68 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 123 



occupying the smaller tributaries at higher elevations (even now, 

 except in one locality on Potts Creek, above 890 m), the ancestors of 

 Dn. ardis had access to the lower elevations (below 625 m) in the James 

 River drainage. Through migrations of C. b. bartonii overland 

 between headwater streams, Dn. ileata was introduced from the 

 New River into the James River, where it is widespread in the 

 Mountain Lake area, and, although less well established in the 

 Roanoke River, it has been found in one locality there. Two sources 

 of evidence provide the basis for the hypothesis that Dn. truncata 

 originated in the Roanoke River and later reached the New River, 

 because (1) only Dn. truncata and Dn. ileata (the latter of which is 

 abundant in the New and James drainages but represented in only 

 one station in the Roanoke) are found in the Roanoke, and (2) 

 Burton and Odum (1945, pp. 192-193) indicated that two "Atlantic 

 coast" fishes are found in the New River system, suggesting that 

 streams of the New River system had pirated tributaries of the James. 

 Now there are data that support equally well a tapping of the Roanoke 

 River by the New River (R. D. Ross, pers. comm.). Even if stream 

 piracy were highly improbable, there is no reason why C. b. bartonii, 

 one of the principal hosts of the species, could not cross the relatively 

 low divide between tributaries of Indian Run or Cedar Run (to 

 Roanoke) and Slate Branch or Toms Creek (to New). 



The monotypic Phymocythere phyma, the closest relatives of which 

 are members of the genus Ascetocy there, is probably a somewhat de- 

 generate offshoot (reduced clasping apparatus and unadorned swollen 

 subterminal area of the peniferum) from the Ascetocythere stock that 

 has forsaken the groundwater environment and ventured into competi- 

 tion with other ostracods frequenting lotic surface habitats. In terms 

 of numbers of individuals, it does not appear to have been spectacularly 

 successful. Supporting the assumption of the close relationship of 

 Phymocythere to Ascetocythere is their apparent commonly shared 

 preference for living among the setiferous areas of the mouthparts of 

 their hosts. 



The single ostracod that seems to have been derived from a stock 

 reaching the area from the west is Entocythere kanawhaensis, which 

 probably arrived on Cambarus sciotensis; close relatives of this ostracod 

 are known from the west and south. 



Four species apparently are derived from stocks that have moved 

 into the area from the southwest although the ancestral home of the 

 two genera is in the southern Appalachians and Cumberland Plateau; 

 thus, any of them conceivably might have had their origin in this 

 region. Ancestors of Ascetocythere asceta undoubtedly arrived in the 

 area on the C. carolinus stock, the range of which is almost congruent 

 with that of the genus Ascetocythere; furthermore, As. asceta is not 



