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 setae two-thirds the length of the inner; that is, about 
one-fourth the length of the caudal ramus. Rami 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 autepenultimate 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 uuequal, 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- 
tliird 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 setce within instead of three. 
