560 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 112 



in clear water, as contrasted with B. mackini and B. gigas, common 

 in the same area, which are usually in opaque water. The temper- 

 ature of the water at the time that collections were made ranged from 

 48° to 69° F. All collections in the State of Washington were made 

 from late March to mid-June. The collections from near Rawlins, 

 Wyoming were made on August 5, 1937, and June 20, 1958. 



The elevation above sea level of the ponds in which B. campestris 

 has been found ranges from about 1,100 feet in Adams County, Wash- 

 ington, to 6,750 feet near Rawlins, Wyoming. 



In 60 percent of the collections, B. campestris is the only phyllopod 

 present. Artemia salina was, however, regularly present in three of 

 the ponds in Grant County, and often the two species occurred simul- 

 taneously; at times only one or the other species was present. In 

 three other ponds in Grant County, B. mackini in small numbers was 

 associated with B. campestris. 



In spite of the scanty data on physical conditions in the ponds, it 

 seems evident that B. campestris is adapted to living in water with so 

 high a content of dissolved salts that only Artemia salina can develop 

 abundantly in the same habitat. This may account for its scarcity 

 in Grant County, Washington (known from seven ponds) where, 

 before the construction of enormous dams and the permanent inunda- 

 tion of approximately 67 square miles of land with water from the 

 Columbia River, there were literally hundreds of ponds that annually 

 produced teeming populations of B. mackini. This circumstance pro- 

 voked the thought that campestris might be only a variety of 

 mackini developing in water of greater alkalinity and density than 

 its more usual habitat. This conjecture seems most unlikely, how- 

 ever, in view of the relatively large number of specific differences, and 

 my failure to find unmistakable intergradations between the two 

 species. 



Remarks. Including the subject of this paper there are now eight 

 species of Branchinecta reported from western North America, and 

 several undescribed species that await the attention of systematists. 

 In addition, redescriptions of several species and a study of the 

 variability of others are in order before an adequate revision of, and 

 keys to, the species of the genus in North America can be made. 



