70 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 123 



species, which, in some respects, is the most primitive of North 

 American branchiobdellids, and, in other respects (its parasitic 

 habitus) , among the more highly specialized. Probaby an early bran- 

 chiobdelhd stock acquired the parasitic habit, is now widely dis- 

 tributed, and has undergone (because of its confinement to branchial 

 chambers of the various hosts [?]) little or no evolutionary diversifica- 

 tion. A possible caveat to this conclusion must be entered because 

 of the lack of taxonomic studies of B. illuminatus; further study may 

 show that some of the widely disjunct populations (e.g., those from 

 the Pacific drainage in Mexico and California) assigned to this species 

 are actually other disjunct species. Xd. formosus is more common 

 in the Ohio drainage in Kentucky, Tennessee (Cumberland drainage) , 

 Ohio, and West Virginia than in Virginia, but the greatest number of 

 the species are from the Great Lakes region, primarily in Michigan. Yet 

 the genus, with the exception of Xd. jormosus, appears to be confined 

 to the southern Appalachians and the Ozarks. Xironodrilus formosus 

 is a primitive species within the genus (and the order), and one must 

 postulate a post-Pleistocene invasion of the Great Lakes from the 

 South and attribute a relict status to the populations in the James 

 and Roanoke Rivers. This conclusion has been reached because (1) 

 the distribution of the species (and the genus) is such that we can only 

 assume that it reached the Roanoke and James by way of the New, and 

 (2) that the genus originated within either the New or Tennessee 

 systems, most likely the New, since the ancient Teays River and the 

 Illinois Ozarks would link the Appalachian and Ozarkian progenitors 

 of the modern species and leave relict populations along the present 

 Ohio as a source for the invasion of the Great Lakes. The remaining 

 widespread species, C. philadelphica, is clearly one of Appalachian 

 origin that has spread from the mountains into the piedmont and, to 

 the north, the coastal plain; the western limits of C. philadelphica are 

 in the interior plateau regions of Tennessee, Kentucky, and Ohio, 

 although there is evidence here that it intergrades with the Ozarkian 

 C. chirocephala, and there is one anomalous record from Wisconsin. 

 It is futile to speculate about the origins of B. illuminatus and Xg. 

 instabilius; they are bound to the ancient invasion of the continent 

 by the ancestors of the order. Cambarincola philadelphica and Xd. 

 -formosus, the former the dominant branchiobdellid species of eastern 

 North America and the latter a primitive species fragmented into dis- 

 junct populations, are, however, derived from early inhabitants of 

 either the Tennessee or New Rivers, more likely the New. 



The remaining two-thirds of the species are at home in the area. 

 C. branchiophila is a gill-inhabiting branchiobdellid known only from 

 the Mountain Lake area, where it is found associated with all the cray- 

 fishes except the burrowing C. carolinus and the introduced Orconectes 



