no. 3602 CRAYFISHES — HOBBS, HOLT, AND WALTON 67 



Cambarus sciotensis — by migrations from the west along the New River. 

 Orconectes juvenilis — by introductions in Potts Creek and in Mountain Lake 

 proper and by recent migrations from the west in New River. 



Despite overwhelming evidence for a lack of host specificity for 

 most of the ostracod and branchiobdellid symbionts, it is highly 

 probable that all of them or their ancestors have reached the area as 

 passive migrants on one or more of the crayfish stocks presently 

 represented in the area. Too, it is scarcely to be denied that the 

 association of certain ostracods with a particular crayfish is in a few 

 instances apparently absolute, and most of the ostracods tend to 

 "favor" one or two crayfish species. Among the ostracods that have 

 evolved nearby or within the Mountain Lake area are the monotypic 

 Phymocythere phyma, and three species of the genus Donnaldsoncy- 

 there: Dn. ardis, Dn. ileata, and Dn. truncata. As nearly as can be 

 ascertained at the present, the genus Donnaldsoncy there has as its 

 center of distribution the Appalachian Mountains, and Dn. ardis and 

 Dn. truncata are confined to the general Mountain Lake region. 

 Dn. scalis, which is also known from two localities in Pennsylvania 

 and one in New York, is largely restricted in the Mountain Lake 

 area to the higher altitudes in the James and New drainages, where 

 Cambarus b. bartonii is the usual host. Dn. ardis, endemic in the 

 James drainage, occurs at lower altitudes primarily on Cambarus I. 

 longulus. Dn. truncata appears to be confined to lower elevations 

 in the Koanoke and New drainage systems, where the principal hosts 

 are C. acuminatus, C. b. bartonii, and C sciotensis. Dn. ileata, known 

 from a number of localities in the New River (Kanawha) system to 

 the west of the Mountain Lake area, occurs in all three drainage 

 systems and on all the species of crayfishes in the area except C. 

 carolinus. In this group of four closely related ostracods it seems 

 likely that three of them might well have evolved from a common 

 stock that became, for a time, isolated in the three drainage systems 

 studied here: Dn. ardis in the James, Dn. ileata in the New, and 

 Dn. truncata in the Roanoke. Dn. scalis probably arose in some 

 stream to the north (its range is too poorly known at present to be 

 more specific), and it has been introduced by C. b. bartonii into the 

 Mountain Lake area at higher elevations, apparently first reaching 

 the James drainage and later being transported, presumably by the 

 same crayfish, to the headwaters of the New River. The absence of 

 Dn. scalis from the Roanoke River can perhaps be explained by the 

 fact that the headwater streams occur at too low an elevation for the 

 species — certainly the maximum elevation of the system in the area 

 under consideration is marginal for the species. With Dn. scalis 



