view triangular, angles broadly truncate and usually each 

 showing three prominences, the bases or supports of three 

 spines, sides concave. 



Diameter 25-30 //. ; length 40 /<. and upward. 



Ponds, near Minneapolis and Stillwater, Minnesota. 



Var. SIMPLEX, Wolle, differing from the type-form in having 

 less spines, usually six; (Plate XLIX, fig. 19,) in end view 

 the truncate angles have only two prominences each. 



