124 Illinois State Laboratory of Natural History. 



ramus, the proximal four fifths of uniform width. At the 

 beginning of the distal fifth of the inner margin is a rather 

 sharp angle, from which projects a long, slender, slightly 

 curved spine, about one fifth the length of the ramus. 

 Beyond this the ramus tapers to a blunt point, hairy at the 

 apex. Besides the spine already mentioned there is a smaller 

 one, only about half as long, having its point of insertion 

 very near and slightly above the first. 



Length of female 1.23-1.28 mm; of male 1.00-1.18 mm. 



D. sieilis closely resembles both D. ashlandi and D. 

 pallidus, differing from the latter, however, in the presence of 

 a hook on the right male antenna, and from both in the details 

 of structure of the fifth pair of feet of the male. 



A very interesting variation was noticed in the inner rami of 

 the fifth pair of feet of the male. In specimens taken from 

 Lake Superior, at Marquette, Mich., the rami were sometimes 

 both one-segmented, sometimes both were two-segmented, 

 and at other times one ramus was two-segmented while the 

 other was one-segmented. Herrick (Herrick and Turner, '95) 

 states that all his specimens had one-segmented rami; also 

 that the process on the right male antenna was shorter than 

 described by Dr. Forbes. 



Although I), sieilis is not at all uncommon, it has occurred 

 less frequently in the collections I have examined than have 

 D. siciloides Lillj., D. ashlandi Marsh, or D. oregonensis 

 Lillj. Marsh ('93) records D. sieilis from the Great Lakes 

 and from Green Lake, Wis., it being the common pelagic 

 species in 1890 and 1891, while in 1892 not a single specimen 

 was found there although the collections were made at the 

 same time of year. The type was described (Forbes '82a) 

 from Lake Michigan and had not then been found anywhere 

 else. In 1890 Dr. Forbes found it in Lake Michigamme, in 

 northern Michigan, as well as in Lake Michigan. His variety 

 imperfectus is D. ashlandi Marsh. In the Yellowstone Park 

 collections sieilis was found in considerable quantities, but 

 as both D. sieilis and D. ashlandi were present, it would 

 require a re-examination of the material to determine the 

 distribution of the two species in that locality. 



