176 BULLETIN UNITED STATES GEOLOGICAL SURVEY. 



In both sexes there are eleven pairs of limbs, which are lamellar, 

 short, and much rounded. A fin-like caudal expansion beginning on the 

 sixth segment from the end, deeply incised at end of last segment on 

 the median line of the body, spatulate, or like the blade of an oar in 

 outline ; edge fringed on the terminal third. 



The sexaal characters are very distinct when the animal is one-third 

 grown, the oviducts being red with eggs, and the males with the frontal 

 tree well developed. 



Length, J-, 23°^°^; 5 , 26^^°^. 



Ellis, Kans., Dr. L.Watson, Jane 26, 28, and 29, October 10 and 22, 1874. 

 In pools of water on the plains in company with Esthena and Lymnetis. 

 A fall-grown male occarred September 27. On October 1-22, females 

 of full size were collected, in company with Ajyus lueasanus, UuUmnadia 

 compleximanus, Estheria clarhii. Ovisacs still with eggs, but empty at 

 the end. 



No striking variation was observed among several hundred specimens 

 of different ages. Dr. Watson writes that they had " red tails". 



Streptocephalus icatsonii, n. sp. — Though this is closely allied to ^S*. tex- 

 anus, Packard (see Hayden's Annual Report of the United States Geo- 

 logical and Geographical Survey of the Territories for 1873, p. 622, plate 

 iv, fig. 13), still the differences here pointed out are constant in numer- 

 ous specimens. 



Male. — The claspers (second antennae) are much longer than in S. tex- 

 anus, reaching, when extended, to the middle of the body, while in >S^. tex- 

 anus they only reach a third of the length of the body. The median lobe 

 of the head, which is very large and long in 8. texanus, reaching nearly as 

 far as the insertion of the basal filamentary appendage of the third joint 

 of the claspers, is, in 8. watsonii, not half as large. The two basal joints 

 of the claspers are twice as long and much slenderer than in S. texanus; 

 the third joint is nearly as long, while the branches and spines of the 

 fonrth joint, though of the same number, are much longer and slenderer. 

 Of the longer branch, the supplementary spine is much longer, and with- 

 out the small inner spine, while the main branch beyond is bent at near- 

 ly right angles, the elbow being much bent, the inside, however, regu- 

 larly curved. At the base of the broader and shorter branch are 

 four unequal teeth ; one attached to the third joint, the other to the 

 fourth, the two terminal ones very unequal, and the fourth square and 

 three times as large as the third, while the corresponding tooth in 8. 

 texanus is long and narrow, and smaller than the one behind it. The 

 genital appendages are long and slender, much as in S. texanus, as long 

 as the three segments followi;ig the one to which they are inserted. The 

 caudal appendages are much shorter and broader than in S. texanus, 

 each blade being broader, and tapering regularly from base to tip, not 

 contracted in the middle, nor curved, as in the male S. texanus; on the 

 other hand, they are of much the same form as in those of the female S^ 

 texanus. 



