SUBTERRANEAN AMPHIPOD STYGONECTES 155 



since they fall much closer to this species morphologically. Their 

 genetic component is still unclear, however, and if these populations 

 do in fact represent intergrades, a close genetic affinity between S. 

 allegheniensis and S. tenuis is more firmly substantiated. 



(3) The range of S. allegheniensis extends well north of the 

 southern limits of Pleistocene glaciation; this extension must be 

 regarded either as the result of a northward migration since the late 

 Pleistocene or as representative of relict populations which survived 

 in deep ground-water, glacial refuges. I view the former as the more 

 feasible interpretation, although a wholly satisfactory conclusion must 

 await an extensive search for populations of this species along the 

 distributional hiatus that exists between central Pennsylvania and 

 southern New York. To my knowledge, ground water outlets of this 

 area have not been biologically explored, and tentatively, I have in- 

 terpreted the disjunction as indicative of the absence of collections 

 rather than the absence of populations. 



On the basis of the foregoing discussion and distributional data, 

 one might suppose that ancestral populations of S. allegheniensis and 

 S. tenuis were once distributed along the Piedmont and higher ground 

 of the Coastal Plain from southern New England southwestward to 

 east-central Virginia. These populations were probably components 

 of a mde-ranging ancestral form, which came to occupy this general 

 area subsequent to fresh-water invasion during the Eocene or Miocene. 

 Migrations westward and northward along the Hudson, Delaware, 

 Susquehanna, Potomac, and, possibly, Rappahannock Rivers would 

 have been the most probable means of gaining access to ground-water 

 habitats of the western Piedmont and the Appalachians. 



Geographic isolation of precursor allegheniensis populations could 

 have followed their migration beyond what have already been given 

 as physical barriers between the range of S. allegheniensis and S. tenuis. 

 Further dispersal has probably taken place since initial restriction 

 of these populations to the Appalachians. Subsequent migration of 

 S. allegheniensis within the Appalachian Plateau of New York has 

 extended the range of this species to the northern margin of the plateau 

 and several populations have been found in ground waters that occupy 

 areas draining into Lake Ontario by way of the Oswego River. More- 

 over, in western Pennsylvania and western Maryland, this species 

 has extended its range westward from the Susquehanna River drainage 

 basin into the Ohio drainage system. 



The upper Chesapeake Bay dissects the range of S. tenuis and 

 presently appears to serve as a barrier to gene flow between popula- 

 tions to the north and east (S. t. tenuis) and those to the west (S. t. 

 potomacus) . The latter subspecies differs from the former subspecies 

 only in the absence of sternal gills on the first pleonites of the male and 



242-803—67 11 



