AQUATIC INVERTEBRATE FAUNA OF WYOMING AND MONTANA. 249 
Antenna moderate, reaching about to the middle of the second segment of the 
cephalothorax. Sixteen-jointed in all adult females, and further especially distin- 
guished by the presence of very long flexible seta; upon the first, third, tenth, and 
fourteenth segments. Terminal set® likewise very long. The seta borne by the first 
segment extends to the twelfth ; that of the third reaches to the fourteenth ; that upon 
the tenth segment extends to the tip of the antenna, and that upon the fourteenth far 
beyond it. All these foregoing set® are borne upon the anterior terminal angles of 
their segments with the exception of that of the fourteenth, which is borne upon the 
posterior angle. 
The first segment is as long as the two following, and very nearly twice as long 
as wide. The second is very short, its length one- fourth its width, and the length and 
width of the third are equal. Of the three terminal segments the penultimate is 
longest, being twice as long as broad; the antepenult two-thirds the length of the 
following; the last is about as wide as long. 
The last segments of the thoracic legs are armed as follows: 
First pair: outer ramus, one spine and two set* at tip, two spines without, and 
three seta; within; inner ramus, two setae at tip, one within, and two without. 
Second pair: outer ramus, one spine and two setae at tip (the inner of the latter 
slender, the outer thick), four setae within, and two spines without; inner ramus, two 
set® at tip, one within, and three without. 
Third pair: outer ramus, two set® at tip (the outer one stout, short, and spine- 
like), four set® within, and two spines without; inner ramus, two set® at tip, three 
within, and one without. 
Fourth pair: outer ramus, two set® at tip, four set® within, two spines without; 
inner ramus, two set® at tip, two within, and one without. 
The fifth pair are two-jointed, the terminal joint with one long and one short seta 
at tip; the basal joint with one long seta without. 
Length, without set®, 1.2 millimeters. 
Grebe Lake, Yellowstoue Park. 
Cyclops thomasi Forbes. (Plate xxxix, and Plate XL, Fig. 13.) 
Cyclops thomasi Forbes, Amer. Nat., xvi, Aug. (1882), p. 649; Cragin, Trans. Kan. Acad. Sci., 
viii, 1881-82, p. 68 (1883); Herrick, Final Report, p. 153 (1884); Underwood, Bull. 111. 
State Lab. Nat. Hist., ii, 1886, p. 332; Forbes, Rept. U. S. F. C., 1887, p. 707 (1891). 
A long and slender species, with seventeen-jointed anteun®, oval cephalothorax, 
somewhat closely articulated, slender abdomen, very long and slender caudal rami, 
and two developed set® to each ramus, the longer of which is about twice as long as 
the shorter. 
The cephalothorax is widest at about the middle, its greatest width a little more 
than half its length. Posterior angles not prominent or produced, except those of 
the last segment, which are slightly produced outwards. Sides of the first segment 
subparallel, rounding slightly towards the front, the segment itself twice as long as 
the other segments combined; the second segment shorter than the third, but longer 
than the fourth ; the fifth reduced to a narrow linear band, as seen from above, the 
extremities of which project a little beyond the lateral outline. 
Abdomen, with furca, a little shorter than the cephalothorax, its greatest width 
one-fourth of its length, including furca. First segment in the female as long as all 
