52 Illinois State Laboratory of Natural History. 



the position of certain spines of the feet, but this is really a 

 difference in description rather than in characters. I find 

 also that specimens collected in Wyoming agree exactly with 

 the Illinois representatives of the species. 



I have noted its occurrence in collections from Quiver, Dog- 

 fish, and Thompson's lakes, and from the Illinois Eiver, 

 near Havana, 111. ; from the Sangamon Eiver, in Champaign 

 county, Illinois ; and from Grebe Lake in Yellowstone Park. 

 Marsh reports it from Eush Lake, Wisconsin, and Herrick finds 

 it in Cullman county, Alabama, and in Minnesota lakes. I 

 have never found it in temporary ponds nor in any of the 

 Great Lakes. From the situations in which it occurs I judge 

 that this species seeks shallow, weedy water rather than 

 pelagic situations. It is not an especially rare species, but I 

 have never found more than a very few individuals in a single 

 collection, 



SPECIFIC DESCRIPTION. 



The cephalothoracic segments are closely articulated and 

 the sides regularly convex. The first, segment is longer than 

 usual and the fourth is semicircularly excavate behind. The 

 posterior edges of the first three segments are irregularly 

 notched and the fourth is smooth. The length of the cepha- 

 lothorax is to its breadth as two to one. 



The abdomen is long, slender, and cylindrical, and is pecu- 

 liar in lacking the usual fringe of spines on the posterior edge 

 of the last segment. The posterior edges of the other seg- 

 ments are likewise smooth. The anterior end of the first 

 segment is but little enlarged. The segments diminish regu- 

 larly in length from first to last. The caudal stylets are about 

 twice the length of the last segment and four times as long as 

 broad. The lateral seta is placed a trifle beyond the middle 

 of the ramus. Behind the lateral spine the stylet is pecu- 

 liarly excavate. The outer terminal bristle, which is set 

 farther forward than in most species, is a short, sparsely 

 plumose seta, l)ut the other three seta' are well developed. Of 

 these tlu'ee the middle one is consideral>ly the longest and 

 the inner one slightlv shorter than the outer. 



