AQUATIC INVERTEBRATE FAUNA OP WYOMING AND MONTANA. 255 



It differs frofli typical nevadensis in the more complete segmentation of the 

 ceph.ilothorax in both male and female, four sutures extending distinctly across the 

 back. It is also a little larger, adult females measuring 2.12 to 2.4 millimeters in 

 length to the tips of the rami, with an ordinary width across the cephalothorax of G.4 

 millimeters. The male is somewhat smaller, about 2.1 millimeters in length. 



The antennaj of the female are long and slender, reaching about to the posterior 

 end of the penultimate segment of the abdomen. The first segment of the female 

 abdomen is as long as the two following together, and the furca is as long as the pre- 

 ceding segment. The female abdomen is not curved as in lacustris, and the sperma- 

 tophore extends downward and backward instead of curving upward, as in that species. 

 The three caudal setaj are all similar and of equal width, the base of each being a third 

 the width of the end of the furca. There is a short, stout, conical spine at the outer 

 distal angle of each ramus, and a small, soft seta at the inner angle. 



The fifth legs of the female are broader in proportion to their length than in 

 lacustris, but more slender than in the nevadensis of Lilljeborg. The last segment 

 of the leg is four times as long as broad and bears six teeth (occasionally seven), four 

 of which are terminal. The inner of these four is commonly the largest, although the 

 third from within may equal it. The inner lateral tooth is close to the inner terminal, 

 and nearly or quite equals it in size, and the outer lateral is nearly opposite. Some- 

 times there are two teeth on the outer margin of this segment. The middle joint of 

 this leg is less than half as wide as long, and the basal is longer than wide. 



In the male abdomen there are five distinguishable segments, as in all the other 

 species,* the second, third, and fifth bearing lateral processes extending to the right. 

 The first three segments are subequal in length. The lateral process of the second 

 has the form of a stout but thin wing or lamina projecting laterally a distance equal 

 to the width of the segment. It springs from a broad and thick prominence of the 

 segment itself; is acute at the apex, with the point a little recurved, convex and 

 smooth in front, as seen from above, and nearly straight behind, except that this edge 

 is irregularly and minutely serrate throughout and deeply emarginate where it joins 

 the segment. As seen from the side this blade is strongly curved downward (ven- 

 trally), like the following. The third segment bears a broad, thin lamina which projects 

 outward and a little backward from its posterior angle as a flat process, as wide as 

 long, curved downward and broadly rounded at the end, quite simple, except that it 

 is strengthened beneath by a ridge of chitin. These processes are in strong contrast 

 to the corresponding ones of nevadensis proper. From the fourth segment spring 

 two processes, the ventral of which is very similar to that of nevadensis, but broader, a 

 triangle in form, with nearly equal sides, with the apex slightly truncate and bearing 

 three serrations, and with the posterior side very minutely roughened. The dorsal 

 process of this segment is a small irregular plate curving forward, inward, and 

 downward. 



Fifth pair of legs substantially as in nevadensis Lillj. 



Abundant in Swan and Flathead lakes, Montana. 



*A recent study of the male abdomen in lacuxtris shows beyond a doubt that the fourth and 

 fifth segments are flexibly articulated and that the fourth is without process, the fifth bearing two 

 processes, as in all the other species. 



