250 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



the others together, broadest in front, its lateral outlines emarginate behind the 

 anterior angle. Posterior margin of last segment serrate beneath and at sides; those 

 of other abdominal segments smooth. Furca as long as the last three segments, the 

 width of the rami about one-seventh of their length. The inner of the two longer 

 setae as long as the entire abdomen, the outer of the two half that length. The ex- 

 treme outermost of the terminal sette two-thirds the length of the inner; that is, about 

 one-fourth the length of the caudal ramus. Kami slightly curved outwards, with one 

 large spine and a few small ones a little beyond the middle of the outer surface, and a 

 vertical comb of small spines at one-fourth the distance from the proximal end. 



Antennae of the female moderately robust, reaching about to the middle of the 

 third segment, without special accessory structures or appendages, the three terminal 

 segments gradually increasing in length, the antepenultimate two fifths the length of 

 the last. The two segments preceding the former, taken together, shorter than the 

 last segment and about equaling the penultimate. 



First pair of legs : outer ramus, two setae at tip, two spines without, and two setae 

 within; inner ramus, one spine and one seta at tip, one seta without, and three setae 

 within. 



Second pair of legs : outer ramus, one spine and one seta at tip, two spines with- 

 out, three setae within; inner ramus, one spine and one seta at tip, one seta without, 

 and three setae within. 



Third pair of legs : outer ramus, one spine and one seta at tip, two spines without, 

 and three setae within; inner ramus, one spine and one seta at tip, one seta without, 

 and three spines within. 



Fourth pair of legs : outer ramus, one spine and one seta at tip, two spines with- 

 out, and three setae within ; inner ramus, two spines at tip, one seta without, and two 

 setae within. 



The outer ramus of the first leg is so foreshortened that the distal outer seta seems 

 to be placed at the tip of the segment, but the usual tooth marking the lateral distal 

 angle of the segment stands between this point and the seta next within, thus show- 

 ing that the spine should be counted as lateral. 



The terminal spines of the inner ramus of the fourth pair are unequal, the inner 

 one a little more than half the outer. 



Fifth pair of legs of two segments, the basal segment about as long as broad, with 

 a strong plumose spine from the outer angle, the terminal segment cylindrical, twice 

 as long as broad, with two terminal setae, the outer of which is as long as the seta of 

 the preceding segment, and the inner a little more than half that length. 



Total length, without setae, 1.33 millimeters; greatest depth a trifle less than one- 

 third the length of the cephalothorax. 



The common Cyclops of Yellowstone Lake, occurring also in various other waters 

 of that region. This well-marked and constant species has a range at least from the 

 Rocky Mountains to the Atlantic region, being, according to Prof. Cragin, a common 

 species in the water supply of Boston. It is also the usual Cyclops of the Great Lakes. 



The original description was inaccurate in two particulars : the outer distal spine 

 of the outer ramus of the first leg was called terminal, and, by typographical error, 

 the terminal joint of the outer ramus of the second, third, and fourth legs were said 

 to have two setae within instead of three. 



