44 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxxii. 



marg-ins furnished with long- plumose hairs and the inner with a few 

 spines. 



Telson cleft to the base, each division furnished with three or four 

 short spines and several long- hairs. 



Length, 15 ram. 



Aroostook Count}', Maine; Caledonia, New York; Marquette, 

 Ann Arbor, and Isle Ro3^aI, Michigan; dredged in Lake Superior; 

 Fort Wing-ate, New Mexico; Wahsatch Mountains; Shoshone Falls, 

 Idaho; Salt Lake City. Utah; Flat Head Lake, Montana; Yellowstone 

 National Park. 



This species closely resembles Gammarus fasciatus^ from which it 

 can be distinguished most easily by the presence of long- plumose-hairs 

 on the lateral margins of the terminal segment of the outer ramus of 

 the third uropods. The other characters in which these two species 

 differ can be appreciated only by a careful comparison of specimens 

 of the two species. The antennae in Gammarus limnaeua are furnished 

 with fewer hairs than in Gammarus fasciatus and the secondary 

 flageUum consists of two to four segments (in Gammarus fasciatus 

 five or six). The propodus of the first gnathopods of the male of 

 Gammarus limnxas is broader at the proximal end than in Gammarus 

 fasciatus; the lateral margins are more convex, and the posterior 

 margin is furnished with spines, which, if present in Gammarus fas- 

 ciatus^ are very small. In the female the propodus of the first 

 gnathopods of Gammarus lininseus is much narrower distally and the 

 palm is more oblique than in the other species. In Gammarus lini- 

 nx.us the spines on the abdominal seg-ments are smaller and the 

 median ones are not raised on distinct protuberances as they are in 

 Gam mar us fascia tus. 



After a careful comparison of the description and figures of Gam- 

 marus limnmus Smith and Gammarus rohustus Smith I have con- 

 cluded that they are the same species. The two species correspond in 

 all characters except the gnathopods, and the differences which are 

 there pointed out are not sufficiently marked to justify distinguishing 

 the two forms as separate species. In Gammarus limiuvus the pro- 

 podus of the first gnathopods is longer in proportion to the width, the 

 palm is more oblique, and the posterior marg-in is armed with more 

 spines than in Gam.marus rohustus. In the second g-nathopods the 

 propodus of Gammarus limndeus \s longer than in Gammarus rohustus. 

 These are differences which appear in specimens of different sizes. 

 From the lengths of the two species which Smith gives {Gammarus 

 limnxus l^-'iO n\n\. and Gammarus rohustus lO-lo mm.) he evidently 

 examined larger specimens of Gammarus lifnna^'us than of Gammarus 

 rohustus, which fact would account for the differences in the gnatho- 

 pods which he found. 



