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 setae upon the first, third, tenth, and 

 fourteenth segments. Terminal setae 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 setae 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 setae at tip, two spines without, and 

 three setna 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 

 setae at tip, one within, and three without. 



Third pair: outer ramus, two setae at tip (the outer one stout, short, and spine- 

 like), four setae within, and two spines without; inner ramus, two setae at tip, three 

 within, and one without. 



Fourth pair: outer ramus, two setae at tip, four setae within, two spines without; 

 inner ramus, two setae 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 setae, 1.2 millimeters. 



Grebe Lake, Yellowstone 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., 

 vin, 1881-82, p. 68 (1883); Herrick, Final Report, p. 153 (1884); Underwood, Bull. 111. 

 State Lab. Nat. Hist., u, 1886, p. 332; Forbes, Rept. U. S. F. C., 1887, p. 707 (1891). 



A long and slender species, with seventeen-jointed antennae, oval cephalothorax, 

 somewhat closely articulated, slender abdomen, very long and slender caudal rami, 

 and two developed setae 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 



