SUBTERRANEAN AMPHIPOD STYGONECTES 137 



difference has been found between populations on opposite sides of 

 the river, although the bedrock occupied by the Greenbrier River 

 immediately between Court Street Cave and Organ Cave is that of 

 sandstone. A few miles further west in the vicinity of Fort Spring, 

 however, the river flows on limestone, and this same limestone is 

 stratigraphically continuous with that which contains caves on both 

 sides of the Greenbrier River. Dispersal under a major river, then, 

 is also indicated for S. emarginatus, but it may occur in a more round- 

 about manner than that postulated for S. gracilipes. 



Primarily on the basis of significant morphological divergence in 

 the structure of the telson, the emarginatus group has been taxonomi- 

 cally divided into two subgroups. Assuming a common ancestral line 

 for the emarginatus group, this line must have furcated at an early 

 time into two branches, each giving rise through subsequent changes 

 to the currently recognized subgroups. Tw^o species make up the 

 gracilipes subgroup and five species compose the emarginatus subgroup. 



On purely morphological grounds, S. gracilipes and S. conradi 

 appear to have shared a relatively recent common ancestry. In 

 comparison to S. conradi, S. gracilipes appears more primitive as 

 evidenced by its possession of a larger number of spines on the 

 gnathopodal propods and telson and more setae on the posterior mar- 

 gin of the abdominal side plates. The greatly reduced ramus of the 

 third uropod of S. gracilipes, on the other hand, appears rather highly 

 specialized. A common ancestor to these two species is postulated 

 to have ranged throughout an area presently occupied by the head- 

 waters of the James River drainage and throughout the eastern part 

 of the headwaters and upper sections of the Potomac River drainage. 

 Migration into caves would have resulted in isolation of populations 

 on either side of a drainage divide composed of several noncavernous 

 ridges; thus, different gene pools could have developed in opposite 

 valleys. 



The five species of the emarginatus subgroup are rather homogeneous 

 in regard to the structure of the telson, but comparatively divergent 

 in the shape of the pereopod bases. Of these five species, only S. 

 mundus is without the shallow excavation in the apical margin of the 

 telson; otherwise this species bears a close morphological alliance with 

 S. emarginatus. S. mundus and S. emarginatus have undoubtedly 

 been derived from a common ancestral form, which was at one time 

 distributed along the western half of the Valley and Ridge Province 

 from western Maryland southward to west-central Virginia. Coloni- 

 zation of cave waters in the Mississippian limestones of the eastern 

 margin of the Appalachian Plateau by populations sharing in the same 

 gene pool as populations that simultaneously invaded cave waters of 

 the Devonian limestones in at least one western valley of the folded 



